Welcome to the new FAQ section of XFire Performance. Answers presented here are based on many years of crossfire experience and the assumption that your 1982 through 1984 Crossfire engine is in good running condition, and there are no major mechanical issues or Check Engine Light (CEL) that is on. These questions are some of the most commonly asked questions about the stock Crossfire and modified engines that we get emails on or calls about on a daily basis. If you have questions that are not listed here, please feel free to send us your specific inquiries via the Contact Form on the Contact page. Due to the high volume of emails and calls, we are unable to troubleshoot your specific issues, that’s what the GM service manual is for. If you do not know how to balance your throttle bodies, have a look at #22 below and watch the two part videos on that subject. We highly recommend that you always refer to your GM Service Manual as your final guide on troubleshooting anything or maintenance. If you do not have a GM service manual, please get one. They are worth their weight in gold in troubleshooting any crossfire car. You can also purchase them from us in digital format on our Shop page. I hope you find these FAQs helpful.
Q: What fuel pressure should I use on a stock crossfire motor?
A: We have found in our testing and as a good rule of thumb to use a fuel pressure setting of 13psi. The GM service manual recommends 9-13psi as the range. The Crossfire motor will not run properly (or to its full potential) on a fuel pressure setting of 11psi or lower, so the 13psi GM recommended setting is best. You can adjust to a higher fuel pressure with a stock motor, but we don't recommend it and it won't make a big difference and probably not needed even if your fuel pump can provide that much flow reliably under WOT. A WORD OF CAUTION We also recommend that you rebuild your throttle bodies if you don't know when the last time this was done. The diaphragm/bladders in both throttle bodies become weak over time and an increase in fuel pressure may be enough to burst them. If that happens, you stand a good chance of catching your car on fire because of fuel pouring over a hot engine.
Q: Is it necessary to know what the fuel pressure is set to on a crossfire motor?
A: Yes, most definitely. The crossfire engine is rather fuel pressure sensitive. It is "extremely" important on a crossfire motor to know what the fuel pressure is set to in order to get the maximum performance from the engine. A low fuel pressure will make the motor run lean especially at WOT and run out of power on the top end around 4,000rpm or greater. We can help you with adjusting the fuel pressure with our fuel pressure adjustment tool on our Shop page.
Q: My car stumbles and runs rough with an erratic idle, what is wrong?
A: This is a very common issue with crossfire engines. The stumbling is probably from a weak fuel pump, low fuel pressure, dirty fuel filter, a dirty fuel pump sock, out of balance TB issue or all the above. We recommend that when replacing any of the fuel system parts to use AC Delco parts only. We also recommend replacing the fuel pump if necessary to the corvette 1985 - 1987 fuel pump which is a direct replacement and has a little bit more flow (gph/lph). Set the fuel pressure to 13psi. The erratic idle both high and low is more than likely caused by a vacuum leak. The typical cause of this is the top plate bolts are loose on the stock manifold and or the throttle body base gaskets are loose and leaking. If you just tighten loose top plate bolts and the gasket is original or very old, it is probably brittle. Also, you will probably crack the gasket further and cause even more leaks. There could be other vacuum leaks and further troubleshooting may become necessary. Always refer to the GM service manual for guidance.
Q: My fuel pressure regulator is blocked on the bottom, how do I adjust the fuel pressure?
A: The fuel pressure regulator located in the rear throttle body on both the 82 and 84 Corvettes and the 82 and 83 Camaro, Firebird and Trans Am engines. The stock fuel pressure regulator was blocked off by GM at the factory. Remove the rear throttle body tower and the regulator "CAN" on the bottom of the tower. "CAUTION", there is a considerable amount of tension on the regulator spring when the four screws holding the "CAN" to base are removed. It is better to use a small clamp to hold it together when unscrewing the screws holding it together so you don't strip them. Now remove the round small anti-adjust plate in the bottom of the regulator "CAN" that is blocking the hole, just punch it out. After removing the plate, the regulator is now fully externally adjustable. Now would be a good time to rebuild the throttle body or at least replace the regulator bladder at this time while apart. You can adjust your fuel pressure easily using out fuel pressure tool available on our Shop page. As always, use the GM service manual as your guide.
Q: I have no fuel spraying from either injector, what may be wrong?
A: There are a few possibilities for this. Check to ensure that the fuel pump is running and in-tank sock filter is not plugged, replace frame rail fuel filter on passenger side, check the throttle body fuses TB#1 and TB#2 (3A), ensure that the injectors are firing by using either a test light or "Noid" light. Also check the fuel pressure is at least 13psi using a fuel pressure gauge. If all these check good, check the ECM connectors and see if the CEL light comes on during key ON briefly, If not, check the 20amp fuse in the battery box. This fuse supply’s the ECM with power. If bad, the engine will crank, you will get spark, but no fuel since the ECM controls the fuel relay and will not power the pump. Also check the HEI module or HEI reference wire on ECM (connector 452, pin2) to HEI for short or open if less than 1.0 volt. The issue could also be the be the HEI pickup coil is bad. Refer to the GM service manual for further guidance if needed.
Q: My motor starts for a short time and then no fuel sprays out of either injector, if I pour gas down the throttle body it will briefly run, what is wrong?
