odd idle + oil cap question
 

so a while back (this year) i replaced my AOS (no smoke or issues. just time+mileage).

prior to that i tried the "remove the oil cap" test, which went fine. no problem getting it off, just a very slight suction.

today i pulled out of the garage and decided to add a little oil (it was slightly low) and when i pulled the cap i noticed the car really didn't want to idle well. i don't remember that happening before replacing the AOS. once i put the cap back on it returned to normal idle.

so my question - when removing the oil cap, should the car idle poorly? if i remember correctly before AOS replacement, it used to idle faster until the cap was put back on and then returned to normal.

Yes, the crankcase is running under a slight vacuum (via the AOS system) to aid ring sealing; taking off the oil cap creates a vacuum leak which can cause the car to run rough.

great. thanks JFP!