bad random misfire after warmup

Ok this has still been a problem for a long time and i have done all the checks and even had to replace the O2 sensor(3rd time) and u-bend(not my choice) Its still misfiring random cylinders under load.
Now its so bad it'l do it in neutral and can almost kill the engine. Still no engine light. But when the engine is cold, it runs fine and as fast as it can be.
When scanned there is no knock or anything when it missfires.
Anyone have a similiar problem yet?

I had this problem with my 90 GP and replaced the crankshaft position sensor. Cleared it right up.

Its been replaced twice now.

But now starting yesterday, it runns like its lacking poer. Oh and it smokes ou the exhaust when it missfires. Does that mean its running rich? Or is it the fuel isn't burning. But anyway when it lacks power. The intake makes a funny noise like its gasping for air. Then when i give it alittle more gass the noise goes away and the power comes back.

Well I had my local mechanic look at it and the fuel pressure looks fine until I revved it up a few times and hten the fuel pressure began to fluctuate by 2 pounds at idle. So we pulled off the vaccum hose to it and it kept doing it. So we think its the fuel pump. I hope it is, I'm tired of chasing this gremilin.

Have you checked your coils? Easier to change than a fuel pump. You need to check for resistance between the poles. I forget the exact numbers, but I want to say around 600 ohms.

Have you checked your coils? Easier to change than a fuel pump. You need to check for resistance between the poles. I forget the exact numbers, but I want to say around 600 ohms.

I didn't check them myself, but i took the coils and the iginition module out and had tested at autozone. I alos took a set from my freinds gtp and tried them on mine and nothing fixed it.



From what I understand, isn't the fuel pump for the gtp a two stage.

Maybe the second stage for the boost is gone.

Also I noticed the other day there is a nasty humming sound coming from the back when the car lacks power.

Change the coils. Thats what I had to do and it cleared right up.

Ok the pump didn't fix it. And the fuel pressure still fluctuates even without the vaccuum hose.

But I found what is causing the missfire. I found an article saying to not use anyother spark plug wires except from GM. Went to the dealership, 60 dollars later missfire fixed. DO NOT BUY AUTOZONE WIRES-NOT EVEN PREMIUM WIRES.

But there still is a problem with power, I'll place new post.

So very untrue for anyone with a 98 GTP. I have them (autozone wires) and Have had NO problems at all.

So very untrue for anyone with a 98 GTP. I have them (autozone wires) and Have had NO problems at all.

Well maybe its just the 99's with the prob. Anyway, when I was searching for 8mm wires, I found an article with a 99 gtp that tried three different types and all of them failed within days. It sounded like mine and the author said he just put the originals back and and no more probs.

Yeah but If I am not mistaken are they not the same engine? Maybe he had mods. I haven't really started moding mine out yet. I would still check out your coil packs. those you can meter out just in case. Let me know what you find out about you loss of power. If we have the ssame symptoms and from what I have read I am thinking it has something to do with the KR and /or timing.

sounds like the coils

Loss of power?
Mine are brand new

Want an "old mans" thoughts? Check the ICM. By the sounds of it, you could be fireing two cy. at a time. Take it up to Auto Zone and have it scanned. Will more then likely come up with a random misfire. It's just my thoughts. But you have to stop throwing parts at it. The cost has got to be getting high. Good luck
http://img.photobucket.com/albums/v425/kustomkid54/1998PontiacGrandPrixGTP004.jpg

Kid are you refering to the loss of power? I have a OBD2 and I am returning no codes everything passes in all three menu options.

It feels like it gets confused and doesn't automatically shift down into third but when I manually do it it takes off

Yeah mines not throughing codes either. The only time it did was when I had the missfires, and I held the throttle to the floor to get the light to come one. But all it said was 300---random multiple misfires.

Like I've said, I've had the coils and module checked before. I'm just gonna borrow the coils and module from my moms 02 boneville(same motor) and see what happens.

ANyway its not like I haven't troubleshot it yet, I'm not just throughing parts at it. I've done the sensor voltage reading and about averything you can do without a scanner. But sounds to me like either the timing is going back and forth or fuel mixture fluctuation.

