My 1999 Mustang GT is running horribly. I cannot figure out what is wrong with it. The check engine light is on and when I went to get it checked out it said it is the O2 sensors , they have a slow response, but I dont understand, I just got new O2 sensors put in 2 days ago! When I start the car it is fine but when I put it in gear the RPMS drop to less then 200 and sometimes the car even dies out. It also happens when I come to a stop, the RPMs drop really low and the car sometimes dies. When I am at open throttle the car drives fine except the exhaust doesnt sound the same, it almost sounds like a helicoptor or something and it never sounded like this before. It normally sounds mean and it actually sounds strange now unless Im really accelerating fast, and even at that it doesnt sound like it did before. Anyone know what it could be?

 

check your MAF screen, see if its dirty, also, clean your IAC. If that doesnt fix it.... I dunno off the top of my head :-/

 

This might sound stupid... but ive done it before.... see if it is plugged in.

 

+1

 

it sounds like something very simple. Bad connection (or lack there of) or a dirty MAF. You'd be suprised what headaches little particles cause in the MAF.

 

I don't know why everyone thinks that every problem revolves around the IAC valve??? C'mon people, put down the book and get your hands dirty with real knowledge.
Your problem could be Long tube headers ran with no cats, or your sensors are bad from the start. If you had no problems prior to knowing of the )2 sensors, it might be your MAF sensor is bad. Check the voltage going in and coming out w/ key in on position.

JUST SO EVERY READER KNOWS***** YOUR AIC VALVE IS ONLY USED AT IDLE TO LET MORE AIR INTO THE PLENUM. IT OVERIDES THE T/B. IT HAS NOTHING TO DO WITH REVVING THE CAR UP OR RUNNING CRAPPY OR ROUGH. JUST SO EVERY ONE KNOWS***OK

 

like every one else said, check your connections, make sure the maf is clean, maybe bad o2 sensors. just because they are new doesnt mean their good.

 

speaking of just because it's new doesn't mean it's good. My buddy with his 72 nova had a bad brake master cylinder. Went to replace it, and we went through THREE 'NEW' master cylinders from three different parts stores and none of them would bleed right. eventually had to order a "NEWWWWWWW" one from the factory


double check everything.

 

like we say in the mechnic world "New means new, it dont mean good". Iv been through 3-5 parts before I got one good. I even ran accross cv axles they didnt put any grease in the boots and it shook like crazy and then locked down on the test drive.

 

we had that happen at a shop I used to work at. I put it in perfectly, test drove it no probs. Forward, reverse etc... the cv snapped later and they tried to say I put it in wrong. It was just a faulty part!