A: There may be several issues causing this condition. Here is a list of probable causes, evaluate each one before proceeding to the next. Injectors not firing, fuel pump, fuel pump sock dirty, fuel filter dirty, dirty injectors, fuel pressure too low (set it to 13psi) or fuel pump relay. Also, it could be the short rubber hose on the fuel tank on top where it connects to the fuel line to the pump/sender assembly; it may be cracked due to age.
Q: I only have fuel coming from one injector, what could be causing this?
A: First, check the two fuses that power the injectors in the fuse box, INJ#1 and INJ#2, both of which are 3amp. Next, check to ensure that the injector itself is working OK by swapping them or swap the connectors and see if issue moves to the other injector. If this is not the problem then check the wiring from the injectors to the ECM and the ECM connector for a good connection to the ECM edge card. If that is ok, then the Injector driver circuit may be bad in the ECM. If you have a CEL light, refer to the GM service manual for more troubleshooting guidance.
Q: Why does my motor bounce between open and closed loop with a 1227747 or 1228746 ECM with headers?
A: More than likely it is your narrow band (NB) O2 oxygen sensor either bad or is cooling off too much causing the ECM issue, the latter usually being the problem. To remedy this from happening, replace your O2 sensor with a heated O2 sensor. You will have to add in a new power source for the heated sensor in the wiring which is simple and always connect it to a switch source so that it only comes ON when the engine is running so you don't drain your battery. There are also companies out there that make a non-heated stock O2 sensor harness conversion to a heated 02 sensor.
Q: My engine idles rough and rich when cold, what is the problem?
A: The most likely culprit, but not limited to this is the Coolant Temperature Sensor (CTS) is bad and should be replaced. It is the sensor that screws into the front of the stock crossfire manifold horizontally and has a small plastic shield around it on a stock engine and same position on the Renegade manifold. Replace the sensor with the newer two pin weather pack type. Do not wrap the threads with Teflon tape if you decide to install the stock type again, the threads/sensor body provides a ground through the manifold and the block to frame. Use Permatex high performance thread seal sparingly instead. Also check the O2 sensor if it is happening in closed loop as well.
Q: Will I gain better performance by installing 80lbph or 90lbph injectors on a stock crossfire engine?
A: The short answer is No, you will not gain anything by installing either injector on a stock engine. Actually, you will be increasing the fuel beyond the motors ability to use it efficiently and the stock ECM can not utilize them correctly or efficiently.
Q: When would it be necessary to install 80lbph or 90lbph injectors?
A: If you have a rather highly modified 355, 357, 377 or 383 cubic inch motor or larger, you need to install these size injectors and increase the fuel pressure to a minimum of 22 psi to feed the motor properly and go up from there if needed. It may also be required to run an even higher fuel pressure with these motors depending on the cam selection. This cannot be done with a stock crossfire fuel pressure regulator safely. Along with the larger injectors and higher fuel pressure, you will also need to install a different ECM to handle these mods. A modified GM1227747 or GM1228746 ECM are the favored choices along with other mods like the Harness Adapter Module (HAM) board that we sell along with the EBL FLASH II from Dynamic EFI also linked from our HAM board page on the Shop page.
Q: Will I see large performance gains by replacing my stock heads?
A: This is a bit of a tricky and somewhat controversial question to answer depending on the size and chamber used. Yes, you will see better performance gains over the stock heads by going to an aftermarket head, the stock heads do not breathe as well as the aftermarket aluminum heads. However, bigger isn't always better when dealing with a wet system like the crossfire engine. The key on head choice is two-fold; one, that the head flows better than stock and two, the velocity needs to remain high to keep the fuel in suspension. When the fuel drops out of suspension your power will suffer.
To retain the higher velocity, you must select a head with a smaller runner. A 170cc or 175cc runner works well on a stock engine. I have used a 180cc Dart Pro1 head on a 357 cubic inch engine and a 383, which later I opted for a AFR 195cc runner works better on a highly modified 383 cubic inch engine which I now have.
My preference rests with the 180cc runner head being the middle ground best choice for a 355 engine and 170-175cc runner for a stock engine and 195-200cc runner for a 383 or higher. Remember, if you lose the velocity and the fuel drops out of suspension, you lose any performance gains by going to an aftermarket head in the first place. It may also benefit you if you're going to replace the heads to port the intake manifold or purchase a Renegade manifold which already has the larger cross-sectional area and 20% more volume to optimize your gains even further. Do your research on this subject before opening your wallet and wasting money.
Q: How can I tell if my throttle body shafts are worn?
A: The easiest way to check your throttle bodies for excessive play is to grab your throttle body linkage and see if you can move it up/down? If you can move it, it’s bad. Sometimes it will rattle if badly worn and WILL NOT hold a balance even if you balance them perfectly, as soon as you press on the accelerator pedal, they will become out of balance and idle. Since you will not be able to balance them properly, the idle quality will suffer. There "will be" play or movement in the horizontal plane in/out which is normal. We may provide a throttle body restoration and boring service down the road, but that is yet to be seen.
Q: I heard that installing two inch throttle bodies or larger will increase the performance of my stock crossfire, is that true?
