how to fix/slow down oil "blow-by" - Vintage Mustang Forums
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post #1 of 17 (permalink) Old 05-03-2009, 09:24 PM Thread Starter
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how to fix/slow down oil "blow-by"

Any tips to control "blow-by"? My fastback is spitting oil out the breather and dip stick. It's not a ton, but it's making a mess. PCV is working fine and "clicks" when I squeeze the tube going back into the carb/intake.

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post #2 of 17 (permalink) Old 05-03-2009, 09:29 PM
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I had that problem with a good engine. Found out the 302 I was using had a good, but wrong PCV valve. Got the correct one and all was well. Blowby will be more of a blue smoke as opposed to actual oil being spit out.

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post #3 of 17 (permalink) Old 05-03-2009, 09:40 PM
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Blow-by is caused by combustion "blowing" past the rings. Unless you have bigger problems with your heads, which is less likely, you can pretty much bet it's the rings.

The only way to slow it down would be to thicken your oil, but it will not help much.

The proper thing to do is to do a compression check. The lowest psi/cylinder is the bad one.

The fix will cost you a set of rings, and oil pan gasket, and a head gasket. You could pull the oil pan and head, remove the bottom of the piston, push the piston up and out, replace the rings and reinstall the piston, oil pan and head.

The problem with that is you might as well replace all gaskets and rings while your at it.

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post #4 of 17 (permalink) Old 05-03-2009, 09:45 PM
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how to fix/slow down oil "blow-by"

Quote:
Originally Posted by AzPete
I had that problem with a good engine. Found out the 302 I was using had a good, but wrong PCV valve. Got the correct one and all was well. Blowby will be more of a blue smoke as opposed to actual oil being spit out.
Good blow-by will be a white smoke. Bad blow-by will show itself as oil spitting out the pcv valve.

When rings go bad or get stuck the resultant pressure inside the crankcase will blow oil ot the PCV valve.
All rings allow some combustion by. Bad rings allow to much. The combustion raises the pressure inside the engine a.k.a. crankcase. Hence the name (PCV) Positive Crankcase Ventilation Valve. When pressure inside is ventilated as hot gas either into a filter out to air or through a filter into the carburetor intake.

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post #5 of 17 (permalink) Old 05-03-2009, 09:52 PM
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Re: how to fix/slow down oil "blow-by"

I don't know if the wrong PCV valve could cause oil to spit out the valve. I doubt it though.
good luck.




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post #6 of 17 (permalink) Old 05-03-2009, 10:00 PM
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Re: how to fix/slow down oil "blow-by"

Quote:
Originally Posted by eimo
I don't know if the wrong PCV valve could cause oil to spit out the valve. I doubt it though.
good luck.


The one I had in my last '66 did for sure. After replacing the PCV valve with the proper one, I drove that engine another 20,000 miles without any more oil being forced out the breather or dip stick tube (sold the car). After that change, I did a compression check and all was real good. With a good but wrong PCV valve, it was not operating properly with the engine vacuum available. This caused the engine to build up to much pressure inside.

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post #7 of 17 (permalink) Old 05-03-2009, 10:06 PM
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cool. Lets hope that is his problem.

Wow, the other end of this tunnel is really dark.....

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post #8 of 17 (permalink) Old 05-03-2009, 10:17 PM Thread Starter
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Re: how to fix/slow down oil "blow-by"

Well, I just swapped the valve from my other Mustang, which doesn't have any such problem. It is a different style, so maybe we are on the right track. I guess I'll know when I take it out tomorrow.


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post #9 of 17 (permalink) Old 05-03-2009, 10:21 PM
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The spring in the PCV valve is calibrated for different motors. So you can't just put any one in.

"don't know if the wrong PCV valve could cause oil to spit out the valve"

The complaint was oil blowing out the breather and dipstick.
Oil can't blow 'out' of the PCV valve because it is a closed system.
The PCV will suck gases and oil vapors 'through' the PCV valve,that is what it is supposed to do.

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post #10 of 17 (permalink) Old 05-03-2009, 10:31 PM
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how to fix/slow down oil "blow-by"

YES, The pcv valve uses vacuum created by the engine sucking air through the intake manifold to suck vapors from the crankcase into the cylinders to be burned with the air/fuel mixture.

...if that is what you mean by a closed system. In this case you with bad rings you will see oil spitting in the filter inside the air filter cover.

When he said breather I assumed he meant one of these...



In this case Oil definitely can spit out of this thing when a cylinder has bad blow-by.

Wow, the other end of this tunnel is really dark.....

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post #11 of 17 (permalink) Old 05-03-2009, 10:39 PM
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Try a heavier grade of oil, my 'ol 351C would push out the dipstick if I used anything lighter than 10W40.

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post #12 of 17 (permalink) Old 05-03-2009, 10:39 PM Thread Starter
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Re: how to fix/slow down oil "blow-by"

That's the breather type I have. It does NOT route back to the air cleaner. but has it's own filter. It is NEW.

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post #13 of 17 (permalink) Old 05-03-2009, 10:50 PM
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That is the same cap I had for the breather on the last '66 with the PCV valve change. Hope the switch works for you.

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post #14 of 17 (permalink) Old 05-03-2009, 11:04 PM
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The only way to know for sure is to check your compression.
Maybe a local auto parts will let you rent one. I paid $35 for mine.

Excessive blow by is generally caused by the piston rings not sealing against the cylinder walls

Wow, the other end of this tunnel is really dark.....

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post #15 of 17 (permalink) Old 05-04-2009, 10:55 AM
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I had bad blow-by, amazing what an engine rebuild can fix .
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