Car Information:
Nissan 300Z
Year: 1992
Engine Size: 3.0L
Trans: Manual

Car has 85k miles and runs great. A vacuum gauge installed after the throttle bodies will read a very steady 20″Hg at idle and 21″Hg at 2000 rpm. However when I put the AC on the vacuum gauge will read 20″Hg but the needle vibrates rapidly +/- 1″Hg from 20″Hg. There are no noticeable drivability problems with AC on. What gives with the vibrating vacuum needle with AC on?

Sounds like valve clearance may be to tight on a valve and the vacuum is being interrupted.

Magic Mechanic
1432 Lee Road OrlandoFL32804 USA 
 • (407) 298-7447