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GTP running rough, no codes though

Z

z28evans

New member
Howdy,
My 2001 GTP (150k miles), all stock, is running rough - esp on cold starts. Buck / surge, feels like a faint random miss. About a year ago (10k miles or so) i replaced all the intake gaskets, plugs, wires, etc. Last night i changed the spark plugs again, did not fix the problem. I've got HP tuners, and it shows no codes (other than one relating to the EBCM being bad). I can't see any vacuum leaks, i replaced the EGR a few months ago. I'm running out of ideas and about ready to sell it.

LTFT - good call, went and played around. light cruise = +16.4 maxed (red) and idle is -10% or so.

For a bone stock car this tells me something is up, what that is i'm not sure

I checked the vacuum lines, did find a few small cracks, nothing major and fixed them earlier today (fixed before logging data).
I also forgot to mention on heavy acc. i get KR and it is lean. It does puzzle me about being rich at idle though.

FPR appears to be good. I did notice that the supercharger is making a rattling sound, could be the coupler - is it possible that this could cause running issues?

Couldn't find any vacuum leaks, sprayed everything down with starting fluid.

Any advice on something to check?

Thanks,
James
 


Did you spray where the intake contact the heads could be a gasket vacuum leak had this before but on abig block chevy. Worse at cold start up as everything got warm and expanded it seemed to get a little better. Good Luck!
 
sprayed around the intake gaskets, vacuum lines, TB gasket, etc. Less than a year ago i changed them all out as well.
 
Does your oil dipstick still have the o-ring seal under the handle? Mine broke and caused a bit of a vacuum leak. Also, the HVAC line tends to crack and break. That's the one coming off the top of the S/C and toward the passenger side of the firewall.
 
Update - with the supercharger belt off it runs great. Nice idle, and runs good for not having boost. Surprised me how well it ran, esp. considering how it was running before.

So, is it possible the supercharger is causing the running issues, or maybe a bad MAP sensor?
 


bump- have the same issue; runs great with the S-charger belt off; rough with it on.....curious as to what's causing it also. New build with tight gaskets; so not that.
 
Update i posted on my local forum:

I spent a lot of time working on the Grand Prix this week. The reason for running lean - dirty MAF was reading, but reading incorrectly (and never threw a code). I only found this after trouble shooting the fule system, etc. After cleaning the MAF the fueling was much better, no more pegged lean. It ran a little better, then threw MAF codes, so i got a new MAF - ran better, but was still not running right. It would stumble, shudder, and throw a Multiple Random Misfire code at anything from light to heavy acc. I went through everything (bought and changed ignition parts, etc). I took the Cat off of the car, but because of some bends i could not see if it was plugged so i cut the welds. To my surprise the cat looked great, so i welded it back up and reinstalled it. At this point I'm checking the internet and trying to figure it out - i hate being stumped.

Long story, but i figured out that the transmission is going out. These trans use a special torque converter that does more than the usual lock/unlock. I found multiple postings where people had the same issue as me, where they could not figure out their misfire, had their trans die and the misfire disappeared after changing the trans. My guess is that the PCM looks at the engine rotation data and detects changes that appears to be misfires but is actually the trans having issues. It gets worse as it gets hot (and worse as i was trying to diagnose it).

So, last night i was racking my brain trying to decide if i wanted to spend ~$3500 and the time fixing things wrong (ABS module, tail lights, trans rebuild) or get something else.

The Grand Prix was in bad shape (worse than i thought), the trans was doing so bad i drove down in 3rd gear and was making plans to tow it back. I went ahead and traded it in, they didn't give me much for it - but it was not worth much and i don't have to deal with idiots trying to buy it on craigslist.

I got a G8

14751493493.274190258.IM1.MAIN.565x421_A.562x421.jpg


James
 
so, did buying the G8 fix your rough running?

i have this same issue and was thinking about installing a g8 but cant afford it at this time.
 


Thanks guys, so far i really like the G8. I liked not having a car payment, but between work and family i didn't have the time to rebuild my daily driver. I've traded the stress of working on the car for a car payment - but so far i'm very happy. This forum was very helpful while working on the GTP - thanks guys :)

The shop is full of "fun" project cars, so the G8 has to sit in the driveway -

normal_g8drvway.jpg
 
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