Oil filter cap blew off

JAACKMC3006

MEMBER
2004 BMW 520
2.2L
6 CYL
2WD
MANUAL
220,000 MILES

I just brought the car listed above 520i. The previous owner said he was coming onto the motor way when the oil filter housing cap blew off. He then got a new cap and had the CCV changed but he said he did not bother getting the hoses done with it. As soon as he started it blew off another brand new cap. I have now ordered a new CCV and hoses that I am going to fit I also got a new oil filter housing cap, but if it blows off again what else could be causing it?

1 Reply

STEVE W.

MECHANIC
13,463 POSTS

Most common reason would be excessive blow-by past the piston rings. Easiest way to check would be to do a leak down test and listen to the amount of air you hear at the cap.
You will hear a small hiss normally as no rings seal perfectly but if you check one and hear a light hiss then go to the next and hear hiss markedly louder it is likely the rings are worn in excess of normal or a ring has broken.

I would also check the housing itself for worn or damaged threads or a damaged housing. Either could cause an issue if they do not hold the cap securely. If you have the old cap you could screw it in and see if it actually torques down properly. Look at the threads very close as well, are they nice and crisp or bent/folded in a way that may show a cause. Maybe even take a close up focused shot of the housing and cap and post it on here to see if it shows something.