A: We DO NOT recommend installing two inch or larger throttle bodies or increasing the injector sizes on any stock crossfire engine. Increasing the fuel pressure to 13-14psi to your stock crossfire injectors will usually show a marked improvement in performance along with balancing your throttle bodies. A WORD of CAUTION; It is always a good idea to do a rebuild on any throttle body that is old or an unknown condition before increasing the fuel pressure to be on the safe side. The bladder/diaphragm inside the throttle bodies can become weak over time and may blow out with the added pressure and present an instant safety issue and possible fire hazard.
Q: What size throttle body should I run on my highly modified 355 or 383 engines?
A: Another tough and tricky question to answer and no real specific answer as it depends on a lot of variables on both of these engines. Generally speaking, a two inch throttle body using either an 80lbph or 90lbphr injector will work fine even on a 383 motor. Just like head selection, on a crossfire, it doesn’t necessarily mean, “Bigger” is always better, depends on other modifications done. However, with that said, there are guys out there running 2.25" throttle bodies with a 383 engine and running just fine. The Renegade manifold will except up to 2.25” throttle bodies as well. I personally run 2.13" throttle bodies on my 383 with 90pph injectors at 30psi externally regulated. We also now do throttle body boring up to 2.15” which will work great with any highly modded 350 or big cubic inch motor.
Another item to consider is that you may/will need to change your stock ECM to either a GM 1227747 or GM 1228746 which is the typical choice and have it modified to accommodate for laptop tuning. We highly recommend utilizing the EBL FLASH II setup from Dynamic EFI with our HAM board. There are other ECM units to choose from as well.
Q: What kind of performance gains can I expect to see if I were to install the Renegade performance manifold?
A: This is a tough question and is somewhat hard to answer and has no absolute values because of various reasons dealing with engine condition, proper maintenance, total miles and modifications that have been done and other modifications etc... Gains will vary, but generally, through our testing in a controlled environment on a chassis dyno we have seen on average a 30HP gain to the rear wheels and some even a little more over a stock crossfire manifold and throttle bodies that are set up properly. We have also seen a 25HP plus gain to the rear wheels over other previous crossfire adapted type manifolds like the old XRam manifold on larger displacement engines. The Renegade manifold dropped one person’s 383 engine 1/4 mile ET by over three tenths of a second over the previously installed XRam manifold and he is now running 12.20s naturally aspirated in his 1984 crossfire.
Q: Can I install a large lift and higher duration cam in my stock crossfire engine and still use the stock crossfire ECM?
A: In short, No. The lift is not really the issue, but the higher duration is the problem. You can not stray too far from the stock duration if you decide to keep and run the stock ECM. You must stay around the 210-214(ish) duration @ .050” for the ECM to be happy. The cam can not be a “thumper cam” that likes to lope which will force the MAP sensor to bounce around excessively and in-turn cause the ECM to wig out and not know what to do and set CEL lights none stop. Also, try and keep the LSA closer to 114 if possible which is more ECM friendly.
Q: Can you send me your tune from your 82 CFI?
A: I have been asked this a lot. The short answer is No. I have spent a great deal of money over the years developing my tune for my 383 engine on a dyno. At times, I thought my car was becoming a dyno queen. The price I would have to charge for my tune would NOT be cost effective to anyone and unless your engine was EXACTLY the same as mine, it still wouldn’t be correct for your engine. Sorry everyone.
Q: My orange Check Engine Light comes on, but then goes off. My car runs bad when it is ON, what’s going on?
A: Well, this is another hard question to answer and be specific. There are a number of things that could be causing your CEL to come and then go off. You must read the codes that are stored in your ECM to figure out what is causing the ECM to set your CEL. The ALDL test port is located under the ash tray and you can short pins A/B together and watch the CEL flash. The flashes correspond to the number of the code set. Short flashes followed by long flashes. Refer to the GM service manual on how to do this procedure and what the codes mean.
Q: My engine when cold has a very high idle rpm, but doesn’t come down much when warmed up, what is causing this?
A: Most likely you have a large vacuum leak. There are a lot of places in a corvette that can cause a vacuum leaks. However, the typical issue, but not limited to is the top plate gasket is possibly cracked or the top plates are lose or both. If the gasket is the original gasket, just replace it on a stock CFI motor. Also, the throttle body base gaskets could be leaking as well. The IACs provide idle control and there are two of them, one on each throttle body. These are essentially a ECM controlled vacuum leak to maintain a proper idle. If more air is coming into to the engine from somewhere else versus through the IACs, it is called unmetered air and the result is a high idle that doesn’t want to come down to normal idle. Ensure the IAC’s are cycling properly as well. Refer to the GM service manual for guidance.
Q: Will I see any appreciable increase in power porting my stock GM crossfire intake manifold?
A: Short answer is, Yes. You will see a good increase boost in power over the stock ports, but you must be careful when porting a stock manifold. The walls are a little thin and you can easily bust through into the water cavity and then you will have real issues. If you decide to port the manifold, go easy, slow and remove small amounts at a time. You will not see an equal boost in power compared to the Renegade manifold which was specifically designed for maximum power for any CFI engine.
Q: I hear a lot about balancing my throttle bodies, but how do I do it and is it really necessary?
