Hello, my 1986 Fiero GT AT has a cold failure and the engine stops if it does not accelerate. The fuel-air mixture is not correct. It has all new sensors, new injectors, the computer does not display an error code. We no longer know what to check.
Before I continue spending money, I would like to test if the problem is with the ECU. My mechanic has a Chevrolet Lumina APV 3.1 ECU that works properly. Externally it is the same, but I don't know if the internal connections match. The idea is to know with certainty if it is the same one that the Pontiac has, so we can test it in the car and see if the fault continues or not. Does anyone know if this test can be done? Thank you!

Most likely it will not work, and could possibly damage your friends ECU.

You can test your ECM

I had 'weird problems' with an ECM in a different vehicle, so I sent my computer to ECU Exchange for testing.

They found a bad power supply which created distortion on the reference and system voltages.

The grand total was around $140

I bet that you will find a ECM service like ECU Exchange in Argentina

If not, this is the service I used in USA:

ECU Exchage
https://www.theecuexchange.com/
https://www.theecuexchange.com/contact-us
Or call (561) 408-6375

------------------
Original Owner of a Silver '88 GT
Under 'Production Refurbishment' @ 136k Miles

Sounds more like your IAC is not working...

As Joe said, do not use another GM ECM not designed for your year Fiero. I don't mean to be harsh but why do so many Fiero owners do not bother to invest in a multi-meter, fuel pressure gauge, ignition tester and a scanner. My suggestion to you is measure your fuel pressure . It should be about 40-43 PSI .

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

quote Originally posted by olejoedad: Most likely it will not work, and could possibly damage your friends ECU.

Thanks, I won't do the test.

One thing that I did not mention is that ECM's can and do fail. Only a dealer with a test jig can test and determine if it is good but often but not always the ECM will set a code that indicates the unit is not working. If you post the ECM part number here a number of dealers here who sell parts can probably give you a quote sell you one and ship it to you.

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

quote Originally posted by Dennis LaGrua: As Joe said, do not use another GM ECM not designed for your year Fiero. I don't mean to be harsh but why do so many Fiero owners do not bother to invest in a multi-meter, fuel pressure gauge, ignition tester and a scanner. My suggestion to you is measure your fuel pressure . It should be about 40-43 PSI .

Not only that - but everyone needs to download a copy of the factory service manual for their car. Those can help diagnose many problems with the troubleshooting guides in them.

ECM failures are pretty uncommon, and I can't think of how it would cause such a specific issue. If the car drives okay then the injector drivers must be okay - idling opens the injectors less than driving does. So if it's just choosing to call for insufficient fuel at idle, it's probably because it's getting bad information or because there's a mechanical problem that it can't deal with.
Do you know that it's a fuel issue? What if you spray starting fluid at the throttle body while it's idling, but don't move the throttle?

Definitely look into the IAC. Also clean out the passage - the IAC inlet is a small hole in front of the throttle.
Check if the EGR valve is stuck slightly open. It should always be closed when idling.

When you turn the key on, the Check Engine light should turn on, blink off for an instant, then turn back on and stay. Does yours do that? This means the ECM is functioning and it believes the ROM is good.
The light then turns off after the engine is started. If there's no light at all, check the bulb.

In addition to the IAC and checking fuel pressure, I would test (not replace, test) every sensor you can. If there's a sensor you don't know how to test, look around or ask - there's probably a way. These old cars use very simple signals. You can do a lot with just a basic multimeter (preferably with decent accuracy, so a step up from the very cheapest).
Replacing sensors isn't a reliable substitute for actually testing and measuring what's happening.

The temperature sensors, for example, can be tested by measuring their resistance and comparing that with a chart like this:

and the MAP signal can be checked as a voltage against this:


Also check the 5V supply that should be found in the connectors for most of the sensors. 5.0V comes from the ECM, so if it's not correct, then the ECM would be suspect (could also be caused by wiring).

This page from the 1986 service manual shows how at least some of the sensor circuits are wired:

This shows where you should expect to find 5.0V. It also gives pin numbers for where the signals go on the ECM connector, if you want to check the signals at that end.

