04 GTP Sputters BAD Under Full Throttle, Fine Normal Driving

[loudandlow]

Hello,

 

I just bought an 04 GTP and of course 3 days later it now sputters VERY badly when pushed at full throttle, which I've done less than 10 times, but the 6 or so yesterday and today have got progressively worse. Seemingly zero problem at about 40% throttle and under, but if you floor it, it bogs down almost immediately. I was thinking fuel filter or cat, but since I'm new to these, I thought I'd ask a community of owners first! Thanks for your help and I hope to help others here one day once I get to know this car better.

[tsilney]

Any engine light on? Sounds like a possible bad cat, could just need a tune up as well.

[RobertISaar]

anything weak in the ignition system can cause this as well.

[White93z34]

I've had a bad coil cause very similar issues. Car would be perfectly fine at idle and up to about 40-50% throttle after that it would feel like it was misfiring and/or stumbling. It was weird in that at no point did I get a SES lamp for anything.

 

I first swapped the ICM with no change, then started changing one coil at a time till I found the bad one.

[Schurkey]

Schurkey's usual advice: Connect a scan tool, look at the data stream. This is especially useful in conjunction with a fuel-pressure test, cranking compression test, cylinder balance test, etc. Since the scan tool is easier to use than the other equipment, I'd probably go there first.

 

A misfire from any cause--weak ignition, vacuum leaks, low fuel pressure, etc. will show up as increased (excess) oxygen in the exhaust, and the computer will command very rich mixture to try to compensate.

 

A plugged cat will show up as lower-than-typical manifold vacuum at part throttle, higher rpm. In other words, the vacuum will drop faster with increasing throttle opening than you'd expect if the exhaust wasn't plugged. The low vacuum is likely to also result in rich-command from the ECM.

 

There are too many other tell-tales to type out.

Edited April 23, 2015 by Schurkey

[Garrett Powered]

the very first thing I do is check the plugs and then scan it

[loudandlow]

Thanks for reminding me to come back and post results. The car had red ZZP plug wires on it. After pulling the boots off the spark plugs, 2 of the 6 had the inside connector stay on the spark plug. I replaced these with autozone lifetime and put in Autolite 605's at .55" gap and the thing runs like a raped ape! The plugs looked fine and in the normal range on the picture charts you find everywhere, and I think the wires had fucked up and were misfiring.

 

And just a tip from a former parts guy who found his way out of that world, just call your local parts store and say you're from the dealership down the street, that the parts guy said you could put it on their commercial account and pay cash and taxes. This is what happens at every shop/dealer with the local NAPA or Autozone when the mechanics or parts guys need parts for their own cars. Get anywhere from 5-35% off! If it doesn't work, call the parts store down the road and try again, but it's not hard to pull that off since it happens everyday for real.