A: The answer to “How To” balance your throttle bodies is an easy process and if you watch the two part series on throttle body balancing with the links below, you will understand how to do it and how simple it really is. Although the videos are not corvette specific, but use a water manometer just like the GM service manual uses, the videos are for crossfire injection and they are good videos. All crossfire injection engines run on the same principle, they need to be in balance with each other to run properly. Lots of people make it out to be rocket science, but it isn’t. Now, is it necessary? The short answer is Yes. The crossfire injection engines are both balance and fuel pressure sensitive. The balance part is to get both throttle bodies in unison with each other so that they both open and close together which will smooth out the idle quality. The fuel pressure is important so that you can feed the engine the proper fuel amount it needs to run and also under wide open throttle (WOT). The recommended amount of fuel pressure is 13psi by GM standards and the range is 9-13psi. Anything less than 12psi and the engine will not run to its full potential.
TB Balance Video Links
Q: My stock ECM is dead, what can I do to drive my corvette again?
A: This is an unfortunate situation, but it happens after almost 40 years of use with electronics that is in a closed box area next to a lead acid battery which the fumes after years of exposure to the ECM corrode electronics and connections. You won’t be able to drive your vehicle without an ECM (computer) controlling the engine. Also unfortunately, a stock ECM for the 82 and the 84 corvettes are extremely hard to find anymore and if you happen to find one, it may not be a good one. Even if a company refurbishes the ECM, they usually are bad or go bad quickly. Now for the good news. We no longer sell our famous HAM board (Harness Adapter Module), but it is incorporated into the EBL Flash II ECM which is the interface board between the newer style ECM along with the EBL Flash II from Dynamic EFI. This will let you retain your stock CFI system main harness and make you engine 100% laptop tunable. Another big advantage of doing this upgrade is that you can tune the engine to accommodate any mods that you happen to do down the road to include a NOS system if you wish. We also offer a FREE stock tune for the 82 and 84 corvettes to get you going right away and save you lots of tuning time. We also suggest going to an AGM style battery to eliminate the battery fume out-gas issue.
Q: Why do you have a 15% restocking fee now on returned items?
A: Short answer is, this was forced on us because of some unscrupulous customers who were taking advantage of us and using our products as troubleshooting aids because they couldn’t figure out what was wrong with their car. Then after using our products and finding the real fix were then saying that our products were defective and not as depicted and wanted their money back, when there was noting wrong with our products. So to that end, we had to start a 15% restocking fee on all returnable items that were not damaged or defective.
We make every effort when someone calls us about an issue to determine if they really need our products. I don’t want to sell anyone an item that they really don’t need, I don’t do business like that just to make a buck. Unfortunately, there are bad customers out there and I can’t fix stupid with any of our products.
Q: What does the TPS sensor do?
A. The TPS sensor is for idle. It tells the ECM where the throttle blade angle is in relation to idle. The TPS MUST be set to .525vdc at idle for best results. To set it, engine OFF, key to ON position. Loosen both screws slightly and adjust the sensor by moving it up/down while watching your volt meter for the proper voltage of .525vdc. You can purchase our TPS test harness in the Shop to make the task much easier. The procedure is in the GM service manual.
Q: I have a 84 corvette and want one of your fuel pressure assemblies, will a 82 corvette fuel pressure assembly work?
A: Technically the 82 fuel pressure assembly will bolt right on to your 84 throttle bodies with no issue and work fine. However, it will NOT work if you try to reinstall your 84 corvette air cleaner assembly. The air cleaner on a 82 corvette is completely different than a 84 corvette. If you look at the pictures in our Shop, you will see there is a difference in design for the two assemblies.
These items below (28-33) are specific and most common issues that come up on CFI engines that I have seen. I have put together a description and some troubleshooting methods to help you understand how they work and would be helpful when using the EBL as well in tuning with some. It does not have or cover any of the fuel pressure issues which are also common of CFI engines. Refer to the GM service manual as always to further your troubleshooting technique. I hope this helps to understand how CFI sensors work.
MAP Sensor
The CFI engines utilize a MAP or Manifold Absolute Pressure sensor. The MAP sensor has a 5 volt reference and ground inputs and in turn outputs a voltage signal between 0 and 5 volts depending on the pressure it senses. This sensor is responsible for measuring the pressure in the intake manifold relative to atmospheric pressure. When the pressure it senses is lower than atmospheric pressure (vacuum), the sensor will output a lower voltage. As the pressure it senses rises to match atmospheric pressure (vacuum drops), the voltage output of this sensor rises. This sensor also doubles as a barometric pressure sensor. When you turn the key on before starting the engine, the ECM takes the reading off the MAP sensor and uses that to calculate barometric pressure. The ECM uses barometric pressure as the basis for calculating fuel and spark delivery to the engine.
When the engine is started, the ECM uses the MAP sensor readings to determine engine load. This is the primary sensor it uses to determine fuel and spark delivery to the engine. The MAP sensor reading is factored in with engine RPM to calculate volumetric efficiency. This is known as Speed Density (engine speed and density of the air charge). Volumetric efficiency (VE) is the term used to describe the amount of air an engine is ingesting vs. the amount of air it can actually hold, expressed in a percentage. If the engine is ingesting the maximum amount of air it can hold, then that engine is considered to be operating at 100% VE. Most naturally aspirated engines never see 100% VE; but engines using specially-tuned intake manifolds can accomplish this. Most naturally aspirated engines typically see up to 80-90% VE without a tuned intake design.
