Vibration after drivetrain work

[dfrost]

I'm getting a low frequency vibration, almost feels like 'growling,' after having a bunch of drivetrain work done. The mechanic that did the work agrees that "something isn't right" but the fault isn't obvious to either of us.

The vibration is present when driving or standing still (just revving the engine), and is most evident just above idle and at ~3000-3500 rpm. The engine seems to pull fine, but doesn't feel as smooth doing it. It felt great before the work was done.

Here's all the work that was done:
- Replaced Guibo, driveshaft (rebuilt from Portland), center support bearing
- Rebuilt shift linkage (feels great now!)
- Installed UUC transmission mounts with Enforcers
- Replaced rear wheel bearings
- A/C repaired with rebuilt compressor, new receiver/dryer, belt, tensioner, and pressure switch. Vibration is present with A/C on or off.
- Replaced tie-rod boots and re-aligned front end

Obviously, the exhaust system was removed to do the drivetrain work, so it is a suspect. The mechanic chose not to replace the exhaust gaskets at the manifold outlets (said they looked fine). I'm wondering if there is a small exhaust leak there.

I checked the exhaust bracket where it attaches to the transmission support and found that it had a little preload, so loosened and re-tightened those bolts to relieve the preload - still vibrates.

The exhaust hangers were not replaced, but seem OK, although one of the middle diamond-shaped ones doesn't seem to be supporting any weight.

The mechanic wondered if the UUC transmission mounts could be the culprit since they're stiffer. Anybody out there have any experience with UUC tranny mounts? I'll remove the Enforcers if necessary.

Any ideas?

[Jeron]

Hopefully it is normal vibration that is being transfered due the UUC mounts. UUC has certain order to tighten the mounts (top then bottom or vice versa) and the torque on the mount nuts is just over hand tight.

Loosen the tranny brace, retorque the mounts and make sure there is no preload on the brace when you reattach it.

Remove the AC belt to eliminate that suspicion. It only takes a minute.

An exhaust leak would be heard more than felt. Did you replace the exhaust rubber hangers?

[dfrost]

Jeron,

Thanks for those suggestions.

I will definitely redo the tranny mount tightening tonight. I've downloaded the instructions from UUC, and (no surprise), they're just as you state. Notably they say: "Overtightening will cause vibration."

The AC belt is a good, quick check if the tranny mount re-torque doesn't do it.

Exhaust hangers were not replaced - something about my car having the "good ones" already - but I will probably replace at least the mid-pipe ones next time I order parts from our host.

[dfrost]

Follow-up with the Fix:

As you suggested, Jeron, the UUC transmission mounts had been over-tightened (at only 16 ft-lbs, it doesn't take much). So I re-did the torque, following the UUC instructions regarding loosening the transmission bracket first to ensure no preload. That clearly helped, but did not eliminate the vibration. But I was clearly in the correct area causing the problem.

So I removed the UUC "Enforcers" (aluminum cups that encase half of each mount) and re-did the torque. PROBLEM SOLVED! Car again feels smooth and 'eager.' These optional Enforcers apparently make the UUC mounts too stiff, at least for my car, but the tranny mounts themselves are fine.

Anybody want to buy a pair of lightly used UUC Enforcers cheap (without the tranny mounts that fit in them)?