Here's an easier way to test. After the vehicle is warmed up, stop and put vehicle in park. Remove foot from brake pedal and sit for a bit. Idle should even out. Then apply brake pedal. Hold down firmly. If it starts to run rough, release pedal and see if it straightens out again after a bit. If this works the way I described, the brake booster diaphragm is the culprit. Good Luck!

 

Just a quick comment:
the idle become smooth as soon as I go to neutral and then become rough again after about 2 seconds when switched back to drive.....all of this with foot on the brake pedal.

Idle is fine in park...have not tried keeping foot on brake though.

 

Did the parking brake test.....no difference....engine idle was nice an smooth when in neutral, but becomes rough when in drive with either foot on brake or with parking brake on.

So I guess cross-off leaking power brake booster.


Any more ideas?

 

I have the same problem and was wondering what the solution to the rough idle in drive turned out to be. Thanks.

 

I have yet to find the solution; still has the same issue. If its a long traffic light...I just switch to neutral....sort of like having a manual trans....if only that were true.....

 

I had the same problem, decided 1 day to unbolt the torque converter from the flywheel...bingo..engine was smooth as silk.
Pulled transmission and replaced lockup switch and torque converter. Problem is, that's all I can tell you for now, I haven't put it back together yet..lol. Probably next weekend.

 

My first thought was the torque converter was not unlocking....so I will be awaiting for an update on what you find-out.
If this is indeed the solution, not looking forward to dropping the transfer case and then the transmission....that is a lot of work to do.

I will be looking for that update.....thanks.

 

eisterd: any update?

 

This is just a shot in the dark and probably get laughed at, but a bad or going out catalytic converter will make your vehicle idle strange and pull your rpm's down and up.

Like I said its a shot in the dark.

 

eisterd: any update?

My '88 with a TH350 has this problem as well. Very frustrating, since mine nearly dies at stoplights if left in drive.