Air coming out of valuve cover oil filler cap

If the spark plug is oil fouled, you either have leaking valve seals, or bad piston rings. Bad rings would be accompanied by excessive blow-by. With the engine running, remove your oil filler cap. If there is air pulsing out of the filler tube, it your rings. If not, it is your valve seals.

Check your PCV valve. Its the device that provides ventilation to the crankcase and valve covers. If its clogged or non functional it will cause the engine to **** or blow air from an area with less resistance, which is probably the filler cap. As its blowing air out of the engine it will shoot a little bit of oil with it

Normal, maybe. Since they started putting the engine case vacuum in the air filter plenum. Yes it happens. Many different cars today will shut down soon if the oil cap is not on. It has to do with the Mass air flow sensor.

Remove oil filler cap which is normally on top of the rocker cover look at underside of cap, the side that goes into rocker cover, and if it is covererd in a white oily substance or if the tube that the filler cap goes into has the white gunge in it yo have oil and water / coolant mixing in your engine

Your question did not specify what "filler cap".
The above response from FTjoe is for the power steering filler cap.

The following RealOEM link is for the engine oil filler cap (Item 4) which is square and black and is on top of the engine. It goes through the plastic cover into the cylinder cover
http://www.realoem.com/bmw/showparts.do?model=ET37&mospid=47720&btnr=11_2172&hg=11&fg=15


The engine oil filter housing cap (Item 3) is shown below:
http://www.realoem.com/bmw/partgrp.do?model=ET37&mospid=47720&hg=11&fg=30

Well -doing that sort of takes out the PCV System
doesn't it

Unmetered air comes in the oil fill hole & gets drawn into
the intake & leans out the fuel mix

You also got PCV system issues with high vacuum

yeah, it's supposed to do this, so keep the cap on when the engine is running

make sure that the filler cap seal is in place and not gone hard over time also has the rocker cover been off at some stage if it has check that the gasket has not moved the other thing to look out for is that the PCV (breather )valve is in good working order if it is damaged the sump compression will rise and push oil out

normally fumes come in to vehicle when stopped because of oil leak from rocker cover but check your fuel filler cap seal etc and also incase the tank is leaking just have a sniff at the rear of the car if its the filler cap or tank filler you normally get smell when tank is nearly full and been around a few corners the fuell filler nect becomes pourous and just spills out a bit if so will need replacing .but most time its an oil leak from rocker box or engine

Find the air intake pipework that runs across the front of the engine and turns towards the rear of the vehicle and terminates in the air filter housing. loosen the fixings on this intake pipework and remove it completely, remove the oil filler cap and plug the hole with a rag to prevent contamination, remove the cover that had the oil filler cap mounted on, this involves the removal of 3 allan screws that are easily visable (more or less in line with the oil filler) then remove another 2 a;;an screws holding the rear of this cover these can be difficult to access as they are close to the bodywork below the windshield, this cover should now lift clear to allow access to the air filter, refitting is just a reversal of the above procedure, it is a good idea to remove any loose dirt before refitting the new filter by using a vacuum cleaner.