Vibrates & shakes when idling -1991 MB 300CE

[Bassmaster]

I just installed new spark plugs, ignition cables and distributor cap in my brother in law's 1991 MB 300CE. It runs much more smoothly now, has stopped mis-firing and has power when you press down the accelerator, but now it vibrates and shakes when idling...not always but often enough to be perplexing. Could this be a bad motor mount? It has 89K miles on it.
Thanks in advance for your thoughts on this.

[glenmore]

Does it smooth out when in neutral?

If those are the original mounts, they probably need changing as well as the tranny mount.

Take a look at the passenger side mount. I think the spec is around 13mm space between bottom of mount to frame.

[Bassmaster]

Yes I think it does smooth out when in neutral. What does that mean? I was suspecting a sticking throttle linkage until I read all the posts about failing motor mounts in this model. Thanks!

[berryrice]

I had a similar issue where mine would do that on my w124. I thought is was a electrical issue. I'd put in in neutral and it would stop "most" of the rough idle...so I thought it might be the tranny. After 1 month if it happening, my car would only start up 95% of the time. After being in the shop twice...it was the dual fuel pump that was faulty. We tested both and there both would work off and on.

If your checking your motor motor mounts...tranny mount and flex disc should be checked out too.

[glenmore]

Originally Posted by Bassmaster

Yes I think it does smooth out when in neutral. What does that mean? I was suspecting a sticking throttle linkage until I read all the posts about failing motor mounts in this model. Thanks!

If it is pretty smooth in neutral and the engine revs smoothly, then it might be the engine mounts. Check clearance of the bottom skirt of the mount with the frame. You might even see the engine torque when someone revs the engine for you.

[big dog 2]

Originally Posted by glenmore

Does it smooth out when in neutral?

If those are the original mounts, they probably need changing as well as the tranny mount.

Take a look at the passenger side mount. I think the spec is around 13mm space between bottom of mount to frame.

Mine was the same! Replaced the mounts as noted above and it drives like new again!

[POS]

The motor mounts, even new ones, are only good for three or four years. Get them replaced; it's a simple job.