Thread: Fluctuating RPMs while driving/idling

'00 GP GT. It's still a minor problem so hopefully I can get this fixed before it gets worse. I've noticed a small fluctuation in the RPMs while the car is either in drive or idling. I'd say it's probably no more than a few hundred RPMs, but if I'm cruising down the highway it's enough to be noticeable. It doesn't happen all the time, just here and there. Seems to do it more than it used to though. Happens with or without cruise control engaged. As far as I can tell it's only a problem when I'm going a constant speed. Only increases RPMs, doesn't decrease.

I'm on a tight budget so I'd like to start with the cheapest possibility, lol. I've read the TPS might be the cause, but that seems to start at around $40 for a new one. Could be worse I guess.

SES light isn't on. Would it still pick up any codes? I recently replaced the plugs, but this started before that and hasn't gone away since then.

I hope it's not the MAF since I replaced it once already, but it was from a junkyard, so who knows. How easy is it to spot a vacuum line?

Edit: I do have an extra MAF floating around somewhere. Wouldn't hurt to give it a try I guess.

Thanks for the help. Hopefully I can figure this out.

Update. Didn't see anything around the vacuum lines. The RPM issue is definitely noticeable on the highway. Feels a little jerky sometimes. It has also stalled twice in the last week or so while waiting at a red light. Any other ideas?

Originally Posted by bluegtp91

Clogged Cat OR if you think its the MAF, unplug it and drive it around...if it drives fine, then the MAF is bad.

And joe, quit trying to diagnose crap...air filter...wtf?

Local has that issue on his '99 GTP. Replaced plugs, wires, MAF, vac lines, fluids/filters, etc.

Ended up being a clogged converter.

You can buy a Magnaflow High Flow unit off of EBay for like 50~ bucks...take it to an exhaust shop and have it welded in, and have the U-Bend taken out of the exhaust right after the cat while your there.

Excellent, thank you. I don't think it's the MAF. I replaced it within the last year, but I also had an extra one sitting in my garage that I swapped out a couple days ago. Didn't fix the problem. I suppose I could still unplug it for a while and see what happens. Seems like two bad MAFs would be unlikely. Parden my stupidity, but what's the advantage of taking out the u-bend?

Edit: I put in my own air intake recently, and this problem existed before I did that, so that's definitely not it...in case there was any doubt...

Originally Posted by bronzemist02

my car had the same shifting problem. I had the solenoid replaced and it fixed it. My truck has the fluctuating problem. I cleaned my maf and it seemed to help a bit. It is still fluctuating on the highway though. I'm thinking maybe my torque converter isn't working right. The truck is shifting into overdrive at about 60 km an hour under light acceleration. I'm getting the trans fluid and filter replaced soon.

I'd like to have my trans fluid and filter replaced also, but it's fairly expensive to have done (at least on my tiny salary), and I'm not sure how easily I'd be able to do it. I've been pretty lucky with this car and haven't had to replace very much in the 2 years I've had it.

Ok, another update. Car stalled on the way home tonight. Didn't want to start again. Finally got it going, took off, and I heard the distinct sound of metal hitting the road. Went back and picked up the piece. Can anyone tell me what it is? Lol...I'm assuming part of the exhaust, but I don't really know for sure. Too dark and freezing outside for me to get under and look.

Doesn't sound any different. I know it wasn't in the road already, it's pretty big. Would have been easy to see. Someone else here mentioned it might be a heat shield. Doesn't affect how the car runs so it's not too big of a deal. I'm a lot more concerned about the surging RPMs and stalling.