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*lakeroadster* · 09-12-2011, 06:27 PM

You asked, What is "Blow By":

If your rings are worn on an older engine, or if they have not yet seated on a new engine, some of the compression leaks passed the rings and pressurizes the crankcase. This pressure relieves itself by breathing out through the "breathers" on the valve covers.

If you replace the breathers on the valve covers with a pcv system, and the pcv system can't keep up with the pressure in your crankcase, then the pressure relieves itself by blowing out wherever is the weakest link. Which means things like valve cover gaskets, fuel pump gaskets, etc.

I learned this many years ago the hard way on a "newly" rebuilt SBC. It ran fine until the first time I rev'd the motor really good, which was after about 300 miles. It blew the gaskets out of the Holley manual fuel pump, blowing oil all over the headers..... I installed a breather in (1) valve cover and added a pcv valve. That fixed the problem in my case.

If the blow by is severe, and is due to badly worn rings, your only real option is to rebuild the motor.

Hope that helps.

09-12-2011, 06:27 PM	#4
lakeroadster Account Suspended Join Date: Sep 2009 Location: High Plains of Colorado Posts: 2,485	Re: PCV problems You asked, What is "Blow By": If your rings are worn on an older engine, or if they have not yet seated on a new engine, some of the compression leaks passed the rings and pressurizes the crankcase. This pressure relieves itself by breathing out through the "breathers" on the valve covers. If you replace the breathers on the valve covers with a pcv system, and the pcv system can't keep up with the pressure in your crankcase, then the pressure relieves itself by blowing out wherever is the weakest link. Which means things like valve cover gaskets, fuel pump gaskets, etc. I learned this many years ago the hard way on a "newly" rebuilt SBC. It ran fine until the first time I rev'd the motor really good, which was after about 300 miles. It blew the gaskets out of the Holley manual fuel pump, blowing oil all over the headers..... I installed a breather in (1) valve cover and added a pcv valve. That fixed the problem in my case. If the blow by is severe, and is due to badly worn rings, your only real option is to rebuild the motor. Hope that helps. Last edited by lakeroadster; 09-12-2011 at 06:36 PM.