Thread: Stumble at idle

ahhhhh im having same problems...has it thrown a code yet?

Well, there's actually a few things that can cause this. The first thing that popped in my head was the Idle Air Control motor, but the MAF could cause something like this as well, particularly the stalling.

Now my last Grand Prix I had, an '89, did something very similar to this and it turned out to be the Crankshaft Position Sensor, but the symptoms you are describing doesnt sound like the CPS in the modern GP's so I wont point to that just yet. The sensor on the '89 GP and the one on the newer GP's are completely different.

The first easy thing you can do is unplug the MAF sensor and then start the car and see if you get the same thing. If you do, plug the MAF back in and you can kinda rule it out and move on to the IAC. If you dont then you may need a new MAF sensor.

Hope this helps out.

Well you can clean the IAC and it still be bad, so keep that in mind. When I clean mine (IAC) however, I spray the crap out of it, use a soft tooth brush and then use some type of light spray lubricant (I use Amsoil MP), but WD-40 would probably work good. But if some of that carbon build up gets in the pintle/plunger mechanism, then it could render that thing dead no matter how much you clean it.

You dont always get a code. Ive troubleshot cars before with completely dead MAFs and had no SES light. Ive pulled the connector off my own MAF before and have run it without any codes.

Furthermore, it takes only just a few minutes to do this test and isnt going to hurt anything and will either remove one item from the suspicions list or he'll find the problem, either one is a good thing.

With regards to your IAC. You arent going to kill it by disconnecting it with the batter still connected. Its a motor, not a sensative electronic piece as with the PCM.

Okay, if the IAC is functioning properly, lets mark it off the list, for now. You can check it with a scanner, but I forgot the IAC count numbers that showed a normally operating IAC.

Theres a couple other things that may cause what you are experiencing which would be the MAF or the EGR. I dont know if you can disconnect the EGR in the way that you can the MAF and see if it is the issue, but it might be worth a shot. Those are the two things I would go with next. First disconnect the MAF and see if anything changes, if not, hook it back up, then disconnect the EGR and see if you get any change.

Wait. I just realized you said you had the throttle body sitting off to the side and connected the IAC back up. What engine do you have? That harness isnt typically long enough to reach anywhere past the throttle body. Furthermore, the IAC is on the throttle body if its a 3800. You got me scratching my head on that one.

One other thing that could be an issue is the purge valve, but Im not sure or as confident on that one but only because I have not see one fail and Im not sure what symptoms it would present with. You could have a vacuum leak somewhere, but I would expect the IAC would show that if that were the case. But that comes back to the numbers that show up on the scanner which I cannot recall at the moment.

What kind of scanner does your buddy have?

Could it possibly be TPS...mine had same symtoms before it threw code.New TPS no more stumble..

I thought about that one myself. But typically the TPS will either just go bad or develop 'dead' spots, which are easily detectable with a scan tool. But an issue with the TPS usually results in some type of symptoms with the Transmission since the PCM uses the TPS signal information for shift control. So while I couldnt say with any certainty that its not the TPS, I'll play the odds on this based on my experiences and say its not something I would suspect at the moment.

well as u kno scotty,mine went bad the other day and i replaced it...and yes it was being screwy with trans too...but however i think i bought a bad sensor. my car tripped another code again...i was way out in the country when this happened and had to drive over 50 miles to a parts store...the whole way to the parts store i had no trans probs.But it was stumbling on idle.i got it scanned and BAM! it was TPS again.I just happened to get a bad TPS from autozone...im sure there are alotta things it could be...but when he said it was idling really low and wanting to stall,i figured id put my 2 cents in.Nevertheless i hope he gets it taken care of...these things can be aggrevating

I wasnt discounting your input, merely explaining that I played the odds on the troubleshooting because it is much less common for a TPS to cause those symptoms than it is for the MAF or EGR or IAC (which he seems to have ruled out). And the reason I played the odds on that was because of the missing information in his symptom report; the screwy trans shifts. Its not something someone would miss or not notice, even if it were to go away, most would say to themselves that has to be related even if it ceased happening.

So I wasnt knocking your efforts or your input, but merely explaing my thought process and why I disregarded the TPS as a possible reason for the issue. Who knows I may have to eat crow on that one, only time will tell.

But when you are troubleshooting you cannot look at every symptom individually. You HAVE to look at it from a global perspective. You cant match the sickness to the treatment, you have to match the treatment to the sickness. That is to say, you have to look at the symptoms and determine what could have went wrong to produce ALL of those symptoms. And if you do not look at it globally, you will get sidetracked in a hurry because some component failures will cause symptoms elsewhere that may look like a separate failure.

For instance, the CPS. Its a two part sensor, one side of which controls spark. But if you dont know this you may look at the symptoms, no spark and determine you have a bad coil pack or ICM. But if you look globally and know that the spark control originates from the CPS, you can make a more accurate diagnosis and you have matched up the symptoms with the only component cut spark before the ICM and coil packs.

Ive been a troubleshooter and an analytical diagnostician for the better part of 20 years and one of the things Ive learned is I am still prone to make mistakes no matter how much experience I gain and that you have to filter reported information very carefully or before you know it you are chasing the wind. But as I stated earlier, I wasnt knocking on your input or your help, just pointing out my process and why I disregarded the TPS as the culprit.

i completely understand what ur saying,in no way did i think u were knockin my input.Im sure u know alotttttttt more about these cars than me.Just wanted to put in my lil bit of info seeing as i had the same prob this week.The original sensor went bad and messed up idle and trans shift really bad.The new autuozone sensor went out just caused bad idle and wanting to stall.Since replaced with a 3rd TPS everything is fine now,car runs great.