Leaking Boost Through Oil Cap

[blueman803]

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?

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

 

[Sharkcus]

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]

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

 

[habitatguy187]

I'm having the same problem, however I haven't done a boost leak test recently it's just that my oil cap keeps leaking (and it's extremely tight) The pcv line has a 80psi check valve in it, and my car seems to run bad until it's FULLY warmed up.

I'm suspecting (and hoping) intake valve seals, but I need help figuring it out. I read that if you do a boost leak test and air's coming out of the VC breather tube that it has to be intake valve seals, anyone care to comment/add to this? Thanks

 

[JusMX141]

It's probably valve seals, but a compression test will definitely tell the tale. If your compression checks out OK, then I'd suspect valve seals.

This is the reason I normally cap the intake manifold, remove the PCV & replace it with a 5/16" hose nipple, then vent both nipples on the valve cover freely to a catch can on older cars running higher boost levels.

 

[habitatguy187]

It's probably valve seals, but a compression test will definitely tell the tale. If your compression checks out OK, then I'd suspect valve seals.

Compression was 139, 142, 142, 145. A mitsu mechanic friend of mine suggested the turbo since my car doesn't smoke much, and only smokes under heavy acceleration. (my wife said she saw it smoke but didn't notice what the color was, may be running rich for all i know )

This is the reason I normally cap the intake manifold, remove the PCV, and vent the valve cover freely to a catch can on older cars running higher boost levels.

Even so don't leaking valve seals constitute a boost leak? Wouldn't that be just a band-aid?

 

[JusMX141]

A mitsu mechanic friend of mine suggested the turbo since my car doesn't smoke much, and only smokes under heavy acceleration.

Could be the turbine seal leaking if the turbo's still original. Tons of hot shutdowns may be taking their toll.

Even so don't leaking valve seals constitute a boost leak? Wouldn't that be just a band-aid?

Sure it's a bandaid, but it's also preventing other issues as increased crankcase pressure will not allow the turbo to drain properly. It's a good fix for someone with a good-running car who doesn't want to pull it down to replace the valve seals.

 

[habitatguy187]

Could be the turbine seal leaking if the turbo's still original. Tons of hot shutdowns may be taking their toll.

To test this I do a BLT from the turbo and disconnect any IC pipe correct? And if air comes out of my VC breather hose then it's turbo seals right? And yes it is the original 14b for all I know with 130k on it.

Sure it's a bandaid, but it's also preventing other issues as increased crankcase pressure will not allow the turbo to drain properly. It's a good fix for someone with a good-running car who doesn't want to pull it down to replace the valve seals.

I'm going to look into this since I've never actually seen a catch-can setup, but I suppose that both lines would go to the catch can and then one line out of the catchcan goes to the intake pipe? I just thought it was bad to disconnect the pcv because it pulls gases out of the crankcase. I'll look it up. Thanks for your help.

 

[JusMX141]

I'm going to look into this since I've never actually seen a catch-can setup, but I suppose that both lines would go to the catch can and then one line out of the catchcan goes to the intake pipe?

I just run them to a catch can with a breather filter on top, although I'm sure you can run that line out of the top back into the intake pipe if you prefer. I like to keep as much oil out of the turbocharger and intercooler piping as possible.

You must be logged in to view this image or video.

 

[habitatguy187]

Don't know if this would matter but my turbo has absolutely zero shaft play, nothing, just thought I'd throw that out there.