Oil cap off, hiss & idle jumps??

[Nixon1]

This happened fairly recently and I was wondering if you guys have any explanation for it?

The car was idling and I was curious about something.... So I decided to see exactly how much pressure was being built up in the engine even though the PCV system SHOULD be operating just fine. So I pulled my oil cap off to let it vent. When I pulled it off, I heard a loud hiss of air escape around the cap, and the engine idle shot up from 800 to about 1300 rpms. It didn't sound too good, so I put the cap back on, but it still idled high and displayed a check engine light. I can't remember what the code was when I pulled it, unfortunately. But anyways, after a few different starts and at different points in time, I tried to duplicate this scenario but I couldn't. Never hissed again and never threw another code, or jumped the idle. So, what the hell could have happened??

Thanks all.