Trust me the nasty missifiring is gone, or at least not as bad. With the old missfiring, the car jerked violently. No its smooth. The intake sound, sounds like it does at startup (dragging shovel), fluctuates when the torque fluctuates.

NEW SOUND. When boost maxes out, a constant pinging, or bangins noise occurs, loud! Knock? That again goes back to the fuel mixture or timing again.

I also noticed when I rev up the engine, the lower radiator hose shrinks shut. Ive heard of bad head gasket causing hose to blow off, what could this be.

NEW SOUND. When boost maxes out, a constant pinging, or bangins noise occurs, loud! Knock? That again goes back to the fuel mixture or timing again.

I also noticed when I rev up the engine, the lower radiator hose shrinks shut. Ive heard of bad head gasket causing hose to blow off, what could this be.
1. misfire code
2. new fuel pump
3. tried diff coils and module
4. new wires
5. checked voltages from sensors

when the boost maxes out your getting a constant pinging which is spark knock. when's the last time you changed the plugs and what kind are they? could be bad plugs or maybe the timing belt streched. have you checked the plugs to see if its one cylinder missing or is it jumping around? timing belt would affect all cylinders and plugs would look the same. if its one cylinder then troubleshooting will be a little easier. it sounds internal by the sound of it, but it's hard to tell nowa days too many damn wires.

Stock GM ignition wires are your best bet..there is absolutely no reason to change them...the factory Delco wires are good enough to handle alot of hp...unless they need replacing of course..aftermarket wires, like Taylor, MSD.., etc..are just a high priced color wire..the only real wire to add that is aftermarket are the PRJ's and they are like $130..otherwise stick with stock..

Also the noise on startup is your EGR system.."dragging shovel" it is recirculating exhaust gases..if you want that noise to go away simple remove the egr system and have the PCM reprogrammed by DHP (Digital Horsepower) to have the codes removed..cya Mik

FOund the prob. The brand new catalyc convertor melted itself shut. And now I think an exhast gasket it gonna go someday. But thanks for the help.

Had the cat removed and it runs perfect, a little too perfect. It says I'm getting 35 mpg on the highway. Is anyone else getting that? Finally got to test it out and killed a 2004 ford lightning. Sweet.

Andydoo,
You didn't change PCM did you? Only happens at high "R's"?
http://img.photobucket.com/albums/v425/kustomkid54/1998PontiacGrandPrixGTP001.jpg

Andydoo,
You didn't change PCM did you? Only happens at high "R's"?
http://img.photobucket.com/albums/v425/kustomkid54/1998PontiacGrandPrixGTP001.jpg
Accually I have a DHP. What do you mean it happens at high r's.

Read this I don't think Big-L wants to type it again.
http://www.automotiveforums.com/vbulletin/showthread.php?t=308015
http://img.photobucket.com/albums/v425/kustomkid54/1998PontiacGrandPrixGTP004.jpg

Sorry,
Mine was missing at about 6200 RPM's
http://img.photobucket.com/albums/v425/kustomkid54/1998PontiacGrandPrixGTP001.jpg

Sorry,
Mine was missing at about 6200 RPM's
http://img.photobucket.com/albums/v425/kustomkid54/1998PontiacGrandPrixGTP001.jpg

Accually when I have a problem, I put the original computer back in. I don't want anyone messing that thing up. But after the missifire was fixed, I put the DHP back in.

I noticed on that thread you linked, someone said about a missfire or loss of power for a second right after it shifts a gear. Mine is doing that when it shifts into third. Thats just a need of a CASE learn procedure causing that? I assumed it was the tranny cause the rpm's drop when it does it.

In Big-L's "book" it says 1998, 1999, ect. You have to do Case learn on all reprogramed PCM. And as I always say.. There are 3 or 4 people on this site that have most of the answers. And two that are never wrong. And if one of those two told me to paint my rims yellow that it would give me 5 extra HP. Well call me twitty.
http://img.photobucket.com/albums/v425/kustomkid54/1998PontiacGrandPrixGTP004.jpg

And if one of those two told me to paint my rims yellow that it would give me 5 extra HP. Well call me twitty.

i think yellow only gives you 2 HP, chrome gives you 5 HP and if you get spinners tack on another 3 HP.