Too much vacuum. can't remove the oil cap at idle

Possible intake gasket vacuum leak?

I suspect two problems. Vacuum when removing oil cap is likely PCV valve. No oil pressure could either be true, and caused by oil pump or clog, or not true, caused by bad sensor.

There is not suppose to be vacuum from the crankcase so having none with the oil cap removed is normal.

Well -doing that sort of takes out the PCV System
doesn't it

Unmetered air comes in the oil fill hole & gets drawn into
the intake & leans out the fuel mix

You also got PCV system issues with high vacuum

You don't add oil with the car running

You broke the vacuum for the crankcase evacuation
(PCV System)

If you have idle problems then they have to be diagnosed

High idle usually indicates a vacuum leak, you may have one of the smog hoses disconnected, that would also cause a suction at the filler cap because the engine is taking in air pretty fast. I have a 93 2.8l and it will run fast idle for a short time when starting it cold, I have learned that is because it is running rich at that time and the computer thins a rich condition with more air so it is not smoggy.

Unless your throttle linkage is hung up, it is vacuum leak.

This is normal. With oil filler cap off, the vacuum pressure in the engine is lowered.

Charlie

That is normal, when the oil cap is dropped the vacuum to the engine drops also cause the idle air valve to close due to lack of vacuum.

sounds like an Idle air control valve or a positive crankcase ventalation valve(or pcv valve). use some intake cleaner in the air intake and take the pcv valve out and shake it it should rattle and move freely. mike