MK2 GTI 8V idle speed drops

[PIPKIN]

Hi all

Just a quick question when I remove the oil filler cap when the engine is running the revs drop.

Is this normal and if not what could be the problem.  If this is a problem how can I identify the specific concern.

Cheers

Paul

[Khare]

It's normal. Engines run on a pressurized system, if you let the pressure out, they don't like it 

[rubjonny]

vacuumised

[Khare]

vacuumised

dam! I was going to use that rather than pressurized but I didn't know which one, so I took a gamble! 

[rubjonny]

if it was a turbo you would have been right 

[Khare]

Ahhhh, NA runs on a vacuum, turbo on a pressurized system. Got it

[rubjonny]

aye engine is in effect a rather big vacuum pump, as the cyls move down they suck air and fuel in so the intake is under vacuum. turbo puts the inlet under +ve pressure to cram even more air in there

[Khare]

Makes sense 

Top man as usual RJ

[PIPKIN]

Cheers for the pointers and assuring me that a bill is not on the way.

Paul

[javalin]

worth pointing out that the crankcase on both turbo and non-turbo shouldn't be pressurised, but under a slight vacuum.  The air intake is pressurised on a turbo though (boost) and slight vacumm on a non-turbo (upto the throttle butterfly of course).

J