I have a 1992 740 (Regina system) and the engine speed when idling oscillates up and down a bit. Not a lot maybe plus or minus 200 rpm. When the engine is cold this is more pronounced.

The throttle body was cleaned not too long ago and so I cleaned the idle air control valve. It was definitely very oily inside. It is the two wire model that does not come apart.

My question is this: when running at idle should the engine be pulling all the required air through the IAC or does most of it still go through the main hose from the air cleaner passed the AMM to the throttle body?

When I attempted to test the IAC by blocking its intake at the air cleaner it really made no difference at all. I pulled the air cleaner top and covered the IAC connect hole with my thumb and I could slowly feel a vacuum build up. On the other hand, if I let the IAC port open and covered the main port to the throttle body, the accordion air hose constricted and the engine struggled.

This seems to indicate that most, if not essentially all the air is going through the throttle body and not the IAC.

What should be happening?