Obviously if there is a problem with the MAP sensor, or the vacuum/pressure readings it is getting are not accurate, this is going to greatly affect the way the engine runs. GM MAP sensors aren’t easily prone to failure, but I have seen them fail if exposed to great pressures such as what could occur if the engine backfired thru the intake. The most common situation that I see that can cause issues with the MAP sensor is a vacuum leak. Any kind of vacuum leak will cause the pressure levels the MAP sensor sees to be lower than expected. This tends to cause the Air/Fuel mixture to go rich (because the ECM thinks the engine is under a load).
On CFI OBD1 applications, there are two trouble codes associated with the MAP sensor. A code 33 will set if the MAP sensor output voltage is higher than expected (indicating low vacuum) and a code 34 will set if the MAP sensor output voltage is lower than expected (indicating high vacuum). As with any trouble code detected, you should NOT assume the presence of either one of these codes indicates the MAP sensor itself is bad. All electrical and vacuum connections to the MAP sensor should be verified before replacing the part. To give a couple examples of what to look for should you get a code 33 would be a vacuum leak, mechanical issue with the engine causing very low vacuum levels, or an electrical problem between the sensor and ECM. If you are getting a code 34 you should look for a collapsed or blocked vacuum line going to the MAP sensor or electrical problem between the ECM and sensor. Normal output voltage of a MAP sensor should be about 4.5 volts or so with key ON, engine OFF; and less than 1.5 volts with engine running at idle. At full throttle you should see MAP output voltage above 4 volts. If there is a fault with the MAP sensor or the readings the ECM is getting from it are incorrect, the engine will most likely run very poorly, lack power, hesitate, backfire, or surge. Basically overall engine operation will most likely be unstable.
More Tuning Information
The CFI stock MAP sensor used on all non-supercharged and non-turbocharged engines is what as known as a 1-bar MAP sensor. This means the sensor is designed to read up to 1-bar of atmospheric pressure difference. Applications using a turbo or supercharger may have a 2- or 3-bar MAP sensor. A 2-bar MAP sensor will read up to 2-bars of atmospheric pressure difference, and a 3-bar will read up to 3. But all of these sensors must still operate within the same voltage output specs as a 1-bar. So in order to accomplish this, the output voltage must be scaled accordingly. This means the output voltage of a 2-bar map sensor with the key on and engine off is going to be somewhere around 2.5 volts. You cannot mix and match 1, 2, and 3 bar MAP sensors. The computer must be programmed to work with whatever type of MAP sensor you are using, or the fuel and spark delivery will not be correct and trouble codes may be set.
Coolant Temperature Sensor (CTS)
The CTS is one of the most vital sensors used by the ECM. It tells the ECM what the temperature of the engine is (or at least the temperature of the coolant running thru the engine). In most applications, the ECM uses this sensor to calculate ignition spark advance, fuel delivery to the engine, engine idle speed, EGR operation, and whether or not to permit closed loop fuel operation. A bad CTS sensor can cause excessive fuel issues at idle because the sensor is telling the ECM the engine is cold and needs more fuel. The engine will smell very rich when that happens
The CTS is a simple thermistor which means its resistance changes based on its temperature. The ECM supplies the CTS with a ground and a small voltage reference signal. The CTS pulls down (towards ground) the voltage reference signal based on temperature and the ECM looks at the amount of voltage pulled down and uses this to calculate temperature. High resistance equates to less reference voltage pull down which the computer interprets as low temperature while low resistance equates to more reference voltage pull down which the computer interprets as high temperature.
In OBD1 CFI applications, two trouble codes are associated with the CTS. A code 15 indicates the coolant temp reading is lower than expected. If this code is present, before replacing the sensor you should check for an open circuit to the CTS wiring. A code 14 indicates the coolant temp reading is higher than expected. If this code is present, check the CTS signal wire for a short to ground before replacing the sensor. If either code is set, or there is a problem with the CTS itself, it is very likely you will notice a running change in the engine. In cases if the ECM is not getting the correct reading from the CTS, it may be altering the spark advance and fuel delivery to the engine incorrectly which will probably cause a lot of drivability issues such as spark knock (detonation), loss of power, or exhaust odor because of incorrect fuel mixture.
The CTS can be tested using a simple ohm meter. In order to test this sensor, unplug it from the wiring harness and measure the resistance across its two terminals. The temperature vs. resistance chart is below…
°F °C OHMS
210 100 185
160 70 450
100 38 1,800
70 20 3,400
40 4 7,500
20 -7 13,500
0 -18 25,000
-40 -40 100,700
Oxygen (O2) Sensor
The O2 sensor is one of the most vital sensors used on the engine. It is responsible for helping the computer make adjustments to the fuel mixture (air/fuel ratio, or AFR) delivered to the engine. The O2 sensors used in GM vehicles are known as a “narrow band” type which means they are only effective at reading close to what is known as “stoichiometric” (or Stoich) AFR, which is 14.7:1 (14.7 parts of air to 1 part of fuel). This AFR has been proven to produce the best balance of performance and emissions in gasoline applications for normal driving and idling conditions. There are certain driving conditions where different AFRs are required and in these instances, the computer actually ignores the O2 sensor input.
