What does it mean when...

[e34.535i.sport]

...you remove the oil filler cap whilst the car is running and it has absolutely no effect?!

On my m30 this played havoc with the idle and the car wanted to stall! Car in question now is 1995 Jag XJR s/charged... (AJ16 engine if I'm not mistaken) and removing the filler makes no difference to the car whilst running. All seems well atm with the car, it's using a little more juice than usual (poss due to wheel change and cold weather) but it idles/runs perfectly and plenty of power no problems.

Very curious to know why there's a difference between the two engines and what this implies!

[whiskychaser]

All post '60s cars cars have to have a PCV system and it runs at a little less than atmospheric pressure. (Its so blow-by and oil vapour out of the sump get 'sucked' back into the engine and burned. I had 1967 MKII Cortina. It just had a pipe coming out and dumped it on the road.). So when you take the oil filler cap off you are introducing air where its not designed to be. I know yours has a blower on it but I bet you still have a PCV valve. Have you tried the plastic carrier bag and rubber band trick?

Apologies for the length of the post and the trip down memory lane

PS I think you have an EGR valve too. Maybe something else to clean?:
http://www.jaguarforum.co.uk/viewtopic.php?t=13239

[repenttokyo]

has zero effect on my M50. When I do it on my Miata it sprays oil all over the engine bay.

[BennyM]

My understanding is that removing the oil cap with the engine running introduces a vacuum leak, just like pulling off a vacuum hose. When I've tried it on my M50, nothing seemed to change. It will lightly suck a plastic bag in, though. So, there's at least some negative pressure.

[632 Regal]

if there is negative pressure good, if no negative pressure bad. Perhaps somehow the computer can see you messing with it? If not I would guess there may be a vacumme leak somewhere.

[Rus]

Not all cars have a vacuum in the crankcase at idle (or at other engine speeds). My 535i has vacuum while my Scion tC (toyota 2.4L engine) actually blows air out of the filler opening. I've checked this on the same engine in my dad's Camry, and it does the same thing. No change in engine operation either. I wouldn't be alarmed if I were you. The fact that the car is using more fuel could be due to winter fuel and cold weather. Just my $.02

[Mr._Graybeard]

...you remove the oil filler cap whilst the car is running and it has absolutely no effect?!

On my m30 this played havoc with the idle and the car wanted to stall! Car in question now is 1995 Jag XJR s/charged... (AJ16 engine if I'm not mistaken) and removing the filler makes no difference to the car whilst running. All seems well atm with the car, it's using a little more juice than usual (poss due to wheel change and cold weather) but it idles/runs perfectly and plenty of power no problems.

Very curious to know why there's a difference between the two engines and what this implies!

Since your Jaguar is supercharged, it does not have negative intake manifold pressure like an n/a engine. I'd suspect the crankcase breather vents to the intake upstream from the blower and air/fuel meter, which wouldn't have any impact on the metered air flowing to the cylinders.

[e34.535i.sport]

Hmmm thanks Mr G that makes me feel a bit better about the whole thing! Still might give this plastic bag thing a go though, sounds interesting!

EGR valve - Sounds cool I will have a mooch around, didn't notice anything that looked like that when I replaced that evil octopus hose but if I have one I'm defo giving it a clean! Think the dude on that thread has an '04 car so coyuld be a more recent introduction... Really need to get myself a repair manual for this thing it'd make life a lot easier!