92 firebird 3.1liter idle problem


Asked by Jul 31, 2016 at 05:16 PM about the 1992 Pontiac Firebird Base

Question type: Maintenance & Repair

I bought a 92' firebird the car has a hard time staying at an idle.My dad is a
past mechanic and we've done everything we can to fix it. So heres what
happens....It will turn on at a higher RPM (2-3k) then when it goes
anywhere lower than 1k, maybe 800rpm it will shoot back up to 2-3k then it
will repeat that over and over. Till it will die.Sometimes it will start up at the
high RPM and then just die.   We've replaced the Idle Air, MAP, Throttle
Position Sensors, New Fuel tank, Injectors, fuel filter, distributor, fuel pump
computer, timing chain, compression tests, new plugs and wires, and some
other things that I forgot. In other-words, we've replaced almost everything.
Someone asked if the catalytic converter has been taken off (in case
something was blocking the exhaust to breath) and it never had a cat when
we bought it, so we dropped the muffler and still.....shitty idle. I hope this
helps you guys out to give options available to us to figure out, but of
course taking it to an actual mechanic shop could do the trip....if it could

2 Answers


When you replaced the IAC (Idle Air Control) valve, did you set the pintle length? GM OBD I is not "plug and play" like the newer OBD II systems. You'll need a shop manual or perhaps a repair manual for the pintle length specification. HTH. -Jim


Vacuum leaks tend to be a constant high idle or rough idle. You seem to be having a surging idle. That may be related to a bad idle control circuit (not just the IAC). After that you need to start trouble shooting as there are a lot of candidates for a hunting idle. Do you have any DTCs being set? It may inform you of any faulty sensors like a bad knock sensor, crank sensor, or head temp sensor that can cause a hunting idle. Even though you replaced fuel pump and filter it wouldn't hurt to test fuel pressure and fuel pump control circuit. If you feel certain the surging idle is not caused by any of the control circuits look for cracks in the exhaust or intake manifolds, or even trapped air in the cooling system.

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