The O2 sensor is installed in the exhaust system, usually close to the engine and samples all or one bank of cylinders. The O2 sensor acts as a small battery as it has the ability to produce a low voltage signal (between 0.001 and 0.999 volts) that the computer uses to calculate AFR. But before the O2 sensor can produce a voltage output signal, it must be hot (approx. 600 deg F). When the O2 sensor is cold, it produces NO voltage output. The most common type of O2 sensor used in CFI cars is the 1-wire, unheated narrow band type of sensor. This sensor relied on the temperature of the exhaust to heat it up, and thus it did take some time after the engine was first started before the O2 sensor started working.
The O2 sensor measures the amount of oxygen present in the exhaust system and compares that with ambient (outside) air to produce a voltage signal. The O2 sensor is calibrated to output approximately 0.450-0.500 volts at Stoich AFR (14.7:1). Leaner mixtures result in more oxygen content in the exhaust system, which will result in the O2 sensor outputting lower voltage (below 0.450 volts). Richer mixtures result in less oxygen content in the exhaust and thus higher voltage output (above 0.500 volts). The computer supplies a voltage bias on the O2 sensor signal wire of about 0.450 volts, and the O2 sensor will either pull down this voltage (when the exhaust is lean) or pull it up (when the exhaust is rich). The reason for the computer supplying a bias voltage is for diagnostic purposes.
O2 Sensor Diagnostics
As I just discussed, the computer supplies a bias voltage signal to the O2 sensor any time the key is on. This is done so the computer can monitor if the O2 sensor is warm enough to start operating. As mentioned before, a cold O2 sensor outputs no voltage signal, and as the sensor warms up, it starts producing enough voltage to either pull up or pull down the computer-supplied bias voltage. Once the computer has determined the O2 sensor is warm enough (READY) to start operating (as detected by it seeing the O2 sensor pull up or down the bias voltage) and other qualifications are met, it will go into Closed Loop mode and start using the O2 sensor to make adjustments to the AFR. If the O2 sensor is not ready to start operating after a set amount of time once the engine has warmed up, the computer may set a Code 13 for Oxygen Sensor Circuit Open in the CFI OBD1 system. This code usually sets when an O2 sensor is worn out or not working correctly, but it may also set if there is an open or break in the wiring connection between the O2 sensor and the computer.
Also in the OBD1 system, there are two other diagnostic codes used for the O2 sensor. Code 44 indicates exhaust O2 lean. This code usually sets when the computer sees the exhaust leaner than expected (as indicated by O2 sensor output voltage). The presence of this code does NOT mean the O2 sensor itself is bad. In many instances this code sets if there is a problem with the fuel system, a vacuum leak, an exhaust leak, or other problem with the engine resulting in it running leaner than normal exists. In some cases, an O2 sensor going bad CAN cause this code. Code 45 indicates exhaust O2 rich. This code usually sets when the computer sees the exhaust richer than expected. Again, the presence of this code doesn’t necessarily mean the O2 sensor itself is bad, so make sure you check the fuel system and other engine mechanicals that would cause the engine to run rich before replacing the sensor.
Misfires, and their effect on the O2 sensor
Misfires or incomplete burns in the cylinder can result in erroneous O2 sensor readings. This is because an incomplete burn in the cylinder can result in more or less oxygen being let into the exhaust system compared to what would normally happen. A misfire on just a single cylinder can throw off the O2 sensor readings which can cause the computer to significantly alter the AFR delivery to the entire engine – which will result in a poor running condition. The 82/84 corvette ECM computers did not have the ability to detect misfires.
CLOSED LOOP: INTEGRATOR AND BLOCK LEARN (Fuel Trims) Valuable tuning information
When the ECM goes into closed loop mode, it will start making adjustments to the fuel delivery using the integrator and block learn functions. The integrator is the computer’s short term (or instant) fuel adjustment, and the block learn is the computer’s long term (or stored in memory) fuel adjustment. In most OBD1 systems, these numbers start at 128; which is 0% adjust. Any number higher than 128 = the ECM adding fuel. Any number lower is the computer subtracting fuel. You can do some simple math to change the 128-based number over to a percentage. If you see 120, then divide 120 by 128 = 0.9375 which means the computer is subtracting about 6% from the stored fuel tables (in the tuning) to satisfy the engine’s fuel needs. You must take both integrator and block learn trim numbers (BLM) into account when calculating the total adjustment the ECM is making to the fuel delivery. So if your integrator (INT) is at 120 and your block learn is at 118, you really have a total trim number of 110 which equals 14% the computer is removing.
Ignition Control Module (ICM)
The ignition control module (ICM) replaced the points and condenser used in early spark ignition systems. The ICM consists of solid state technology which means there are no moving mechanical switches or parts inside the module to trigger the ignition coil. Instead, modern electronic components such as transistors (and/or a small computer) are used to switch an electrical power signal output on and off in order to trigger the ignition coil so it initiates a high voltage spark discharge at the correct time. When GM introduced the engine computer in the early 80’s, the function of timing advance was handled electronically instead of mechanically.
