I have a 99 grand prix GTP. I have a P0300 code. (Random multiple cylinder misfire) This code has been coming on maybe once every couple of months. I just kept clearing it because it barely misfired and didn't cause a problem. A couple days ago I was getting on the gas pretty good and then the P0300 code came back. Now while driving normally the car runs fine, but under a load or at WOT the car mifires badly. My coils are new, my plugs are new and my wires are new. None of the wires are burned either. I checked my fuel pressure and that is fine as well. I tried a new ignition module and that isn't it either. Its starting to mis when I start it up and then a little bit at an idle. Any help would be GREATLY greatly appreciated. Thanks,

I had this exact issue last year except mine was the #6 plug code everytime.

Turned out the porcilin on my new iridium plug was cracked right out of the box...I can't tell you the frustration I went through trying to track down the cause..

hmmmm... so no P0300 code right? How did you figure out that it was exactly that cylinder? Or that problem.... ITs killing me over here.

My car was doing that EXACT same thing...i put new plugs and wires...didn't do it...cleaned my MAF and it has been running strong ever since with no problems...i'd suggest tryin that...get a can of MAF sensor cleaner for like 7 bucks at a parts store...you hafta get "tamper-proof" torx bits...25 size maybe? and just clean it real well..if it works but you keep having to clean it...might be time for a new MAF...good luck...

~Stauffie

check your plugs first.

I would say MAF as well if it's acting up at WOT only. Give it a good cleaning, if that doesn't work, try picking up a used replacement one from Ed Morad.

gtped@msn.com

U&LIM gaskets no doubt

well see its been doing that ever since I bought the car on and off but not as severe. I put an SS intercooler and a machined lower intake on in december and of course all new gaskets.... so I really don't think it could be that cause the problem is still here.

Oh... does anyone know the proper procedure to clean the MAF? I want to be careful and do it right... esp. since that might not be the problem. lol

Here's a pic of the sensor: MAF Sensor Remove the screws, lift it out, spray it, let dry, re-install.
Here's the stuff you should use: MAF Cleaner

thanks guys... ok tried it. Def. not the MAF... I did notice this time though that it only misfires when I make boost. Even if its only 1 lb. Does that narrow it down anymore???

are you serious?

Do you have a Powrtuner or access to a scan tool? If so, you could see which cylinders the misfires are highest.

Maybe your motor has an aftermarket cam in it that a previous owner put it, and you never knew

yes im serious. ive had a few cars that i worked on come to me with multiple misfire code on and it always turned out to be the gaskets.

not to thread jack ... but since people were talken about a MAF ... could that cause why my car hesatates sometimes mainly when i start it up ... i am thinking it could be the maf because i have herd people saying that sometimes with an k&n filter sometimes the oil gets on the maf and messes everything up ... just tryen to fig out the problem

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Alright I don't believe that it is the intake gaskets because A: I had this problem before I changed them and still have it after B: It would be running badly all the time, not just when I make boost.

okay update: I cleaned the MAF. its fine. I'm trying a MAP sensor soon.
my plugs are fine. I don't have a scanner but I have since replaced the wires and coils, tried a new ignition module checked my fuel pump and its none of that stuff. It only does it when I make boost. When I first start the car up and go down the road when its cold it won't really do it, but after a few minutes it will start missing again even if its only 1 lb of boost.

Do you have access to a scanner? At this point I have a feeling it could be a few things, but you won't be able to figure it out without a scanner.

For example: I was having an issue where under different engine loads, the engine would stutter and fall all over itself. Scanning the car revealed the the MAF sensor was intermintently dropping off to zero and freaking the pcm out. Replaced sensor and the car's been good ever since.

So, instead of guessing for a predictable but intermintent problem... I'd get a scanner on there and see if any of the sensors are acting weird.

ok... I just don't know where to find someone who has one... Ahhhhh this problem is killing me

The MAP is a good possibility. When you go into boost, you should be over 100 kPa MAP readings, maybe something weird is happening and your MAP sensor is not registering that you are into boost?

I have one on the way from ZZP... it was only 20 bucks so I figured it was worth a try...

A few other things to check, that can cause a PO 300:
If you haven't tried this already, it's worth a shot: Take the car for a ride with the MAF sensor unplugged; if the misfire is gone, replace the MAF Sensor. Clear the code, and take the car for at least three trips, shutting it off for at least two minutes between trips. If the code doesn't come back, good to go!

Vacuum leaks: Check all vacuum lines, and check for leaks around the intake mounting surfaces, MAF sensor, throttle body, EGR valve, EGR adapter and feed pipes, and injector o-rings. Remove the EGR valve, and make sure it isn't partially stuck open. Check the PCV Valve for improper installation, and damaged/missing o-rings.

A damaged serpentine belt or belt driven accessory can cause engine load variations sufficient to set a misfire DTC.

Extended idling or short trip driving can leave deposits on the heated oxygen sensors, causing them to respond slowly to exhaust oxygen content, affecting fuel control and causing a misfire to be indicated at idle.

Best of luck in tracking down your problem; I know all too well how frustrating it can be...

thanks for the info... I am gonna start narrowing things down one by one... haha. I had a question though. Does anyone know what the proper intake vacuum should be on a GTP? I was gonna check that just to be sure. Also, I read about a lot of people blocking off their EGR. Since that is a hell of a lot cheaper than buying a new EGR and since I don't have emissions testing around here, would that fix the problem if it is my EGR acting up???
I can't tell you how much I appreciate all your responses... I got like 2 on clubgp lol

Vacuum at idle: around -9psi or -18inHg at idle, according to this thread: Questions About Boost

well its not the map and my vacuum is -9.5 psi at idle.... I noticed today when I started it, its misfiring at idle some and it smells like gas around the back header/EGR area. Any suggestions? What about blocking off the EGR? Some ppl have done it so i was wondering if I should?

Get a light back there and see if the injector o-rings are leaking...

I just had someone around here who was having issues when his injectors weren't sealing correctly all the time. Slightly different situation, but the symptoms are similar.

Pull out the spark plugs, keeping them in order; chances are they're probably gas fouled by now, maybe enough to see which cylinder is misfiring. You may have to put in a fresh set of spark plugs; if it still misfires, try unplugging the Mass Air Flow Sensor first, and see if it runs better.
X2 Slick's comment about the injector o-rings; be sure to check the fuel feed lines as well.
Speaking of EGR, did you remove it and check to see if it is partially stuck open? You might want to try that also.

T20