Thread: 1990 Pontiac Grand Prix LE Rough Idle!!?!!

I have a 1990 Pontiac Grand Prix with the 3.1 MPFI v6, has 103,076 miles. It has a new fuel pump and filters, newer battery, newer Alternator, new coil packs and ignition module, plugs wires, IAC. The car runs with a mis fire i cant seem to figure out why. The hotter it gets the worse, trys to stall sitting still. Car has all the power in the world on the highway. Today I disconnected the battery for 20 min, drove around and around trying to reset the computer, ran terrible then improved to its normal self. Parked for 30 min, went out started it up ran perfect, no miss. Drove some more, problem came back. Also the problem gets worse with turn signal. Thanks I pulling my hair out trying to find the issue, please help!

i have a 91 gp sitting in my garage with the same problem. I havent had the time to fix it completelly. Have you noticed if its running rich in idle? Mine throws a lot of black smoke on the back when this happens. I replaced the map and it got better but not fixed. I need to check the vacuum lines too. The rich mixture could be getting your plugs dirty like mine and causing missfire. And yes unplugging the battery only makes it worse because these models take so long to reprogram themselves.

havent noticed it running rich, doesnt smoke at all, checked the plugs when one of my coils went bad, ended up rplacing all three and module because they are such a pain in the a** to get at. Thanks for the reply any little bit helps, love the car, mad its pulling this on me.

UPDATE: after some people told me totry seafoam i broke down and dropped the 10 bucks on it. I didnt notice any differece for a number of days other then the smell of the chemicals. After some steady driving the problem went away. car ran great for 2 days. I drove the tank till the 1/4 mark and filled it up at a trusted gas station. After a 1/4 tank of fresh gas the car began to run bad again. Not sure what the issue is! Is it a blockage or build up of carbon in the system the the seafoam helped but didnt fix or the raise in gas quality from the chemicals?