CAR: a 98 grand prix gt 3.8. 157k miles
ISSUE: p0300 -random multiple cyl misfire, p0306 -cyl 6 misfire. some stalling at idle.
HISTORY: car sat for a lil over a year, i bought it with a 153k miles 3 months ago. it needed a battery, starter, all new steel brake lines and the 12v ign feed to the shift and tcc solenoids. it ran great and would spin the tires at will from a stop or 5-10mph roll.
it started stumbling/hesitating when warm if i accelearated hard off the line and wouldnt spin the tires at all anymore and also stumbled if i was cruising up hill at 40-50mph as the extra up hill load came on and i tried to maintain speed, i could WOT and it would clear up for the most part. these are the only times it stumbles.
i found out the plugs were original ac irridium with 154k on them, carbon cracked in the porcelin, i changed them out with ac delco platinums. i found a set of new out of the box duralast plug wires and coil packs in a junkyard bonneville pass seat. so i put the wires on at the same time. it was still stumbling and also started stalling at start up and de-accelaration with low idle (400-600rpms) untill fully warmed up. i changed the fuel filter, cleaned the IAC solenoid and replaced air filter. MADE NO CHANGES. after being frustrated, i turned the idle set screw up to let it idle around 800rpms. stalling wasnt an issue any more but the stumbling is. i even swapped out all 3 coil packs, seemed to fix it for 2 days, then it CAME BACK. i thought maybe there was some crap in the tank from sitting over a year, so ive tried lots of lucas fuel treatment with 87 octane, 89/10%ethanol and 91/10%ethanol with no change except in gas mileage. it seems to run fine cold, but 5 min later warmed up it comes back.
recently it triggered a p0306 cyl 6 misfire, i cleared it and 2 weeks later a p0300 random multiple cyl misfire came on.
any ideas or advice? ive thought maybe the plug wires got pinched, pulled or something wasnt right with #6 since i didnt buy them new, they were new, clean and sitting in the jy bonneville seat with the box on the floor board and old wires still on the engine, same with the coil packs, but after getting the multiple cyl misfire code, that idea went away.
not sure what these have for idle vaccuum, but this is reading between 5-7 in/hg of vac at idle and cruising. fuel pressure is around 50psi if i remember correct (whatever normal pressure is) and i believe the platinum plugs are gapped at .60" or whatever autozones computer says to gap.
