Oil Cap off, idle jumps??

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.
 
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My buddy has a 2000 Grand Prix GTP that he leased and during the lease period he took the car in to the dealership for regular oil changes. On one oil change, the mechanic forgot to put the oil cap back on and as my buddy drove away from the check engine light came on. He pulled over and popped the hood to find the cap missing and oil all over the friggin engine compartment. He turned around and took the car back, they topped it up and replaced the cap.

The cap does serve a purpose other than protecting the valve from dirt getting inside the valve covers and contaminating the oil. If you check the upper inside of the oil cap and work you way up the treads you'll see there's a rubber gasket on the cap. That gasket protects from air getting in or out. Seeing as the whole pump works on a vacuum/pressure system, of course if you take the cap off you'll hear air hiss. The oil pressure system works the same way as your gas tank unit. Ever notice how when you stop for gas and when you open the gas cap there's a hiss sound? That means you're getting good vacuum in the tank.

I hope that by your removing the oil cap while the engine was running didn't create some instability in the engine and now you have a leak somewhere because a gasket or hose was blown or something.
 
The cap doesn't prevent all air from entering the crankcase - it simply prevents unmetered air from entering via the valve cover. Air has to enter the crankcase to make up for what gets pulled out through the pcv valve/manifold when under vacuum. That makeup air is supplied via the hose running from the throttle body to the valve cover nipple. So air passes through the maf, through the hose to the crankcase via the oil filler nipple, and out the cranckcase into the intake through the pcv valve. This approach pulls any oily vapors into the engine and combusts them - something the epa mandated many years ago. Under full throttle when there is no vacuum, at higher rpm the crankcase produces pressure. This pressure is relieved via the pcv valve - pressure in the crankcase forces air into the intake (which at w.o.t. is at atmospheric pressure), and it also forces air back out the filler neck nipple into the intake through the throttle body. Over time, that's how oily grunge builds up in the throttle body and IAB solenoid. So that little hose between the throttle body and valve cover is very important and it's flow is bi-directional. When the engine is producing manifold vacuum, air flows through it TO the crankcase. When the engine is not producing vacuum, air flows from the crankcase into the throttle body/intake.

You can ventilate the crankcase simply by putting a breather on the valve cover - but it's messy (over time), it's tampering with the emissions control system of the car, and in many cases, if the pcv is left hooked up, it causes idle issues as it can act like a vacuum leak.

The idea of hooking manifold vacuum to the fuel tank via the charcoal cannister and the purge valve is not to put a vacuum on the tank. It's simply to keep gasoline vapors from venting to the atmosphere. The major source of unburned hydrocarbons used to be fuel evaporating from gas tanks that were vented to atmosphere. With efi and high pressure pumps/return lines, all sorts of vapors are stirred up in the tank. Venting to atmosphere releases too much unburned gasoline - so the purge valve/vacuum cannister takes those vapors into the running engine - so they're disposed of by combustion, and cleaned up by the cats. It wouldn't take much vacuum on a fuel tank to collapse it - they're not that strong.
 
Good or bad is a subjective judgement in this case. If you're fuel injected, normally aspirated - there's no benefit to changing the factory arrangement. If you don't mind unmetered air entering the intake under vacuum condtions, and the potential for hot oil mist throughout the engine compartment, go for it. But it's not a choice I'd make for mine.