97 Grand Prix GTP with Intermittent #4 Cylinder Misfire

**1998xGTP** · 05-21-2009

Off topic but it hurts the crap out of my eyes to try and read that.

**NaturallySmashed** · 05-22-2009

it hurts even worse when you can't fix the problem haha

**wgardi** · 07-22-2009

I have struggled with a cyclinder 4 misfire in my 97 grand prix for several years! It all started when I had my son replace the spark plugs. It's never run correctly since! I've replaced plugs, wires and the #4 coil. I think it is a wire issue, but I can't isolate it. When I fiddle with the wires, and the metal sleeves on the plugs, the misfire changes--sometimes nearly going away. I didn't replace the wires with the GM stock wires, but used a NAPA version. I've wondered if I need to just buy the expensive GM wires and see if this will fix it. NAPA tells me their wires are more than adequate. Any advice on this?

**dngrrngriv** · 09-01-2009

i have a similar problem, but the more i read, the more i think that it is a weak valve spring. my car starts and runs great, UNTIL i get to about 4400 RPM, then #5 misfires about every revolution until i let off it. i too have replaced all plugs and wires, along with installing a new coil. another thing that i heard about and did, was to remove all the coils, and the module, and clean up the surface of the module and mount plate, coat them both with a very lite layer of dielectric grease. nothing i have done has changed how my car runs. ANYBODY HAVE ANY IDEAS???

**foley28012007** · 09-02-2009

I have a 2004 gp natrual asperated 3.8 I'm getting a misfire in cylinder #2 I have changed the plugs, wires, and all 3 coils. I got good fuel pressure and had the icm tested good. It seems to start happenning when the engine gets about have way to operating temp (i hate temp gauges with no numbers), I asuming that is where the thermostat should open. I took it to a local shop and they had it inside from about 10 AM to 8:30 PM that night, they found nothing bad other than the misfire. I am at a lost on where to go next.

**wgardi** · 09-04-2009

Mine seems to have something to do with the engine temp as well. It seems on cooler days it will run without any misses, until it heats up. Also, if I'm going up a hill, just before shifting down it will miss pretty bad until it shifts, then it has plenty of power. This makes me think it isn't a valve or spring as it seems it would be more consistent with a mechanical problem like that. I'm still leaning towards wires. Do you have metal sleeves on the part of your wire that goes over the plug? When I changed my wires, after putting the plug cap on, I pulled the metal sleeve up on each cylinder wire so it wasn't sitting all the way down. (I did this thinking the metal sleeve may have been sitting too close to the engine block, causing some kind of electrical naughty or something) The car didn't miss at all for a while and I thought I'd solved it. Then it slowly started missing again. Has anyone tried pontiac service to see if they've solved this?

**foley28012007** · 09-04-2009

I call the pontiac dealer here in roanoke, va and they just told me it could be a lot of things, like a bad injector or wiring. They told me to just worry about that cylinder. I tryed to change the crank position sensor. When I finished I try starting the car and it sounded like I was 180 degrees off timing. But it was so late I had to call it quits for that night. The next morning I thought I would try it before I went to work and nothing. She wouldn't even try to turn over. I took a break tonight to think thing out. Starting on it again tomorrow morning. Hope I have better luck.

**00gpgtp** · 09-04-2009

does anyone know when the pcm switches the pump to "high speed mode"? i too am having a misfire on 4 (sometimes on 3 but i think its just picking up 4's since they are across from eachother) but my symptoms are really weird. it seems to really misfire in park or neutral. as soon as i put it in gear and theres a load on the engine, it clears up. also will misfire sometimes on decel.

my thought is, if the pump is in "low speed" when idling maybe it has something to do with that, and if it switches to "high speed" when its in gear maybe the little extra fuel pressure clears it up??? just a thought.....

i have changed so many things that ive just given up. driving it is fine, power is there, idles fine in gear so stop lights and such give me no issues. i have not yet changed my lim gasket (which im sure is horrible) or looked under the rear valve cover. hopefully i will be finishing my intercooler tomorrow which will be replacing the lim gasket and give me a chance to get in the rear valve cover easier.

so in short, im thinkin if bypassing the pump speed deal so its always in high speed mode (which im told doesnt matter whatsoever, pump doesnt care). thoughts??

**jp2001gtp** · 09-12-2009

same problem here switch coils changed plugs to copper plugs, even lower thermostat change wires changed down pipe do to bad cat and new 2.5 pipes and mufflers.

**victorrleo** · 09-13-2009

Sorry to ask but is problem solved? And what is a LIM leak. New to the whole S/C V6 thing. Had Grand National and have always worked on and raced old small blocks.

**jp2001gtp** · 09-13-2009

problem not solved

**00gpgtp** · 09-15-2009

well we finished my intercooler install the other day (everything in the motor) and i replaced a few things while i was in there including the rear valve cover gasket, lim gasket, sc gaskets, and new vacuum fittings and lines. i was surprised to see the stock lim gasket wasnt in all that bad of shape, lets just say ive seen pictures of way way worse so i was kind of disappointed and was expecting noticeable failures in it.

after getting everything done, the car runs better then it ever did. no more misfires, and gets about 19.5-20 in vac., when before it was about 18. im thinking one of these things is what fixed it, but leaning towards replacing the vacuum lines and lim gasket. just thought ide share to say that one of these could cause the problem.

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