It's possible to build or buy a cable that can hook a laptop PC up to the car's ALDL port. Then you can install WinALDL (free) and it will allow you to watch all of the sensor signals that the ECM is receiving and show what the ECM is doing. This is very convenient and gives a lot of information. WinALDL will show you what signal is *actually* reaching the ECM - so even if you have a wiring problem somewhere, you'll see it. If you only test the sensors themselves then you could still miss a wiring fault.

Did you replace the Throttle Position Sensor? If so, did you adjust it so that the voltage at idle position is 0.50 volts? An improperly set TPS can add or subtract fuel if its reading doesn't match other sensor tables.

quote Originally posted by armos: ECM failures are pretty uncommon, and I can't think of how it would cause such a specific issue. If the car drives okay then the injector drivers must be okay - idling opens the injectors less than driving does. So if it's just choosing to call for insufficient fuel at idle, it's probably because it's getting bad information or because there's a mechanical problem that it can't deal with. Do you know that it's a fuel issue? What if you spray starting fluid at the throttle body while it's idling, but don't move the throttle?

I agree OP has more troubleshooting to do and it is good to do some basic investigation rather than replacing parts. However these ECU's are now 40 years old. They will fail. One failed in my Fiero, and I had 2 fail in my 84 Regal. And when they fail, odd things happen.

quote Originally posted by armos: ECM failures are pretty uncommon, and I can't think of how it would cause such a specific issue. If the car drives okay then the injector drivers must be okay - idling opens the injectors less than driving does. So if it's just choosing to call for insufficient fuel at idle, it's probably because it's getting bad information or because there's a mechanical problem that it can't deal with. Do you know that it's a fuel issue? What if you spray starting fluid at the throttle body while it's idling, but don't move the throttle? Definitely look into the IAC. Also clean out the passage - the IAC inlet is a small hole in front of the throttle. Check if the EGR valve is stuck slightly open. It should always be closed when idling. When you turn the key on, the Check Engine light should turn on, blink off for an instant, then turn back on and stay. Does yours do that? This means the ECM is functioning and it believes the ROM is good. The light then turns off after the engine is started. If there's no light at all, check the bulb. In addition to the IAC and checking fuel pressure, I would test (not replace, test) every sensor you can. If there's a sensor you don't know how to test, look around or ask - there's probably a way. These old cars use very simple signals. You can do a lot with just a basic multimeter (preferably with decent accuracy, so a step up from the very cheapest). Replacing sensors isn't a reliable substitute for actually testing and measuring what's happening. The temperature sensors, for example, can be tested by measuring their resistance and comparing that with a chart like this: and the MAP signal can be checked as a voltage against this: Also check the 5V supply that should be found in the connectors for most of the sensors. 5.0V comes from the ECM, so if it's not correct, then the ECM would be suspect (could also be caused by wiring). This page from the 1986 service manual shows how at least some of the sensor circuits are wired: This shows where you should expect to find 5.0V. It also gives pin numbers for where the signals go on the ECM connector, if you want to check the signals at that end. It's possible to build or buy a cable that can hook a laptop PC up to the car's ALDL port. Then you can install WinALDL (free) and it will allow you to watch all of the sensor signals that the ECM is receiving and show what the ECM is doing. This is very convenient and gives a lot of information. WinALDL will show you what signal is *actually* reaching the ECM - so even if you have a wiring problem somewhere, you'll see it. If you only test the sensors themselves then you could still miss a wiring fault.

Thanks for everything!
I have already spoken with my mechanic so that he can go step by step checking all these recommendations. Thanks again!

Well finally, I had to buy an ECM, because I haven't been able to find a mechanic who could test my original ECM. What I didn't know was that it is sold without Prom, so now I don't know if the problem persists, it could be the old Prom and not the ECM. If the fault is not fixed with this change, could someone tell me where I can buy a Prom with shipping to Argentina? Rockauto doesn't have it, and Fierostore sells it very expensively. On the other hand, does anyone have an ECM pinout diagram they can share with me? Thanks!

proms don't really go bad, AFAIK...