G8RB8

Hang on. In gear?

Hilton

Just to check - are you sure the TT isn't spinning with the clutch in? (i.e. clutch isn't dragging at all?)

How's the clearance between the upper bell housing and the frame, or the front end of the TT and the sides of the tunnel?

Those volvo mounts raise the engine up a long way, and coupled with new transmission raising the aft end of the TT, might be enough to hit the tunnel?

Black Sea RD

There has been a lot touched along the way, not just the TT. Brian's description does not sound like a classic TT problem. However, if there is something found with our rebuilt TT, we stand behind our products 100%.

The latest description with the engine popping under load sounds like something going on with the engine.

Brian, have you double checked the spark plug wires and spark plugs? I replaced my spark plug wires with a new set on our 1989 once and had rough running issues. A friend carefully checked the new plug wires and found a faulty one which after it was replaced smoothed it out.

Very sorry to hear about this and hope you will find out the root cause shortly.

Best regards,

BRB-83-911SC

In gear, out of gear, car stopped, all combinations - the vibe is there, just amplified quite a bit under load.

Slides into reverse with no grinding, so I think my clutch is disengaging fully.

All clearances have been chacked, and there is plenty of room at all potential points of contact. I will be checking that again.

Thanks Constantine. The "popping" is just off idle, no load, car parked. Under load while driving, the engine sounds fine, no audible popping or wierd exhaust note like you would expect if not firing on all 8. It pulls stongly all the way to redline - very smooth/linear power delivery - just that resonate vibration (which is felt in the body of the car, and creates a droning/buzzing sound) that crescendos at ~1300, ~2600, ~3900, etc. Also, at idle (750) engine feels like it has a lumpy cam.

I have to wonder if the engine vibes are "normal", and there is some sort of MM failure that is causing the resonance through the body of the car (this, understanding that the misfire is not normal and needs to be fixed - just not sure that it is contributing to the vibes).

What exactly do bad MMs feel like? And if the mounts sit high like they should, and the engine rocks, could they still not be doing their dampening job?

BRB-83-911SC

It's finally time to close the book on this one. The vibration issue was cured this past Saturday after installing new Porsche motor mounts. The engine now spins from idle to redline with no vibrations felt throughout the range.

After going through EVERYTHING (as noted in detail above), plus much investigation into engine running condition (compression and leak-down tests, oscilloscope diag of the ignition system, injector flow/spray tests, and air/fuel ratio tests), the only theory left to test was that somehow the 2 year old Volvo motor mounts became damaged when the transmission/torque tube was out. This was supported by the fact that the vibes felt/sounded like some sort of mechanical contact with the chassis.

The theory is that when the transmission and TT were out (for several months), that the engine, now supported only by the motor mounts, took a different "set" causing some sort of interferance. There is a witness mark on one of the mounts that may point to the issue (picture below). In any event, the new Porsche mounts seem to have done the trick.

That said, I do still have a slight off idle random misfire, and the A/F ratio (tested via Innovate LM2 with 2 O2 sensors in the tailpipes) does indicate slightly lean off idle and at cruise. WOT richens up nicely. I will continue to hunt this demon down.

Thanks for all of the support and suggestions along the way. The car is such a joy to drive now.

M. Requin

Thanks very much for the followup, Brian.