View Full Version : Bad idle in gear

09-08-2009, 09:29 AM
Well I was driving the SC home from my girlfriend's place late one night and the CEL came on. I never see the CEL so it worried me. Next thing you know the car started surging and running rough. The light went off and then the car ran fine, until it started surging again, etc.

If I start the car up it runs fine in park. As soon as you put it in gear it starts shaking and running rough. When you put it back in park the idle will smooth out after 5 seconds or so and it will sit there and run great until you put it in gear again. Any idea where to look? I sprayed ether all around the engine bay assuming this could be a vacuum leak, but I didn't locate any. The only code that came up was for the charcoal canister and I've had that for as long as I've owned the car. Any help would be appreciated. I need to get the bird running since I broke my SHO too....

09-08-2009, 12:12 PM
Are the rough idle and the surging part of the same problem, or do they occur separately? An intermittent bucking with a CEL sounds like a problem with the ignition module or a sensor. However, a constant rough idle sounds unrelated.

09-08-2009, 02:29 PM
The surging started the same night the rough idle did. I haven't left the driveway with the car since, but I'm assuming the surging is still there. When the surging started the car seemed way down on power, however I was babying it since I didn't know what was wrong.

The weird part is that it will idle in park perfectly, but once you put it in gear....

09-08-2009, 03:32 PM
Is the tachometer acting strange at all? If so, it would suggest an ignition problem.

You could have a failing IAC ... although they usually fail in the other direction: idle way too high.

One often-overlooked sensor that can give some problems is the BAP (barometric atmospheric pressure) sensor. You could try unplugging it before you start the car and see if it runs any better. It's possible to do an actual test if you have a multimeter that can measure frequency of a signal.

Stupid Canuck
09-08-2009, 06:14 PM
I have a similar problem and don't know the answer but my car has several mods. Could be, in your case, as easy as cleaning the MAF element.

What are the vacuum readings in drive and park? And what are the tach readings?

09-11-2009, 12:50 PM
I'll check the vacuum and tach readings when I get a chance (at work). From what I recall, the vacuum and tach is fairly normal in park. When you put it in gear the idle is very low and the vacuum starts dropping to almost nothing. When you put it back in gear, everything slowly goes back up to normal.