rough/surging idle

[goober544]

I ahve a 1993 grand Am SE coupe. I love the car but one problem i can not fix cuz i dont know what the problem is.
When in park, the car very infrequently idles roughly; meaning the engine surges to a high, then drops down to an extreme low, goes back up high, then drops back down low again. If you have the lights on, you can see the lights grow brighter and then grow dimmer in tune with the surges of the engine. Everytime i take it to a garage the car doesnt do it. Nobody seems to know what the heck I'm talking about. Does anybody have any suggestions as to what is the problem? Once again, the car ONLY does this when it is in Park. If I shift out of park to any other gear it runs fine but only in park. Thanks a lot

[swalt]

Four things I would look at. Your IAC (idle air control) valve is dirty and needs cleaning or replacing. Your TPS (throttle position sensor) is acting up and will need to be replaced. You may have moisture getting in the vehicle and it is corroding the wires on the passenger side. Check your passenger side floor pan to see if it is wet. If it is, you have water likely getting into the wiring connections on the firewall. You need to pull out the glove box and the lower kick plate. Check the wiring connection that is mounted to the firewall, pull it apart and check the connections in it to ensure none of them are green. Also disconnect the plug on the computer mounted to the right pillar and check them for corrosion. If you have water coming in, do a search in this forum and you should find many posts of where to find the leaks. One other area to check wire connections is in the engine compartment. Check the wiring that runs by the transmission dipstick. Those wires are prone to being torn from rubbing on the dipstick and can cause all sorts of electrical problems.

[Slade901]

I agree on loose wires. Check the battery cables for tightness and other wires.

Have your alternator tested as it may be partially failing. Check the wires on the alternator itself to make sure they plugged in good and the other alternator wire secured by a nut is secured properly.

[Hoags]

Does the car idle rough if you put on the A/C [of equipped]? If putting on the A/C then I would think it may be a vacuum problem. Just my 2 cents worth.

[goober544]

No My AC does not work so its never on. ALso, I just had my alternator replaced a week or two ago so its not the alternator. My dad said the TPS cant be it, as my car does NOT idle all the time, just very irregularly. It can go a few weeks without doing it, then do it every day for a few days. Extremely random. But i am looking into the IAC and the TPS anyways. Thanks for the suggestions.

[swalt]

A TPS can intermittently fail, especially if moisture builds up in it.

Just curious, does your problem seem to happen in any particular types of weather, such as rain or dampness?

[goober544]

no it just randomly fails period. theres been no consistent factor, it just does it whenever it feels like it apparently. im not entirely sure if moisture is the problem but i am going to have it looked at regardless

[Woohan]

I have that exact same problem. I have put a IAC valve and it didnt really solve anything. Mine happens when its warm engine wise regadless on the weather. I did a intake gasket change and it helped with engine drivability, but nada for idle. just missed really bad.. After driving for a bit and comming to a stop the engine runs real ruff and if you let up off the breaks the surges. When you put in park after driving sometime it's hunts for idle for a sec or two. Im wondering about the wiring, but I havent noticed to much water ..... I havent changed the TPS and checked it with a meter and when moving to WOT it reads then tward the top it Overloads then reads again.

[Igovert500]

Irratic idle can be caused by plugs, vacuum leaks, and dying o2 sensors. 02 sensors typically throw check engine lights...take the car to autozone and have them run the ECU for free to see if there are any codes stored.

Also turn the car on, let it idle at warm temps, and pop the hood, listen for air escaping from vacuum leaks.

And how long ago did you replace your spark plugs?

[Herbstleyd]

if youre engine is the 3300 V6 take a look at this

when it start to go up an down (the rpm) look the light on the transmission

P
R
N
D
2
1

the light near the P (when in park) will shut down when the trouble start
just press on the light and the light will goes on and 2 sec after the trouble will stop

it work for my in my GA '92 V6 3.3

[Woohan]

Ok I was out in the car yesterday and was checking the MAF sensor plug try to clean it out a bit( I had corrosion on my ECM harness under the dash (I repaired it). I recently replaced the TPS and harness so I decided to get a look at the harness for the MAF and what do you know but the B- wire was alittle brown in color, and the C+ wire was corroded), But before I hooked it back up I decided to start it up and let the computer use its generic figures for it, funny thing is that the engine didnt miss that bad for a min, then it started to search idle for a few second it warm up some and just be for the fan switch kick on the car ran really smooth, as it cooled back to temp it the began to search again.

So I thought hmmmm lets see what does it do when you drive it. Mind you it still had a slight miss, hit the gas a bit and the car picked up fine, alittle better even and was alittle smoother up crusing. Slightly pulsing as I cruised about 40-45 mph, but not like before. The SES was on of course, So I took it back home and hit park, it searched a sec then idled. Pluged it back in (the MAF) and back to the roughness it went, but the light was off.

Well Since I had noticed that the plug had corrosion on it I decided to check others. I went to the passenger side and too out the glove box and panels from underneath. Im beginning to get the water that triclkes down the intake flap so I thought well the ECM middle connector had corrosion lets look at others and so I grabbed the connector that hooks the out side to the inside (flat connector) and pulled it appart and wow there was alot of corrosion, so I cleaned it as good as I could with electronic cleaner and WD40, its better and my idle is alot better, but its not a 100 percent contact because if you move the wires around the idle will change a bit. Now for the MAF I noticed after fixin (well sort of) that connector that it was sorta running like it was in generic mode so I went and unpluged the MAF and not a change just a very short idle change because the light came on. Im thing short somewhere, maybe in the MAF let me know what you all think.

[streethoboz]

I've had the same problem in my '01 GrandAM GT you people are talking about. My problem delt with the IAT sensor but, my problem was that a wire was pulled. While the wire was pulled my car would idle screwy in drive or neutral or completely die on me in the worst places. Try looking at the IAT sensor wires... Just a thought.