Valve cover building up excessive vacuum on 97 318ti

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

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

Hi, all you need to do is replace the engine breather valve on the intake manifold and the problem should be sorted.

I doubt the oil is boiling in the sump or your engine would show signs of over heating. Change the PCV valve in the valve cover as it may be stuck closed causing internal pressure to build up and escape through the filler cap.

why do you need to remove the filler cap when the car is running???
There is a seal around the distributor. Mark where the distributor is and remove it. Then replace the seal. Put it back together in the same location as before you removed it. It is also possible that your valve cover gasket is leaking and dripping on the distributor.
Cooling problem: Try washing the outside of the radiator where the cooling fins are. Spray it down with a really good degreaser and rinse it off. WARNING: Do NOT use high pressure because you will need a new rad after.

have you had the alternator or fan belt checked as they will squeel when they are loose and you accelerate.
This may not be the problem but it is a cheap fit if it is the problem.
hope this has helped

Make sure your PCV is not restricted causing pressure to build up in your crank case. thts what this sounds like. somethign is causing you to build up pressure.

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.

Make sure the pcv valve is working correctly and allowing the engine to breath