Rough Idle when in Drive and stopped only
 

Like the subject states; rough idle when stopped with trans in Drive. No problems taking off from a stop or in normal driving.
If I put the trans into neutral, idle smoothes out just fine, put back into drive and idle gets rough again.
I thought it might be the IAC; removed it and path and tip are clean, cleaned them anyways and same as above.

Tonight when at a stop I switched on the AC thinking that with the added load the engine would stall...nope....idle seemed to smooth out just a bit...not as good as when in neutral though.

Thinking now it that it could be a vacuum leak or the IAC has a bad spot or something.

What else????

I believe that I can eliminate the vacuum leak; checked hoses and did not find anything and I replaced most of the hose this past year anyways.

So is it the IAC or something else?

I had a grand am that did the same thing. The rubber in the motor mounts were shot. More common issue on front wheel drives.

I did noticed that there is a small change in the idle between DRIVE and NEUTRL, slightly higher in NEUTRAL and smoother. Idle change every time I switch between the two.

Is there some method to test the IAC?

I will take a god look at the motor and trans mounts today.

Usually iac is problematic on cold start up, when getting off gas pedal and engine rpms drop to settle in at idle. Problems with iac would be any time the engine is at idle.

So if not the IAC then what could be causing this? Is this possibly the torque converter locking-up at idle? I do not feel any change.

I doubt that it is fuel pressure as I would think the rough idle would be present regardless if in drive or neutral.

Is this possibly the torque converter not unlocking at idle? Would a scan tool with data capture tell me this?

I am at a loss.

Still looking for suggestions as to what to check....anyone????

Does your truck have an EGR valve? It was either 2001 or 2002 when they stopped using one, but if you have one, it could be the EGR valve being partially clogged & slightly open.

How are your plugs/wires/cap/rotor? Are they all AC Delco?

Something to check. If it runs rough when you are stopped in drive, do you have your foot on the brake? When you shift to neutral, do you take your foot off the brake? Could be a leak in the diaphragm in the power brake booster. When stopped in drive with the brake applied, it causes a vacuum leak. When foot is removed from brake pedal when shifted to neutral, leak is gone. When you take your foot of the brake and step on the gas to go, vacuum leak is removed. If brake not applied, when driving, no vacuum leak. Easy way to check. Put vehicle in park, set parking brake, shift to drive, with parking brake holding vehicle. If it idles smooth, That is your problem. Hope this helps.
Larry

No EGR valve. Plugs, wires, distributor all replaced mid last year.

When stopped, have foot on brake and do not remove when switching to neutral. Did notice one time that when switched to neutral brake pedal went down further...just a tiny bit, but enough that I noticed it. Only did it once and I have tried this multiple times and has not repeated.

Will try with parking brake on, but I think I have to have foot on brake in order to change gears due to safety lockout...but will try and report back.

Thanks.