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Old 08-09-2010, 08:33 AM
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sbc engine blow by

i'm trying to figure out a problem a friend has.. first we did a dry cyl pressure test and all cyl's are in the 150 to 155 psi range.. its a sbc400 bored 40 over and mostly stock from there. problem is engine internal area is being pressurized and blowing out valve cover filters and dip stick.. has a pcv on one side and filter on other.. he mentioned to me the other nite he went for gas (3 miles) and at the station he noticed a small puddle under the dip stick on the ground and frame is covered... any ideas ????

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Old 08-09-2010, 08:40 AM
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How many quarts of oil did he put in it to start with? It may be overfilled.
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Old 08-09-2010, 12:24 PM
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blown'n oil

well cobolt thats a good question that i don't know.. i do know when he changed the oil he mentioned there was only 2 qts left.. its a wonder he didn't smoke test the motor. so even if he over filled it, it shouldn't blow at least3 qts out..
i'm ck'd the returns on the heads and see no problem there.. thats without pulling anything off except the valve covers..
bill
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Old 08-09-2010, 12:43 PM
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is it a filter on the valve cover or a oil filler cap
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Old 08-09-2010, 12:58 PM
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Quote:
Originally Posted by 27 t
is it a filter on the valve cover or a oil filler cap
the reason i ask is i had this problem at one time.. i was running a pcv with an oil fill cap. the eng would build up presser and blow the dip stick out. all so leak out of what ever orifice was week enough to push oil out.. or even if the breather was plugged. maybe he forgot to cut a hole in the grommet some are sealed and need to be cut open...
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Old 08-09-2010, 06:36 PM
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Quote:
Originally Posted by delawarebill
i'm trying to figure out a problem a friend has.. first we did a dry cyl pressure test and all cyl's are in the 150 to 155 psi range.. its a sbc400 bored 40 over and mostly stock from there. problem is engine internal area is being pressurized and blowing out valve cover filters and dip stick.. has a pcv on one side and filter on other.. he mentioned to me the other nite he went for gas (3 miles) and at the station he noticed a small puddle under the dip stick on the ground and frame is covered... any ideas ????
If it's done this since it was built it's a good bet some or all the rings are in upside down. Will hold good cranking compression but goes nuts when running.

Another good one I've seen is over-bored to one size and running a smaller than that bore piston, this really upsets the ring seal but it'll rev like crazy and holds good cranking compression.

You can also consider a cracked cylinder wall or piston, or wall flex; the 400 doesn't take too kindly to over-boring and unfortunately if you have a core shifted block oh-forty could be at the problem stage.

Bogie
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Old 08-09-2010, 06:57 PM
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Do a leak down test at TDC as a compression is not a good idicator of ring seal but a leak down test is.
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Old 08-10-2010, 06:52 AM
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will pass on the info

he did call and mentioned he put new covers on with baffles and 1 filter and 1 pcv.. said its much better but not cured..
from his statement to me this tells me alot.. some of this i didn't know... no baffles???? not good... guess i'll need to go up there and see for myself what he's doing... as far as the rings, the motor was pro built about 8 yrs ago.. he finished the car last yr so its been sitting way too long for me...
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Old 08-10-2010, 03:49 PM
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Quote:
Originally Posted by delawarebill
he did call and mentioned he put new covers on with baffles and 1 filter and 1 pcv.. said its much better but not cured..
from his statement to me this tells me alot.. some of this i didn't know... no baffles???? not good... guess i'll need to go up there and see for myself what he's doing... as far as the rings, the motor was pro built about 8 yrs ago.. he finished the car last yr so its been sitting way too long for me...
Yeah that's a long time to leave an engine sit, especially if your not in the desert southwest and the engine is sealed in that dry environment to keep sand and critters out. Metals trade electrons especially given a little moisture in the air. Some years ago (OK, really decades) the Navy did a study to find why spares pulled from storage had almost the same failure rate as equipment in use across the same time span. Most stuff being full of dissimilar metals which like to trade electrons, it was found that in storage especially where the humidity was not kept extremely low, that unused parts and equipment corroded at close to the same rate as in-service parts and equipment over the same time period. So one can take this to an engine where aluminum is in contact with cast iron and steels of various types. Toss in a little humidity and you've got corrosion taking place which changes the chemical composition of the machined surfaces leaving pits where material is eroded out and that eroded material forms grit that gets caught between moving parts sanding the interface. Next thing you know super smooth surfaces are being randomly reshaped by this process and for an engine this means oil and compression gets around the rings. Also seals harden with time, so valve stem seals no longer do the job as well as they should and crank seals become weepy especially when helped by a blow-by pressurized crankcase.

Baffles are important on the vents leading out to the atmosphere or to a PCV valve. They provide a place to slow the escaping gasses which entrain a lot of oil. Slowing the velocity gives a chance for the oil to condense and fall out while the baffles provide a place for it grab and form droplets large enough to fall back into the engine rather than be carried out with the exiting air stream.

Bogie
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Old 08-10-2010, 11:46 PM
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my 67 chevy truck with 327 does the same thing. only when you run the hell out of it. park it and open the hood and see oil.
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