Need help diagnosing engine shake. Please!
 

Greetings AW, it's been awhile since I've posted anything on the forum. Hope everyone is having a wonderful day. Let me get down to this:

I've had my A8 4.2 for a bit over a year now. Great rock-solid car, and have loved every second of it. No large issues except for the common valley pan gasket leak that happens. Other than that, basic maintenance, no complaints.

With that said, something strange is occurring on cold starts (yes, this only really happens on cold starts). NOTE: Before anyone implies that this is engine related, there are no codes, the engine does NOT have a misfire, no rough idle, no skipping whatsoever. As a matter of fact, I just did the sparkies 3 weeks ago.

Here's what's going on:
-Start car (engine idles fine)
-Once the engine bogs down to idle -> time to go right?
-Put car in reverse... This is where the shaking/shuddering starts. (Once again looking at the tach, it's not the engine doing rough idle)
-Put the car in drive -> shaking lessens, but is still there.
-Here's the catch, put the car back in park... shaking gone.

Once again, I wish to clarify, this is ONLY an issue at cold start. If the car is warm or even slightly above cold, this is non-existent.

There you have it. I'm hoping that any of you fine gentlemen will be able to give some possible causes for this little nuisance, if anyone has any further questions, please ask. I will try to answer as constructively as possible to them. Much will be appreciated. Thank you.

Regards,

TeamHate!

Vacuum hoses loose or leaking?

Well, ltooz, which ones?

If it is a vac leak, wouldn't it throw a CEL? How can I locate the culprit? Also, why would a vacuum leak have an effect when the car is cold, and not when it's warmed up?

Thanks.

Did you scan for codes? Could it be a motor mount?

As my experience with Audi's, at idle and in gear at cold is normally hoses around the throttle body and manifolds leaky. The air temperature in the engine bay is cold causing the air sucked in cold hence it's not well controlled, then the engine bay is hot, the air sucked in warm hence it's better.

Bad motor mounts as a possible, especially the torque mount (farthest one forward on USA passenger side near whole alternator assembly low down) that would explain why you feel it more in gear. Search for more info, including Mister Bally's prior post w/ pictures. Blew one on my C5 4.2, and blew two so far on my (higher torque) D3 W12.

Maybe bad flex joints in the exhaust. Under the car and pretty easily found/seen. The temp would change quickly as things get hot and then different possible vibration result.

Maybe a combo of both of those. Neither one would typically involve codes--really a pretty classic old school mechanical as the miles and years go by.

+1 on suspecting motor or transmission mounts as the symptoms worsens under load. If you have some one go through the gears, park, reverse, drive while holding the brake and watch if the engine or tranny shakes you maybe able to track it down to the specific mount. After that, i would have a good look at the drive shaft.

Please keep us posted.