Blowby?
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Cargo Master
Joined: Jul 2010
Posts: 2,099
Likes: 8
From: York, Pa
Blowby?
I have a 390 that likes to put a little bit of oil residue out of the fill cap. It wasn't connected to the aircleaner but is now and still puts a tiny bit in there. The Pcv valve does appear to work by sound. The motor runs great plenty of power no smoke and starts really easy. It has around 70k orig miles. Is this too much blowby? Or am I missing some other type of crankcase ventilation?
FTE Leadership Emeritus
Joined: Jul 2002
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From: Long Island USA
What kind of oil are you using? Synthetics are known to be "volatile" - meaning some of it evaporates. It hits the cooler air, and condenses, leaving slime all over 
If you put your thumb over the PCV valve, does it pull hard?
If you put your thumb over the PCV valve, does it pull hard?
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Cargo Master
Joined: Jul 2010
Posts: 2,099
Likes: 8
From: York, Pa
We are just using regular old conventional oil. I haven't tried putting my finger over the pcv. I did shake it though and it does rattle. The 390 we have in the 74 never puts any oil on the valve cover. I'm just confused I dont believe this motor is wore out it runs way to good.
Thanks
Thanks
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Cargo Master
Joined: Jul 2010
Posts: 2,099
Likes: 8
From: York, Pa
I think it does. Its a 1968 motor but we swapped the valve covers with '65 valve covers to make it look right with the truck its in. They are the older pent roof style I believe they are called. I'm sure they do have a baffle in them.
I guess I really cant compare with the '74 since it has a set of the tall orig "cobra le mans" valve covers. Being taller maybe they help keep the splashing in.
I guess I really cant compare with the '74 since it has a set of the tall orig "cobra le mans" valve covers. Being taller maybe they help keep the splashing in.
FTE Leadership Emeritus
Joined: Jul 2002
Posts: 42,561
Likes: 421
From: Long Island USA
Just remove the breather and see if the baffle is in there... you never know if it was removed or fell off ...
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Cargo Master
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From: Spokane, Wa.
My 65' 390 does the same thing. I suspected blow by as well since it has 114,000 miles on it and never been rebuilt in 45 years. Anyways, when I did a compression test in anticipation of a bent valve I came up with low numbers and a lot of variation. Then I did a second test with just a little oil on top of the pistons to seal the rings and got much higher and more consistent numbers. The difference between test 1 and test 2 will tell you how much blow by you have. If it doesn't change much then your rings are good and the splashing is from something else. I hope this helps
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