Possible bad motor mount
The most probable cause would be the idle control on the engine. When you put a car in gear, there is a "load" put on it (the transmission is using power) and the idle speed normally drops. The idle could be dropping to the point where it becomes unstable and causes the engine to vibrate.
Fast idle motor is not working. This is allowing the idle to fall below the threshold of what it should be when the A/C is on.
low idle and uneven idle
The engine is running C O C K
I would look for a vacuum leak.
It is the engine that is vibrating and it is just being felt in the steering wheel. Can be a vacuum leak, or miss in the engine.
because your to fat
The cause of high engine idle and surges in the 1989 Corvette is due to..Read More
Timing belt. misfiring engine
If it vibrates at idle and runs good under under full throttle, you have a vacuum leak. It can also be defective spark plugs or plug wires.
Out of time.