Thread: 99 GTP Troubleshooting- rough idle, misfire? No codes (warning very NOOB)

Hello everyone,


Thanks for taking a minute to look at my thread. I hate to post another rough idle / help me thread with so many others on the forum but I couldnt find an answer to my issue after reading through them. (Though I did find some good pointers) Please bear with me as I am very new to diy engine repair etc.

Car is:

99 Grand Prix GTP - Stock @ 150k miles
-No mods other than k&n box filter

So here goes- My car started started idling rough about a week ago. I wasn't sure what caused it but all I know is that the only changes performed on the car at the time was an alignment and an oil change. I have been doing a lot of reading on what the issue may be but I am still stumped. The symptoms are: Rough idle, stumbles while driving, stalls, misses(?) not sure what a miss is, sputtering and other adjectives that describe poor or rough throttle issues. The car is getting poor fuel milage at 17.6 mpg and I THINK I can smell fuel. not sure tho but would that indicate an exhaust issue?


So far I have:
-Changed fuel filter
-Cleaned MAF
-Cleaned IAT
-Cleaned TB (partial)


So far there are no codes or anything I can really work with to pinpoint the issue. Its pretty general at the moment but Im hoping you guys may have some insight or ideas that I should look into.


Here is a video of the rpm gauge at idle, to see the issue look at 4:22 on the vid. Its nothing much but it randomly spikes over 1k without any throttle input.


-Click here to see video-

My only thoughts left are vacuum pressure leak somewhere or bad plugs / wires. Im hoping thats the prob and not a bad piston or injector...waaaay beyond my knowledge there.

Other symptoms that may be useful:
- car is running a bit hot
- can possibly smell fuel
- poor fuel milage
- rough throttle performance
- stumbles and wants to die sometimes
- possible flux in current, noticed pulse in lighting/display components...very infrequent
- can hear whining while radio is on that matches rpm or acceleration somewhat like supercharger whine but through speakers.

Thanks in advance for any insight everyone. I am stressin over this because winter is coming and Im on a rather tight budget from just being laid off. I had funds saved up to go see my bud after he got back from a tour over seas but this may eat those funds right up


I will keep everyone updated as I attempt further diagnostics / fixes and will hopefully provide a thread with closure so others can learn who have similar issues. Also, please dont hesitate to direct me to an existing thread if you feel that it can help. I appreciate all your help.

Let me know if you have any questions or need any more info.

Thanks in advance

-Nick

Have you cleaned your IAC ?

The stalling, poor idle, etc etc.. makes me feel it's maf related. Unplug that puppy... get the car running and stable (some fight a little until they learn, others run great from the first crank w/o a maf). Drive it a little (don't beat it or hit boost) and see if the issues go away. If so, likely your maf is heading out the door on you.

Hey guys,

Thanks for your replies.

@GTP in RTP- Sorry when I listed what I've done with the car so far in regards to troubleshooting I said that I cleaned the IAT but I meant to say IAC. I removed it and cleaned it quite thoroughly removing all the carbon build up. Hopefully cleaning it removed it from the list of possible culprits.

@BillBoost37- I tried unplugging it for a short bit to see if there were any noticeable responses but havent had a chance to drive with the sensor off. I will give that a try here soon and will update to see if your thoughts are correct. Im hoping they are Do you feel it would matter if I ran this test with a cold engine over a hot one? Also, do you think using MAF cleaning spray could help the issue? Please let me know if you have any other ideas you think I should look into and I will keep you updated.

Thanks for your insights guys

-Nick

kinda figured you meant iac

Okey dokey, update time. Car is still acting up big time however I think I made some progress. As per BillBoost37 recommendation to run the car without the maf plugged in triggered the computer to give me a code. Not sure if it was solely because I tried running the car without the sensor or if that assisted in finally putting a finger on what the computer deemed the actual prob was but its a P0171... maf or seal leak. So new maf is in the mail and Im going to check the hoses again and dig around to see if everything is sealed up properly.

In addition to checking for leaks Im planning on Sea Foaming the engine to help clean it out a bit. From your guys experience do you think that if I run sea foam through my car the smoke will come out of a hose if there is a leak or is that all going to go straight out the exhaust?

Im hoping the new maf will help. Ill keep everyone updated once its installed. Thanks again

you got the code because you un plugged the maf and ran the car, thats normal. if you clear the code, or un hook the battery for 30 minutes that will also clear the code. plug the maf back in and the code should not set for the maf again.

if it ran the same with out the maf as when its plugged in, thats not your problem.

you may want to cancel the maf order you placed.

Okey dokey, update time. But first, @scottydoggs. Thanks for your insight, I was thinking along the same lines. It did seem like a forced code however I was also narrowing the symptoms down to a faulty maf given that all the other sensors are clean or new and there are, as far as I know, no leaks. So, with all that said new maf is in and idle has smoothed quite a bit... however still idling a little rough and stil getting a random rpm pulse while idling in park as well as a stumble or miss while driving. Not sure if Ive quite driven it enough for everything to smooth out on its own given that the computer is still increasing the mpg (still at 11.7) but I will keep an eye on it through the weekend and update if that was the fix. Does anyone else recommend anything that I should keep an eye on? I do know that my computer and my fuel gauge are not matched up, any reason for this? Im getting the low fuel warning with a quarter tank left.

check for a vac leak. use some carb cleaner, spray all the lines, and line end fittings, if the idle jumps up you found your broken vac line.

has no one seen his symptom of RFI increase with RPM? get a squirt bottle and fill it with salt water, spray along your spark plug wires. bet you its your wires grounding out somewhere, especially with your radio frequency interference.

that or you have a cracked or shorting coil