There are two types of ignition systems used on OBD1 CFI. One was standard computer controlled distributor ignition module. The essential functions of the ignition module remain the same. The ignition module accepts inputs from the ECM and distributor pickup coil and outputs switching (on/off) voltage used to activate the ignition coil.
When cranking the engine over to start it, a reference pulse is generated by the pickup coil inside the distributor and is sent to the ignition module. The ignition module interprets this input signal and generates an output signal to the ECM that is used to determine engine RPM speed (known as distributor reference pulse) this is important. Any time the engine RPM is below about 400 rpm, the ignition control module is directly controlling the amount of spark timing advance. When the ECM sees the engine RPM go above a preset level (usually 400rpm), it considers the engine to be running and supplies a 5 volt signal which goes to the bypass circuit in the ignition module. When this happens, the ignition timing advance function is transferred from the ignition module to the ECM. The ECM supplies a pulse signal along the EST (electronic spark timing) circuit which commands the ICM to fire the ignition coil accordingly. When setting base timing on a distributor based system, the ECM supplied voltage is removed from the bypass circuit (pulling the EST wire) so the ignition module assumes direct control of spark timing advance. This puts the engine into “base timing mode” so you can set initial timing by turning the distributor.
The ECM uses the distributor reference pulses to determine engine RPM which it uses to determine when to fire the injectors, as well as a basis for most other calculations needed to determine fuel and spark delivery to the engine. In the CFI OBD1 systems, there are two basic trouble codes associated with the ICM, codes 12 and 42. Code 12 is used during the Diagnostic Circuit Check procedure to test the code display ability of the ECM thru the check engine light (series of flashes indicating code numbers). Code 12 indicates that the ECM is not receiving the distributor reference pulse (engine RPM) signal from the ICM. Without a distributor reference pulse, the engine WILL NOT RUN since this signal is used for determining when to fire the injectors.
A code 42 indicates there is a problem with the bypass circuit or wiring. When the engine RPM is below 400, the ICM grounds the bypass line and thus the ECM expects to see this circuit grounded during these conditions. When the engine rpm goes above about 400 rpm, the ECM applies 5 volts to the bypass line. At this point the ignition module should no longer be grounding the bypass line. If the bypass line is open or grounded, the ICM will not switch to EST mode (computer control) and a code 42 will set. If you have a code 42 present in the ECM, you should check the connections at the ECM and ICM. If the connections are good and free of corrosion, you should check the bypass wire for an open circuit or short to ground.
There are three other wires that connect the ICM to the ECM besides the bypass wire (which is tan w/ black stripe). One is either red w/ black stripe or black w/ red stripe - which is a ground supplied to the ignition module by the ECM. Another is a white wire, which is the EST circuit. The last is a purple w/ white stripe which is the distributor reference pulse generated by the ICM that the ECM uses to determine engine RPM. If there is a short to ground or open circuit in any one of these three wires, the engine may not run at all, or it may fire while cranking but immediately cut off as soon as the engine RPMs climb above 400rpm. These four basic wire colors have been used by GM for the same functions since computer controlled timing and the ECM were introduced in the early 80’s up until modern model year vehicles.
Testing the Ignition Module
The ignition module can only be tested using specialized test equipment, but even then, a faulty ignition module may not test bad if it is not hot. Most of the time when ignition modules start to go bad, the number one symptom is the engine stops running when it gets hot and will not restart (no spark while cranking) until the engine cools off again. If the ICM is at fault, this is most likely caused by a break of one of the internal circuit connections inside the ignition module due to heat expansion. By the time you remove the ignition module from the car and take it to a facility that can test it off the car, the module cools down enough for the internal break in the circuit to reconnect – and this may result in the module testing “good”. It should be noted that this same scenario can also be caused by a faulty pickup coil. The pickup coil and most crank sensors can be tested using an ohm-meter, consult your vehicle repair or service manual for resistance specifications and testing procedures for these components.
Throttle Position Sensor (TPS)
The job of the TPS is to tell the computer where the position of the throttle blades are at any given moment. This sensor is vital in helping the computer determine if the throttle is closed, open or nay other position; or how fast the throttle is opened or closed. The throttle position sensor is a simple potentiometer on a wiper arm that uses ground and 5-volt reference inputs to produce a varying output signal depending on the position of its detection arm or shaft. At rest, this sensor outputs a relatively low voltage signal; as the arm/shaft is turned (as it would when the throttle opens), the output voltage increases. If this sensor is out of adjustment or is failing, the result could be stalling, idle surge, flat throttle response, hesitation, or erratic engine operation. The TPS on a CFI engine is adjustable, it must be set correctly in order for the computer to function normally. It is located on the rear throttle body or drivers side and secured by two screws.
