Thread: '97 GTP, rough idle

My sisters GTP developed a rough idle recently, and she said it acted odd while driving sometimes, so right away I figured the TPS was shot. We decided to replace the IAC at the same time for good measure (I liberally sprayed the IAC passages with carb cleaner as well). Fired it up, and not a damn thing had changed. I discovered that it won't free rev above 3500-4000 either (it acts like a rev limiter is kicking in). It revs and runs good otherwise, no stalling. I noticed it didn't build boost at WOT, it did at partial throttle though. I'm stumped, any ideas?

Does it not rev over 4000 in neutral only? If so then it's fine. It is the Rev-Limiter.


Try changing the plugs...And possibly the wires. Also make sure the new plugs are gapped right. .060 I think.


You've pretty much covered everything else.


Oh....Check for vacume leaks also. But usually with those you will get a SES light.

I actually didn't drive it, so it was in Park. I asked her to give it some pedal on her way home to see how it ran but she forgot. I figured it should idle higher with a vacuum leak, idle speed is normal it's just really rough. I was also thinking possibly the MAF, but there's no difference when I disconnect it. Next time I'm near the car I'll pull a few plugs out and see what they look like, come to think of it I have no idea when the last tune up happened. Thanks for the response

EDIT: come to think of it, is there some sort of process to make the computer recognize a new TPS?

Originally Posted by oldschoolpontiacs

I actually didn't drive it, so it was in Park. I asked her to give it some pedal on her way home to see how it ran but she forgot. I figured it should idle higher with a vacuum leak, idle speed is normal it's just really rough. I was also thinking possibly the MAF, but there's no difference when I disconnect it. Next time I'm near the car I'll pull a few plugs out and see what they look like, come to think of it I have no idea when the last tune up happened. Thanks for the response

EDIT: come to think of it, is there some sort of process to make the computer recognize a new TPS?

I have had a vacume leak before and it does not idle higher. The PCM has a disered idle speed and usually stays in that area.

The RPM's will only go up to 4000 and stop only when you're in park or neutral. Or they should anyway.

I have never heard of a MAF going bad on these cars and not throwing a code or SES light.

Try cleaning the MAF. That is also a common problem on these cars. That could be it right there.

Okay will do, thanks for suggestions guys

Hey, sorry it's taken so long to update, I got hit with some out of this world virus that's had me in bed the past 4 days. So the vacuum lines are all in decent shape and the MAF screen looked fairly clear of debris. I pulled the front 3 plugs just to see what they looked like, they all had burn marks on the insulators and the gaps were a little wide (worn electrodes) but I didn't see anything that would indicate they weren't doing their jobs. I regapped them and put them back in, with no improvement as I expected. It definetely seems like a timing/ignition issue to me, like just one cylinder is firing intermittantly. It smooths out with higher rpms. I got some more details from my sis about how it started, she says she pulled out to pass someone and it happened while she was doing that. She heard some sort of "backfire" noise and then it began to idle rough. There are so many possibilities right now I don't even know where to begin, blown head gasket, bad valve, blown piston, bad coil pack, bad cam sensor...I guess I should do a compression test to start with. I sure wish those back 3 plugs were easier to get at...

You guys won't believe this, but the #2 piston gave its spark plug a little love tap, providing the spark plug smoosh effect you would expect. Somehow mechanically everything is A-okay, so we put new plugs in (and wires for good measure) and she purrs like a kitten now.

I would imagine that you chippd a piston and that is why the spark plug was the way it was.


That's the only thing that can cause that. Enless the plug was hit on the end and closed the gap up or the wrong plug was installed. But if the wrong plug was installed Wrong length) it seems like it would have happend to all of them.