Hi everyone, I am new to ThirdGen. I would like to ask for some help with my 1990 Firebird, 3.1 V6 (original drivetrain). It has just over 127,000 miles on it and I have owned this car since brand new. The car has been reliable all these years until recently. The problem I am having is that it seems to run OK at startup when cold but once it warms up near operating temperature it starts running rough and flucuates up and down at idle, although not a huge fluctuation. When I step on the gas in park I hear it backfire sometimes. The idle gets so low for a second that it sounds like it is going cut out. There are no service engine lights coming on at all. I just put a new distributor cap and rotor on yesterday (the cap was corroded inside). I installed new wires about a month ago because one of the wires came off a wire separator and was leaning against the exhaust manifold for about 25 miles down the road. I seemed that the car was running fine until this happened (maybe a coincidence). After installing the wires and cap/rotor, it still is running rough. I haven't replaced the spark plugs yet and that would be my next move but I checked a couple of them and they seemed normal. Keep in mind, I have a broken driver side motor mount and I have not driven the car in a few months until I replace it. So I have not tested the car at driving speeds. I thought about old fuel causing the problem but I added fuel stabilizer awhile back when I knew I wouldn't be driving the car for some time. Could it be the fuel pump or pressure regulator, I guess it could be anything? Additional information I thought you might want to know is the fuel pump, catalyic converter are original. I replaced the fuel filter about 7,000 miles ago. I had a new ECM installed at the dealer about three years ago. Any suggestions would be appreciated. I would like to drive my bird again. I am trying to figure this out before I have to take it to a mechanic. I have a Pontiac dealership about two miles from my house. I believe the hourly rate now is about $100 plus parts. Let me know if you need more info. Thanks!

 

Does the SES light come on when you turn the key to the on position (not start)? Might be throwing codes but has a burnt out bulb or bad wiring.

Have you checked your O2 sensors yet? The engine will run fine when it's cold because it's in open loop and not using the O2 sensors for tuning because they take time to warm up to operating temp. Once the engine and o2 sensor(s) have warmed up it will go into closed loop. Closed loop uses the O2 sensors in combination with other sensors to supply the ECM with information about how the cars running. Check the O2 sensor(s) or just replace them. Should be only 19$ each.