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Old 03-06-2012, 09:01 AM
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Excessive pressure in valve covers

I have a 350 sbc that I completely rebuilt that has 305 heads on it. I have been noticing the valve cover screws (center bolt) have been come loose quite often. Yesterday I was out driving the truck and was getting into it and noticed smoke behind me. Pulled off the side of the road to notice the 2 center valve cover bolts have come about an 1" out and spued oil everywhere. So I tightened them back down then about 100' down the road my breather blew off rubber gromet and all. The driver side of the engine have a PCV valve and the pass. side is the one with the straight open breather. With nothing on either side at idle i have a constant stream of air oil vapor mix coming out of the valve covers, and even more so at higher rpms. I have good compression on the cylinders I have tested (150 psi). So I dont know it is my valves not seating all the way to let exhaust in or if it have to do with the 305 heads I have on it? Can anyone please help I am completely stumped!!!

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Old 03-06-2012, 09:20 AM
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sounds like you have your engine plugged up and not venting, do you have the PVC and Breather system Installed? Other than that, blow by from the rings is the only way you will pressurize the crancase, windage presurises it some though.
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Old 03-06-2012, 09:39 AM
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Did you buy the generic PCV or breather grommets that you have to cut the bottom out to let anything thru?
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Old 03-06-2012, 10:26 AM
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Yes one valve cover has a PCV valve through the carb and the other side has a breather on it...even with nothing at all, just open holes in the valve covers, I have a constant stream of air/oil vapor coming out of the covers with pretty good pressure behind it.
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Old 03-06-2012, 11:08 AM
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Quote:
Originally Posted by chevy355ss
I have a 350 sbc that I completely rebuilt that has 305 heads on it. I have been noticing the valve cover screws (center bolt) have been come loose quite often. Yesterday I was out driving the truck and was getting into it and noticed smoke behind me. Pulled off the side of the road to notice the 2 center valve cover bolts have come about an 1" out and spued oil everywhere. So I tightened them back down then about 100' down the road my breather blew off rubber gromet and all. The driver side of the engine have a PCV valve and the pass. side is the one with the straight open breather. With nothing on either side at idle i have a constant stream of air oil vapor mix coming out of the valve covers, and even more so at higher rpms. I have good compression on the cylinders I have tested (150 psi). So I dont know it is my valves not seating all the way to let exhaust in or if it have to do with the 305 heads I have on it? Can anyone please help I am completely stumped!!!
To start with oil consumption, blow by, and compression pressure are separate issues. As an engine ages they get rather conflated but when new there are specific problems that each relates to.

But before we tear things open a reminder that there is always blow by hopefully less when the engine is fresh compared to aged but its still there. If the crankcase isn't venting the blow by pressure will climb till it makes a vent.

You need to start at the breathers to be sure that they are open, many start with closed passages that need to be opened up, the same with grommets, there are versions that are molded closed and need to be opened when put into use.

The center bolt valve covers may have oil separator baffles under the breather holes, these need to be checked to see that they will pass blow-by gasses.

The PCV circuit needs to be checked to see that the receiver port at the intake does draw a vacuum. The valve needs to be checked for installation direction, and sizing to the engine displacement and cam timing.

Technically both sides of the venting are supposed to be closed, this means the PCV valve connects a rocker cover to the intake manifold by way of a PCV valve. On the other side the cover should be hsed to a connection inside the air-cleaner. This way there is no way for crankcase vapors to escape to the atmosphere.

There is always the possibility that assembly damage or improper ring installation can be the cause of excessive blow by. A broken ring or a ring installed upside down or out of order between the top and second, or lack of sufficient gap, even the use of the wrong size ring for the overbore dimension are all things I've seen over the years. But these require some deep diving into the engine so the best thing is to start with the outside stuff first to be sure the vents are functioning.

Bogie
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