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.
Friday, April 6th, 2018 AT 2:30 PM