1997 80-series Vibration while Stopped in Gear (2 Viewers)

[CJK]

@JTinNM Not sure if you done it recently, but I would check your electrical grounds.

I just replaced my battery cables + grounds (block to battery, block to firewall, t-case to body) and the engine is idling very stable again in my limited testing.

My leading theory is that I had a bad/weak ground = weak spark at idle --> rough

 

[manofthewoods]

Its a very common 'harmonics' issue. I'd wager as soon as you increase engine rpm a couple hundred it quits, right?

Replacing motor and transmission mounts can help in some cases. Also....be sure your engine is in good 'tune' and that it runs and idles smoothly (no missing or erratic idle).

If the motor mounts have never been replaced.....odds are good they have cracks. The rubber dries out and loses its dampening capability.

Here is what mine looked like:

View attachment 3513513View attachment 3513514

Good photos.
If my memory serves, mine looked something like yours, maybe a little better. I didn't have the vibration issue prior to my rebuild/"reset" but no way I would put in used mounts after a rebuild.

My gut tells me that our big six should idle relatively smooth even with lousy mounts.
Certainly cleaning up the grounds is a cheap & good idea no matter what. Plenty of good ideas in the replies above as well. And, mounts as old as ours (if original) ARE begging for replacement, regardless of mileage.