Hey guys, it's been awhile. Unfortunately, I'm back because I'm having some problems with by car. 02 GTP, little less than 120k on the clock. I'll list all of the problems, although I don't know for sure that all of them are related.

For the past few months, I've been getting poor MPG. It went from ~19 with normal driving (not flooring it but not granny-ing it either) down to ~15 mpg with granny-ing it. Money has been tight, so I hadn't been able to try working on it until lately. I changed the spark plugs about 3-4 weeks ago, but after using about a full tank I hadn't seen any improvement.

Then, a couple days ago, I was leaving work and it started idling high. In Park/Neutral, it idled a little above 2k RPMs, and in drive it was about 1600 RPMs. It was noticeably more difficult to stop than normal because of the higher RPMs. Once I got on the road, I noticed it didn't want to shift up. I had to let fully off the gas before it would shift, and when it did, it would sometimes surge forwards a little, just a single surge. It would shift down just fine.

I let it sit until I had a day off and either used my girlfriend's car or my mom's car in the mean time. I went to drive it today, and not only is it still idling high and not shifting up unless I let fully off the gas, now, when I slow and it wants to downshift, it surges forward several times while I'm decelerating. I simply drove around the block in my neighborhood, and when I parked it at home, it was idling at about 1900 RPMs in gear and it jumped up to 3000 RPMs when I put it in both neutral and park.

What's really odd is there is no SES light. I was going to drive to Autozone today and have them scan it to see if maybe it was throwing a code and the bulb was burned out or something (I figured it was worth a shot), but with it acting even worse than it had been, I just went around the block.

I also cleaned the K&N filter I have a couple months back to try and help the MPG, so I know the filter is good.

Any suggestions? The car not wanting to shift up and surging on downshift is what's worrying me more than anything else...

I literally just got back in from fixing a similar issue with wierd idling issues that sound almost identical to yours. At first i was leaning towards the IAC. And someone from the forum advised me to test them. Upon testing they turned out fine. It gradually got worse. Then it started acting up on starting. After driving it and then going in somewhere and coming back to start the car it would take several cranks to start. Problem for me was the fuel pressure regulator. Good way to check is to pull the vacuum line from the regulator and see if its wet with gas. For me it was and upon replacing it all of 30 mins of time with a towel two flathead screwdrivers. I drove several times back and forth to town. Car is not idling funny not shifting crazy. And seems to have fixed the issue. I would check that.

If the SES light comes on when you turn the key the bulb is good.

Unplug the TPS and see if the motor's rpms do anything. If it does then it's the TPS.

No changes then with it plugged in un plug the maf. Listen for changes.

If it drops it's the MAF. When you cleaned your filter did you over oil it? If so it could dirty the MAF and cause problems.

If still no changes start searching for a vacuum leak.

Best guess is you probably have a vacuum leak or a bad sensor, but I'd get the check engine light fixed so you can get some codes, if it has any. I've seen GM cars do strange things when the check engine light burns out, or is removed for any reason.

@pontiaclivson, do you have a pic that shows where the fuel pressure regular is, so I can check it? (Or Blackarrow? I'm sure you do. lol)

And Herc, I just checked and the SES light does work. I hate it when I overlook the simple things like that...

A dirty MAF is one of the things that I was thinking, especially now that the idle is crazy, but would that or the TPS affect how the car shifts? I will check them regardless, but I was just wondering if they would affect the shifting or if that might be a different issue.

Here's a pic of the fuel pressure regulator, with vacuum line attached:



From this write up: http://grandprixforums.net/injector_oring.php

I don't know but it's worth getting the idle issue fixed and go from there.

http://www.grandprix.net/forum/ubbthreads.php?ubb=showflat&Number=213500&#Post213500 theres the post i posted about the issue. and i described alot of the issues im not super mechanic. so i dont have explanations of why it does those things but if i had to guess the extra gas going through the vacuum line into the throttlebody makes it rev higher to try and offset the extra gas. it shifts hard because the engine is reved higher and not going any lower. and it would also explain your bad gas mileage. just sharing from experience. part was 52.99 at advanced. and immediately after changing it the engine sounded different and didnt feel as rough as it had. i hope it helps i was gonna get pics but was beat to the punch and glad i was too. cause would be ashamed to put pics of my dirty engine bay up.

LOL, yeah...no worries on what you got. Guarantee that I got you beat.

At least yours isnt missing pieces!


Notice shiny new fuel pressure regulator

Please let us know what the issue turned out to be when you figure it out.

Well, here's the latest. I went to move the car into my garage and had trouble starting it. It turned over, but took several tries to catch. That could just be because it sat there for almost a week, and after starting it a couple times it seemed to start fine, so I'm not too worried about that.

I unplugged both the TPS and the MAF sensor and the RPMs didn't change.

I pulled the vacuum line off the fuel pressure regulator, and there didn't seem to be any fuel. However, since I haven't driven the car for several days, could that be why? Should I go drive it? Or was it running for 5 minutes long enough to tell me whether or not that was the problem?

While it is in idle, the RPMs would be at about 2200. They would try to drop, but as soon as they got down to about 1500, they would come back up. Seemed like it was trying to keep itself from stalling. It only seemed to do that after I put it in drive and back to neutral or park - if I don't ever put it into drive, it idles high but stays steady.

This will probably sound like a stupid question, but if there is a vacuum leak, can you hear it? I've never had to deal with one so I'm not sure. While the car is idling, it sounds like I may be hearing a sucking noise. It's hard to tell, though, because with the engine idling so high, it's making a lot of noise in a small space like the garage (well, the front half is inside, the back half is outside so I don't die lol) and with the loud *** K&N intake, there's a lot to listen over. Of course, the noise I'm hearing could also be coming from the intake itself.

I've never been able to hear vacuum leaks, and that was with my bone-stock GT. Best advice is the most repetitive, unfortunately: chase the lines. Something else I've noticed is that these vehicles are starting to see the IAC fail quite a bit. I guess those sensors are reaching the end of their life. Good news? OReilly has Borg-Warner sensors for $37. That's not to be implied that your IAC is bad, because I'm not sure that's the case. With the Camaro, my car was WANTING to stall, which is not your symptom at all.

How do I check to see if the IAC needs replacing?

Hit it and see what happens. If the idles comes down you've found your problem. They may not always come unstuck with a tap but it works often enough to suggest it.

Another way to see whats going on is using a scanner and view the live data. See what the commanded idle is vs actual. Then it's really helpful if you can run a test on the IAC and adjust the idle from the scanner. I don't know if you or a friend has one with that feature set, but if so it's very helpful.