vacuum on oil filler cap.

[seany260]

Was doing a service when I noticed there is suction on the oil filler cap when the engine is running. Car rpm's dip and idles erraticly when cap is removed. I've noticed also normal idling is a little low I think, sitting below the 750 mark on the rev counter.
I've removed the breather pipe and the crank breather assembly on the back of the block and both are clear.
Has any one experienced this or is it just normal for a 2006 1.4i.
In my experience engines only have vacuum on the filler cap if there is something wrong with the breather system.
Cheers.

 

[cheshire cat]

don't think you have a problem there should be some vac --when you remove the cap you are effectively weakening the mixture

 

[chrisRibiza]

Totally normal.

The cap needs to be correctly installed or there isn't enough pressure inside the engine.

 

[RUM4MO]

Just a thought for the OP - how do you think the crankcase gases get purged so that they do not escape to atmosphere - correct some air is drawn through the crankcase, and doing that will lead to a slight depression in the crankcase spaces - like the cam chest area!