Thread: GTP stutters.. struggling to accelerate

Good afternoon. my 2002 GTP sedan started struggling to accelerate last week. doesn't have to be in boost. The engine stutters and shakes a bit when I try to accelerate anything over I'd say 15% throttle or so. Idles fine. No CELs, no other issues. What I have done to try and diagnose:

Brand new this week:
AL606 iridium plugs
ZZP 10.5mm wires
ignition coils
ICM

Catless downpipe installed recently as well as PLOG. stock supercharger pulley. EGR delete. Stock injectors/fuel system.

I was almost sure this was going to be an ignition problem, as it has the classic plug-wire/coil stuttering issue, but it's not the case. The only other thing I can think of is a clogged injector or something fuel related??

Open to recommendations on next steps to trouble shoot. I have searched through everything I can find on this forum and others related to this issue and most of them pointed to ICM/coil issues or clogged CATs which fixed their problems. Unfortunately I am still on the search for root cause. Thx

I'd try some stock wires and maybe watch misfire by cylinder with a scanner.

It may not be tossing a check engine light but it can still be misfiring.

if your old plug wires worked/ looked good when they came off, put them back on.

the old wires did work fine, but when I went to do the tune-up on it, one of the wires pulled right out of the boot, so I bought new ones. What is the likely-hood that these new 10.5mm ZZP wires are causing it? any recommendations other than the AL606s?

Thx for the quick replies gents

edit: I do have HP-Tuners Pro as well so I can try and so some scans later today to watch for misfires

could be a bad coil too. if you got a t app you can scan for misfires

and to think I was doing an ignition system "upgrade" !!

Do people often run into issues with ZZP wires?

iirc they used to, but i think they fixed that issue. im not a fan of zzp stuff in general. i got nothing they sell on my car. lol

Ha.. yeah, I really like how they're downpipes and PLOGs fit up, but this would not be the first time I have had an ignition-related issue AFTER I have installed one of their ignition components.

Probably going to pick up some stock replacement plug wires at Autozone later today.

I have not done a compression check. But I would assume if I have a cracked head I would see the oil and coolant mixing? Fluids are currently spotless

Just replaced the new ZZP 10.5mm wires with some new AC-Delco 7mm wires from Autozone. still having the exact same issue. Any chance the Autolight XP606 iridium plugs could be an issue? I have not changed the gap. They are gapped as they came right from the package.

Well..... found the issue.. After scanning with HP-Tuners and not seeing anything, I decided it was time to pull the plugs to see how they looked, and try to find evidence of any potentially not firing. When I got to cylinder 6 and removed the plug, here is what I found:



I specifically remember when I bought the set of 6 last week, the gentlemen at autozone dropped one of the individual plug boxes on his way to the counter. I almost guarantee it was the same plug from cyl. #6, and he prob put a hairline crack in it.

Good news: replaced the plug, problem fixed, car runs great.

Lesson learned (and I should already know this...): do not throw parts at it (new ICM, coils, and 2nd set of plug wires) until you've investigated all most likely root causes first. Cost me an extra $248 which could have been avoided had I have just pulled the plugs the first day I noticed the misfire.