Opening the oil filler cap during idle: what's normal, and what's not?

[r1de]

Hey all,

I know that various different things can happen when you pop the oil cap while idling, based on different kinds of problems (for example, hard to open, easy to open and oil sputters, easy to open and engine starts to run rough, ...).

Has anyone compiled a list of common "if this, then most likely that" symptoms of popping the oil filler cap on idle? Seems like a good cheap diagnostic tool to add along with other symptoms when identifying possible problems.

Cheers,

[Charles.waite]

Opening the oil filler cap to diagnose anything is some bush league shit.

/shortrant

Long answer: about the only thing you can tell by popping the cap at idle is if the PCV is functioning properly. The cap should be getting sucked down onto the valve cover because the crankcase is under heavy vacuum at idle.

[r1de]

LOL. That's a pretty good answer, actually. In the case of the PCV failure, it's a pretty good indicator though - as is the opposite case of oil coming out of the crankcase (and/or engine running rough at idle with filler cap shut tight), right?