c6 clunk

when I go from park to drive or reverse I get a pretty good clunk. My transmission guy said this is because I have a low vacuum problem. The c6 needs 18inhg vac. He is right I am only reading about 10inhg at the manifold. Has anyone had a similar problem?

..bad ujoint or slack in the ring gear most likely

.....or possibly a transmission mount has broken or turned to mush. Then maybe a loose spring hanger or clamp. Of course, it could be a loose wheel or sloppy axle splines.

What's your idle speed? If it's above 5-600 rpm, then the clunk is just all the loose parts going from a slack position to a loaded condition.

AND you do need about 15"Hg at idle to operate right:eek::eek:

I have new U joints, rear end seems to be tight and new motor mounts. Everything seems to be pretty tight or at least that's what I am being told, haven't actually checked it myself. I can get about 15"hg if I bump the timing up to about 14 BTDc but then having trouble getting the idle down to the 5-600 range.
Thanks for your replies.

All of the above suggestions are good.

But one thing we did "back in the day" at the dealership was to pull the driveshaft and check the play in the splines on the slip yoke to the transmission output shaft. If the play is not excessive just pack some heavy grease inside the slip yoke to dampen the noise. It's easy, and might be worth a try.

John thanks. The clunk isn't a noise but more of a jerk like it has bad U joints of play some where.

what is your idle speed??? too high it will clunk.

You got some really good answers.
Kurt