1997 80-series Vibration while Stopped in Gear

[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.

 

[FIDO96]

So after a lot of searching, the consensus is the vibration while idling under load is most likely old engine / tranny mounts. I’d this the reason to get an engine hoist or just make do with floor jack.

 

[Kernal]

Old thread revival, but FWIW another cause of a vibration with the engine at idle, transmission in gear, foot on the brake pedal (at a stop light for example) that was not discussed in the thread can be due to broken exhaust system rubber isolators.

 

[FIDO96]

Old thread revival, but FWIW another cause of a vibration with the engine at idle, transmission in gear, foot on the brake pedal (at a stop light for example) that was not discussed in the thread can be due to broken exhaust system rubber isolators.

I replaced those this summer. I watched a Timmy the Toolman where he does a test to see how high the engine “jumps” when you brake, throttle in D & R. Mine seemed nominal.

 

[CJK]

TLDR - it could be may things. Rubber isolators dampen engine vibration but you might be treating symptoms if you only address those.

I haven’t fully solved the issue (still no bueno when in reverse) but my idle speed was too low caused by TPS being at -2% when it should have been 0%.

I chased a weak ground and weak spark (corrosion on plug wire). Pulling spark plugs in next couple weeks to see if that it adding to it.