Car accelerates on its own & idles high

I have a 98 Grand AM SE with about 65K miles. It will want to accelerate on its own after I drive it a while, the idle revs up prety high, etc. Looks like the cruise control is engaged. When at a stop, it will rev up to 1500 RPM and when I let of the brake, it will go on its own. In park, It will exceed 3000-4000 RPM. Normal idle is about 750-800 rpm. I shut it down and restart and everthing is fine for a while. It's seeems to occur more when it's humid. I've had the fuel injector and throttle body serviced, a new TPS sensor installed. I've done some research and it appears that it's an IAC problem. However, my mechanic thinks its either the cruise control self engaging or a bad ECM. Well, I've disengaged the cruise control; unplugged it. Could this be a problem with the ECM? He said that Pontiacs are known for having problems with their ECM. Help...I'm clueless..and sorry for the wordiness...:banghead:

Welcome to AF!!

There may be two problems here but they lead to the same conclusion. Check for a vacuum leak. High idle usually is caused by a vacuum leak but the second issue is that it may be coming from the intake manifold gaskets. Usually a vacuum leak will manifest itself at the start but if the intake manifold gaskets are going bad the idle may increase after the car is warmed up. Check the seating surfaces around the manifold and check the vacuum lines, one could be cracked and causing the issue.

Welcome to AF!!

There may be two problems here but they lead to the same conclusion. Check for a vacuum leak. High idle usually is caused by a vacuum leak but the second issue is that it may be coming from the intake manifold gaskets. Usually a vacuum leak will manifest itself at the start but if the intake manifold gaskets are going bad the idle may increase after the car is warmed up. Check the seating surfaces around the manifold and check the vacuum lines, one could be cracked and causing the issue.

So, I shouldn't worry about the IAC or ECM? Sorry, but I just want to be clear on this becuase I plan on taking it in to the shop tomorrow afternoon. This will be the 3rd time to try and get it fixed. I'm still :banghead: but I really do appreciate your expertise.

The reason I keep going to the IAC is because I appear to be having similar problems as listed in the following thread, http://www.automotiveforums.com/vbulletin/showthread.php?t=359341

Well, I took the car in and they said there were no vacuum leaks on the intake manifold. Also, they said the IAC, MAF and TPS are all ok. They told me the only other thing that it could be is a bad ECM. At a loss here...I told them to go ahead and replace the ECM; approx $600...Am I getting worked over by them??? HELP!!!

Yes you are. You could have purchased a reman unit or a used one from a wrecking yard for a lot less. Parts America has one for either engine size for around $122. They are not hard to put in either. If this fixes it I will be very surprised. I guess anything is possible but generally there are other issues before an ECM goes bad. Let us know if this fixes the problem.
Good Luck!!

Well, looks like I spemt $650.00 for nothing. No change in how it is performing. They said they test drove it, etc. Well, I test drove it and at about 7.5 - 8 miles into it, the RPMs stuck at 1500 cruising at 55-65 mph. Felt like cruise control was engaged. I pulled over and while in park, it revved up to 3000 RPM. So, what next???

GTP dad is right you have a leak somewhere but, I would take that car right back to that shop a demand an explanation or somebody would br giving me some money back!

Here is something you can try if you car is a V6 get a can of starting fluid and while the engine is running carefully spray around the intake plenum and the throttle plate if there is a leak you will find it. You will here the fluid get sucked in and the idle will change this should help you pinpoint your leak.

Rthumperboy is correct, I would go back and tell the stealership that you want them to either correct what they said they would or you want your old computer and your money back. Obviously they didn't correct the problem and it is worse now than it was before. If you use starting fluid you may get a fire especially if the engine is hot. Use a can of carb or throttle body cleaner to check for a leak.

OK, I brought the car back in. This time it was running a little hot as well; reservoir overflowed. Had them look at it, etc. When I got word from them they said that the flash to the ECM was flawed somehow. Something about how they used my old one for something and somehow it affected the new one:wtf:

They said my original ECM had a "virus" or something like that...I've never heard of this before:shakehead . Anyway, why would it surface now and not anytime prior top this in the 8 years Iv'e had it :screwy:.

I think I dropped the soap on this one....This has become a nightmare. Missed 2 1/2 days of work also. I just want my car fixed.

I repost with the latest...........

Here's the latest...I picked up the car yesterday evening. I drove for about 16 miles or so and all appeared ok. I drove it to work today, a 26 mile commute, and everything was fine. I'm hoping that it's fixed for good. They said they just had to re-flash the new ECM???

Hope all is fixed with your car. Never heard of having a virus on a car computer, don't think it can happen. Find a new shop they are ripping you off.

Hope all is fixed with your car. Never heard of having a virus on a car computer, don't think it can happen. Find a new shop they are ripping you off.

Yeah, I'm gettin' that type of feeling:swear: :crying: :screwy: :nutkick:

I couldn't find anything regarding a virus on an ECM. Live and learn....

Find another shop or dealer. There are no viruses in the auto computer systems if there are you are the first to get one. The place that worked on your car doesn't know what they are doing and they are taking you for a ride. The overheating is probably due to the car not firing properly. I still say you have a problem with the intake manifold!

I don't know what they did but the car seems to be running fine now. I made it to work and back today, a 52+ mile round trip, with no problems. Anyway, I will definately find another shop. Thanks for all of y'alls input.

Sounds to me like the last time you took the car in they finally figured out the problem and made up the BS about a problem with your original ECM to cover the fact that they misdiagnosed it and sold you an ECM that you didn't need.

All appear that I have the same problem, I have changed the IAC valve and the problem was didn't correct...
question ; what is the ECM, I'm from Mexico and the english not is easy to understand to me
gracias
I have a 3400 v6 grand am

Sammy, Its pretty much what I told you, the same thing in your PM a vacum leak inthe air intake or a manifold leak.

If the IAC you replaced was turned or pushed in, it'll screw it up.
Also with a new IAC I think it needs calibrated or set at a certain distance...
Thats it !!
The new one needs to be pushed in so the distance from the tip, to the metal mounting bracket,
is a little less then 1 1/8".

Give that a try and see what she does Sammy.