Thread: 04 GTP misfire..a fun one..

2004 Grand Prix GTP CompG 111k miles, ZZP PCM, 180*, CAI

My issue started several weeks ago when I took over this car from my wife since I bought her a new vehicle. I commute to and from Atlanta 3 times a week. I had noticed the shifts bounced a little so I contacted ZZP to have another PCM sent to me that had firmer shifts/upgraded software.

I got the computer in and the car was running great, really smooth, fast/firm shifts. Life is good.

Then after about a week or so I started having issues with the car not wanting to start. I could turn the key and the engine would just spin, turn the key off and then try to start again and it would catch but not start. On the 3rd try it would finally catch.

Before I could start diagnosing that issue within a day or 2 I blew up my tranny. The car spent a few days in the shop getting a rebuild done. $2500 later I have a nice shifting tranny but still a hard start.

So when I get the car back I head down to ATL for class and all appears to be well until I get into the throttle/boost. The car starts to miss. I figure it was time to do plugs again as this is usually what occurs when they get old and it may fix the hard start(and it was time). So being the person I am I decided to upgrade to the NGK Iridium plugs (I got a new K&N filter as well). I got them next day from ZZP (they have always treated me right) and got them installed. The car was still hard to start but once running was very very smooth.

The next day I rode down to Atlanta and never had a problem with any misfires. Unfortunately on the way back home that night it started misfiring again when I got on it or even just a part throttle/heavy accelerate. I figure it was the NGK plugs, the things aren't recommended to be gapped and they were at .34 or so. Sooo I pulled a couple of plugs and gapped them at .55.

So I go to work the next day and damn it the car is still missing/not starting. So I figure the NGK plugs are junk and go buy some Autolite 605's, which I have always used. During the process of getting these installed I broke 3 of the NGK plugs and got ticked off I threw the rest away.. $$$ down the drain.

So the Autolites are in and again the car is still not running right. At this point I've done what I can do and off to the dealer I go. After waiting a couple of hours the service guy says the mechanic checked everything out and all looks good. The only thing they found was a misfire on #1 cylinder and that I might have wiped a lobe on the camshaft. $700 to tear the engine down to see what is going on. I say thanks and leave.

I got on my message forums and also emailed ZZP about my issues and the responses that came back were mainly about the MAF sensor. This was along what I was thinking too so I replaced the MAF. $150 and no change.

So now I decided the hell with this and get a rental and drop the car back off at the shop that did the tranny for a second opinion.

They called me back the next day and told me the MAF was not sending/receiving a signal to/from the PCM. They said it could be a bad MAF, wires, PCM. But seeing as how I just changed out the MAF that probably wasn't the issue (the old MAF was avialable to test as well), they checked the wires and all was ok, so that left the PCM.

I contacted ZZP and Tim was able to send another PCM to the shop. The shop called today and advised that the car was ready to be picked up. So I take the rental back to Enterprise and they picked me up in my car to take me back to the shop to settle up. I got in the car and it fired right up..which now that I think about it always had when it was warm. The car was hard to start when it had sat for a long amount of time.

So I get on it part throttle and the miss is gone..WOOT! but then I get on it a little harder and the miss is back at the higher RPMS 4-5k. I'm thinking ok maybe the PCM just needs to relearn. So we make it back to the shop and settle up and I leave. I stop by Wally World for about 15 mins. Get back out to the car and start it..the stupid thing catches on the first try but then dies. It starts immediately on the 2nd try.

So here I am worried that this thing is going to end up right back where it was to begin with. I am at work now waiting for the car to cool down before I go try to start it. If it starts then I am going to go buy some plug wires to see if it fixes the high rpm miss. If it doesn't want to start it will go back to the shop.

Update:
Took the car back to the shop and got another rental that same day. I had to leave for a week for firearms training and during that time they called that week and told me they found a fuel pressure issue during starting and suggested replacing the fuel pump.

So here I am today ($2000 more broke, not including tranny) with a new fuel pump and the starting issue and part-throttle miss is finally gone. BUT now I have a misfire at high rpms when I'm accelerating. I can do 70mph on highway, go to pass someone and the damn thing starts to miss.

I am going to replace the plug wires and I've read something on here about the cat might be plugging up. I dunno...

Any thoughts?? I'm sure I've left something out but will post it when/if I remember it.

Thanks!

B.

Yes, when pump was done.

B.

Dude, you got a plug wire issue. I had the same thing.

I just had the same issue basically with my 04 gtp, change the wires before you do anything. I changed my plugs and it did the same thing. ran fine until I put a load to it and it would miss and sputter. put new wires on and it's perfect now.

Thanks for the suggestions. I am definitely replacing the wires but am trying to get a good deal on some PRJ's or similar. Probably overkill but they look nice. I think this miss at high rpm is just another separate issue from the no/hard starts and mid throttle missing. Since the pump was replaced all of that stopped happening.

Anyhow thanks for the help, very very much appreciate it!

B.

Ok, got some PRJ wires ordered. Hope to have them this week sometime.

B.

Spray the wires with some water out of a spray bottle. If theyre bad they'll start arching all over the place.

yeah, if the wires don't do it then I'll go from there. I like this site..lots of traffic.

Thanks!

I'd change the wires too. Have you looked at the plug end that connects to the coil?
If you take the wire off a coil, any coil, and look down inside the boot, I think you will find that the metal termainal on the inside is rusty. All of mine were like that when I got my car and I put new wires on before I had a problem with it. I did have similar problem in my 86 MonteSS with the same symptoms that you described and it was the plug wires.

Ok, got some Autolite 7mm wires on there. My PRJ wires fell through. I'll drive it tomorrow and see what happens. Wish me luck!

I'm located just outside of Chattanooga, TN.

B.

The car appears to finally be fixed. Drove to work this AM and it didn't miss once when pushing it. I've got to go to ATL tonight (and back home) so I'll know for sure tonight.

Thanks for the help!

B.

Anybody have troube with a 2004 supercharged grand prix with a misfire in cylinder 3 and 4. Expecially under heavy load with pedal 3/4 to all ther way down. Check engine light is blinking. Both spak plugs showing misifre.

New plugs and wires and pcv valve.

Compression is good.

You can tell a little rough miss during idle but no codes during idle.

Anybody have this trouble before? Bad gas mileage.

I do know the cat is plugged or getting plugged. So I plan on ordering one of those. Somebody said one time the exuast on the back of the converter should be 100 degress hotter than the front? That they shouldn't be the same temp. Mine are the same temp.