In order to adjust the TPS, you will need a digital volt meter. You will also need the proper tool to loosen the retaining screws. If you are using a digital volt meter, then voltage will need to be measured across the blue and black wires going to the TPS. With the engine OFF and the key ON, the voltage with closed throttle should be set at 0.525 volts. If the voltage you get is not within spec, loosen the TPS retaining screws and adjust as necessary. Normal voltage reading for wide open throttle (WOT) is above 4.00 volts. The closer to 0.525v the better and the voltage will vary slightly as you tighten the two screws. Keep readjusting as necessary while tightening the screws.
There are trouble codes that are associated with the TPS, code 21 and code 22. One code will set if the TPS voltage is too high when the computer expects to see it lower. Another code will set if the TPS voltage is lower than the computer expects to see. The TPS code for low voltage is the most common and will usually set if the TPS is out of adjustment or the sensor has failed. The first thing you should do when you get a TPS code is to check adjustment and signal output of the TP sensor before replacing it. Be sure to wiggle all connections while watching scan data/voltage readout to make sure the problem is not a loose or bad connection.
There are circumstances that could occur with a failing throttle position sensor that may not set a trouble code. One of the most common symptoms of a failing TPS would be a tip-in hesitation or stumble when you apply throttle to take off from a stop. This can be caused by a dead spot in the TP sensor’s internal circuitry on the arm wiper, which usually causes the output voltage signal to not change (or it drops out) when the throttle opens. Unfortunately, this type of failure is not easy to diagnose without the proper tool – a digital waveform scope. Most digital volt meters and scan tool displays will not respond fast enough to show this type of a glitch; but some higher end meters may. If you do find this fault, then the obvious fix is to replace the TP sensor.
Idle Air Control Valve (IAC)
The IAC valve GM uses for most of their engines is a stepper motor actuated valve. A stepper motor is a device that moves in counts a predetermined amount per electrical signal it receives by whatever device is controlling it. The GM IAC has a pintle that extends or retracts into or out of an idle (bypass) air passage in a throttle body or intake manifold attached to the engine and is in fact a controlled vacuum leak. Typically, as the IACs pintle extends, air flow to the engine is restricted as the air passage is shut off. As it retracts, air flow to the engine is increased as the air passage is opened up. This air passage is a simple bypass for incoming air to take around the throttle blade.
Diagnosing problems associated with the IAC aren’t simple. There are many other causes that can make an IAC valve appear faulty. In many cases, there may be only one or two trouble codes in the ECM/PCM assigned to the IAC, but these trouble codes can set for a variety of reasons. Basically, this trouble code sets if the ECM cannot make the engine idle at a set (desired) speed by control of the IAC valve’s position. There are limits set up within the ECM that only allow it to move the IAC in and out of the idle air passage so far. The IAC valve position is referred to as IAC counts. Scan tool data indicating “0” IAC counts mean the IAC valve is fully extended (shutting off idle airflow to the engine); and “255” IAC counts mean the IAC valve is fully retracted, allowing as much air to enter the engine via the throttle bypass passage as possible. Some ECMs may never allow the IAC to reach 255 counts. Generally, anything you see over 160 counts should be considered to be a near- or fully-open idle air passage.
On a normal operating engine, it is typical to see high IAC counts (100 or more) when the engine is idling cold and during the warm-up cycle. As the engine warms, the IAC counts should decrease. By the time the engine reaches operating temp (fully warmed up), the IAC position should drop into the range of about 20-50 counts in park or neutral. If the scan data you get reports counts lower than this, then that can indicate one or more of the following problems exists:
·There is a vacuum leak allowing un-metered air to enter the engine
·The throttle stop screw is adjusted incorrectly (throttle being held open too far; more on this later)
·There is a problem with the throttle cable or cruise control system that isn’t allowing the throttle to close all of the way
·The IAC valve itself is faulty
If you are using a scan device and see the scan data/data stream or EBL WUD reporting IAC position higher than 20-50 counts on a fully warmed up engine, this could indicate one or more of the following problems exist:
·There is carbon buildup on the IAC pintle, or in the idle air passage restricting air flow
·There is carbon buildup on the throttle blade or throttle body bore
·The throttle stop screw is adjusted incorrectly (throttle resting closed too much)
·There is a mechanical problem with the engine resulting in lower-than-expected vacuum levels at idle (this will require the IAC to open further so the engine gets the required amount of air to maintain the preset idle speed)
·There is increased load on the engine (such as what would occur if the automatic transmission was shifted into gear)
·The IAC valve itself is faulty
Any one of the above issues can cause an IAC fault code to set in the ECM. As you can see, there are many issues other than a faulty IAC valve that can cause a code to set. So before replacing the IAC valve, you should check all of these possible issues first.
The IAC valve cannot be fully tested using conventional electrical testing means. You can measure the coils, but that can be OK and still be bad because of a frozen shaft. There are special tools available that are designed to test GM IAC valves, but I have discovered most shops don’t have these tools anymore. To be quite honest, you don’t really see many IAC valves fail. When they do, they usually freeze up or get stuck in a fixed position.
The ECM (RESETS) the IAC valve when the car is operated at normal road speeds (35mph or more for a couple miles). During this time, the IAC valve is typically extended out all the way (IAC counts = 0), thus closing off the idle air passage. This helps the ECM “learn” the position of the IAC valve. Anytime the IAC valve is replaced, this “learn” procedure should be performed. If not, you will experience a fast idle.