Oil cap and engine behavior

Hi!!

I've noticed a weird behavior when i take off the oil cap in my GTI.

I've never seen this in any other car I've had before.

Here's a video.

https://vimeo.com/147145191

You're creating a vacuum leak when you do that. Working as expected.

Why would you take your oil cap off while engine is running?
Please inform?
Surprised oil did not spray out.

For no reason at all.
But while I was unsure what the right behavior should be, I know it won't hurt the engine.
Oil would spray out if you run without it.

Thank you for the clarification.
I guess the wrong behavior would be engine turning off, right?

Have never removed a cars oil cap while running but have had a grass push mower come off by accident. Oil sprays everywhere.
Best to only remove when car is off.
Don't want you to get oil all over engine. Would be a mess.

You won't really have oil spray out since there is a baffle in these motors, but you will get oil vapor. I've seen some cars stall when removing the oil cap while running. I've also seen some assholes use it as a negotiation tactic - puffing air out the oil cap when its removed is excessive blowby due to bad rings.

Some old dry sump engines (air cooled Porsche 911 engines) required checking the oil level while the engine is running. So you would need to remove the oil cap to the oil reservoir to add oil if needed, which would cause a vacuum leak with the associated rough running and low idle (think pre drive by wire). You'd have to manually idle up the engine using the idle screw on the throttle body to within spec, prior to taking an oil level reading. Then once done, close the oil cap and readjust the idle.
Pretty much all other engines I've ever come across require the engine to be turned off to check the oil level. So there really is no reason, or at least one that I can think of, to remove the oil cap while the engine is running. If you notice that there is no change in behavior when you remove the oil cap while the engine is running, then that would be a cause for concern.

Don't do that.

Morphine said:

Don't do that.

Click to expand...

Could you explain why not?
I'm curious and wishing to learn.

mroski said:

Could you explain why not?
I'm curious and wishing to learn.

Click to expand...

Engines work best and are designed to run with a vacuum on the crankcase, so they have a PCV system which allows 'air' out but not back in (one way check valve) and dumps it back into the intake manifold to burn off (on cars that need to meet emissions laws, otherwise it goes out the exhaust). This 'air' is blow-by from combustion, oil vapor, etc from inside the crankcase that is pressurized by the pistons moving up and down.

When you open the oil cap suddenly the system is sucking in a bunch of oxygen rich air and also now has a non-negative crankcase pressure. This affects how the motor runs. On a car with a MAF (we don't have one) you will be drawing air into the intake behind the MAF sensor - causing the vehicle to suddenly run very lean, which makes the idle jump or the car stall.

If you're interested in why negative crankcase pressure promotes more power there are a lot of good race engine article floating around. There are upsides and downsides to this, especially on a street car (dealing with wear).

Thank you very much!