Blow-by Blues

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May 6, 2005 | 11:50 AM

albatrosss

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Blow-by Blues

I am experiencing a significant amount of blow-by in my recently rebuilt 460 (long block from a rebuilder). I know that blow-by can be caused by faulty rings, are there any other possible causes for blow-by. The engine was running great then in one day the smoke appeared from the breather.

Please help, the manufacturer is giving me a hassle about the warranty claim.

May 6, 2005 | 12:06 PM

Bear 45/70

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From: Union, Washington

A bad PCV system can cause it.

May 6, 2005 | 06:47 PM

albatrosss

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Is there a test to specifically determine blow-by, or exclude the pcv system?

The compression test shows normal pressures (125-145 lbs)

May 7, 2005 | 12:45 AM

Bear 45/70

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From: Union, Washington

If you pull the PVC valve out of the valve cover grommet with the engine running. You should hear it sucking air. Put your thumb over the inlet of the PVC and the engine RPMs should drop and vacuum should hold your thumb to the valve. Shut the engine off and shake the valve, it should rattle. Only test I know, if in doubt change it the are cheap. Your case the RPM drop test is your primary concern. One other thing, with engine running remove the beather for the other valve cover, if engine speeds up you have plugged breather filter in the air cleaner.

May 7, 2005 | 08:38 AM

greenhighboy

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From: Missouri

Worn valve guides cause a lot of blowby also. I have had a 390 and a 460 have severe blowby and both motors had bad guides. I rebuilt a Marshall long block a few years ago and both heads on it were junk.

May 7, 2005 | 10:00 AM

albatrosss

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Is there a test for worn valve guides that would rule our the rings, or the guides?

May 7, 2005 | 11:07 AM

Bear 45/70

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From: Union, Washington

No test for the valve guides, you can check rings with a compression test, first dry compression test and then with some oil squirted into the cylinders, if compression increases, rings are bad.

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May 9, 2005 | 07:35 AM

DailyDriver

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From: Bakersfield Ca.

Quote:

Originally Posted by Bear 45/70

... first dry compression test and then with some oil squirted into the cylinders, if compression increases, rings are bad.

Not necessarily. Compression will increase even if the rings are good. A large increase=bad, a small increase=good. No increase at all means a bad valve. I don't have my shop manual at hand but there is a percentage of how much compression is increased when adding oil to the cylinder to determine if the rings are bad.

This may be redundant but, a bad PCV valve doesn't cause blow-by. All engines have it, some more than others. The PCV system is designed to suck out, (ventilate) the blow-by gases from the crankcase and direct them back into the engine to be burned. If it's not working then the engine oil will become contaminated and can make the problem worse. Signs of oil coming out from around the breather is a sure sign of either the PCV valve/system not working or the blow-by gasses are overwelming the PCV system. FWIT