Leaking Boost Through Oil Cap

blueman803 · 01-09-2006, 12:11 AM

Alright in the process of doing a boost leak test I took the oil cap off and notice that air is hissing and whistling through there. The intake was pressurized to 20 psi and it would bleed to 0 within 30 seconds. Important things to consider when this test was done.

1)No PCV line to the intake, so a leaky PCV is not an option.
2)Connected off the throttlebody, so turbo oil seals are not suspect.

This leaves two suspects, piston rings and valve seals, correct?

I took the valve cover off so I could see when each valve is lifted. Regardless of which cylinder is on the intake stroke, the hissing is still there. This leads to suspecting the rings correct? The compression test returned values all around 195. So basically I have a hissing sound, loosing pressure somewhere, and perfect compression. The engine was just changed to synthetic oil, with 1200 miles on the rebuild. I am hoping its not the rings, since I really would prefer not to do that again. Can bad rings pass a compression test? What can I do to narrow it down?

Does it even matter?

blueman803 · 01-09-2006, 04:46 PM

So is it normal for pressure to leak past the rings or pistons during a boost test? I have no leaks anywhere else.

Sharkcus · 01-11-2006, 08:45 AM

I don't think I ever took my oil cap off when doing a leak down test, but I would assume it would bleed out of the vent on the valve cover. I think it would be more beneficial to do a compression test. If that comes back fine, your rings aren't leaking any more than normal.

I wouldn't worry about it if your compression is fine. I hope that helps.

gmentals · 02-08-2006, 06:00 PM

So did you ever figure out what was happening? I am having the same problem.