I have a 1991 Firebird, 3.1L V6. I’ve been experiencing this problem since I bought the car almost 2 years ago. It starts and idles fine cold, which is about 1100 rpm, but after a while and after it starts to warm up, it starts to idle really low (500 rpm - 650 rpm) but when you stop at a red light or for any kind of traffic, it decides to surge violently to about 1500 when in gear and throws a check engine light. If you put it in park and it’s not in gear, it shoots to 2500 rpm and continues to slowly climb until you shut it off. The highest it’s gotten for me so far was about 3000 rpm before I shut it down. That also throws a check engine light.

So far to try to remedy this (obviously with no luck) I’ve replaced the EGR gasket, Idle Air Control, all of the vacuum lines (also tested with carb cleaner and nothing’s leaking) and I’ve replaced the upper intake gaskets. I have a feeling the charcoal canister and/or the purge valve are the problem/problems but I figured I’d ask the people who actually know what they’re doing lol.

Other replaced parts include: ECM (old one was dead when I got it), alternator, starter, injectors, all fuel lines (main, vent and return), all brake lines, all brake pads and lower ball joints. Haven’t had to touch anything else so far.

Any help would be greatly appreciated

Thanks!

 

When the surging episodes occur, what is the error code causing the CEL?

 

^That^

 

it throws code 34, but I’ve replaced the Idle Air Control twice now.

edit: meant to say it throws code 35. Typed In the wrong number.

 

The error code is not specifically for a problem with the idle air control, but with the idle control SYSTEM. That includes wiring/connectors, unexpected feedback, and the IAC itself. You've replaced the IAC, and we can only guess that was because of the code, so the IAC was a suspect. The new IAC evidently did the same thing, so it was replaced again. The problem with the original IAC is likely still the problem with the second and third. If the connectors, wire harness, ECM drivers, throttle body, or intake system have issues, you can replace the IAC until the world's supply is depleted and still have the same symptoms. It would be very revealing to connect a scanner (like an OLD GM-Tech-I tool or SnapOn MT2500) and see what the ECM is commanding for idle RPM, how many IAC steps are invoked, and what the ECM thinks the engine RPM is.

Also, be aware that the factory tachometers are not always completely reliable after a few decades, and a diagnostic tach is a far better method for diagnosis and adjustments.

 

Understood. Thank you.

 

The V-6 has some variations in the control scheme from the V-8/TPI system, but the basis is mostly consistent. The IAC can be set to the fully closed position by invoking field service mode in the ECM, and that would effectively reset that to a default position for the first run cycle. Of course, starting the cold engine will cause the ECM to add steps and increase RPM until the coolant temperature and IAT are happy, any the start timer has elapsed.

If there are other vacuum leaks and/or a worn throttle body, all bets are off until those are resolved.