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Old 07-22-2020, 11:29 AM   #1
shinzon
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Default Boost From Dipstick

I was doing a boost leak test today on my 93 940T and I had a major leak from the dipstick. If I remove the vacuum hose that connects to the PCV system from the intake manifold, the leak from the dripstick goes away.

Problem is, I just replaced every piece of the PCV system with new parts last week... Did i get an oil separator with a bad check valve or something?

As a bonus, the only other leak the system seems to have is from the charge bypass valve on the turbo. Is this a bad diaphragm? I don't think there's a gasket that goes between the turbo and the CBV...
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Old 07-22-2020, 01:40 PM   #2
dl242gt
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Have you checked the fitting on the intake manifold for being clogged? Probe it with a piece of wire and you can spray some intake cleaner through it.

The cbv diaphragm can fail causing large intake leak. It also bypasses boost and boost response is worse if that fails. You can buy separate parts fo fix the cbv.
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Old 07-22-2020, 03:04 PM   #3
shinzon
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Quote:
Originally Posted by dl242gt View Post
Have you checked the fitting on the intake manifold for being clogged? Probe it with a piece of wire and you can spray some intake cleaner through it.

The cbv diaphragm can fail causing large intake leak. It also bypasses boost and boost response is worse if that fails. You can buy separate parts fo fix the cbv.
Thanks for the reply.

I actually had the intake manifold off the car when I put a head gasket on it recently and cleared all the ports on the intake manifold. It WAS clogged--two of them were actually--but not anymore.

Is it supposed to be like this? Is there a check-valve/PVC valve in the oil separator on these cars? Thinking about it, any boost pressure on that line will be sucked back to the pre-turbo intake pipe by the larger hose. Positive pressure being pumped into the lines while the turbo isn't sucking is something that'd never happen during normal operation.

I took the CBV apart and the diaphragm was in good condition. I added a thin layer of copper high-temp silicone round the edge of the diaphragm and that seems to have fixed the leak.
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Old 07-22-2020, 03:28 PM   #4
2manyturbos
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To answer your actual question, no, there is no check valve. Any pressure the crankcase receives should be going out of the large hose on the separator box to the intake tract.
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Old 07-22-2020, 06:55 PM   #5
shinzon
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Originally Posted by 2manyturbos View Post
To answer your actual question, no, there is no check valve. Any pressure the crankcase receives should be going out of the large hose on the separator box to the intake tract.
So nothing at all to prevent boost from entering the crankcase?
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Old 07-22-2020, 08:52 PM   #6
philski o'flood
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boost from dipstick ?
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Old 07-23-2020, 02:43 PM   #7
ZVOLV
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Do a cylinder leakdown test.
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Old 07-25-2020, 11:20 AM   #8
stove
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Head gasket replaced because engine overheated?

If yes, sounds like collapsed rings, which is pretty much what always happens when an engine gets overheated.

Last edited by stove; 07-25-2020 at 11:52 AM..
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Old 07-25-2020, 12:22 PM   #9
DET17
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Originally Posted by shinzon View Post
So nothing at all to prevent boost from entering the crankcase?
The inlet side to the turbo has atmospheric pressure at best. This connects to the inlet of the compressor; hence no positive pressure.

The CBV tip is a good one; that side is pressurized with boost. You can isolate the hoses one by one (to your intake) and see if your dipstick stays in the tube for a quick test drive. Divide and conquer.

Worst case: you have excessive blowby past your rings..... and likely won't be able to stop this issue without new piston rings.... a hone, and perhaps an overbore depending on the amount of belling in your block. Miles on this engine?
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