What is Blow-By?
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Junior User
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What is Blow-By?
I recently bought an '82 F-150 Lariat 4WD with a 300 Straight Six under the hood. The guy I bought it from said the only reason that he was selling it was because of the "blow-by". He has a garden hose running from the top of the motor where you put the oil in over to front of the truck an out through the grill. After bringing home the truck I fired it right up! After running a few minutes sure enough the was blue smoke coming out of the hose. After playing around with truck I noticed it still has plenty of power and everything seems to be just fine. There are no weird or strange noises so my questions are - What exactly is blow-by and how much is it going to cost to fix?
Thanks,
Chris
Thanks,
Chris
Laughing Gas
Joined: Feb 2004
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blowby is caused by worn rings and cylinders.on the compression stroke gases blow by the worn parts into the base.if it gets bad enough it will start to blow oil out of the engine.the only way to fix it is a new engine or have yours bored and rebuilt.engines can last a long time like this.i had some i drove for years.
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From: houston
Everything 312 said is correct. However, the first thing I would check is the PCV system. Be sure it is functioning and that all the hoses and connections are in good order. If you don't already have a manual for your truck, get one. All engines, even brand new ones, have blow-by. Naturally there is only a small amount when new and it increases as the engine wears. A working PCV can handle a considerable amount of blow-by. Usually, if an engine has enough blow-by to over come the PCV, it will also show other signs of extreem wear. Most common would be high oil consumption, generally poor running and low power. You indicate that it runs good with good power. Good luck.
Fleet Owner
Joined: Sep 2005
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From: Lake Tahoe, Nevada
Originally Posted by srercrcr
fyi... the pcv routes the blowby into the air cleaner so it can run through the combustion process.
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I stand corrected. Thanks.
That makes me wonder why the crankcase fumes are directed to the air cleaner to be eventually burned, yet the fumes coming from the PCV are routed to the intake manifold to be eventually burned as well.
That makes me wonder why the crankcase fumes are directed to the air cleaner to be eventually burned, yet the fumes coming from the PCV are routed to the intake manifold to be eventually burned as well.
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Logistics Pro
Joined: Jan 2002
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From: suburban atlanta
Crankcase fumes *ARE* routed thru the PCV into the intake for burning. The connection to the air cleaner is for supplying air to the crankcase, flowing thru the PCV, to the intake. The PCV stands for Positive Crankcase Ventilation. Fresh air routed thru the system allows the engine a metered amount of air, known to the engine designers, that is regulated by the vacuum level in the intake manifold. This fresh air is supplied via the air cleaner.. keeps out bugs, you know.. and generally comes in on one end or side of the rocker/cam cover. The 'other end' of the system is the PCV, connected to the intake manifold. So clean air comes in one end, and blow by and any other vapors are routed thru the PCV for combustion.
tom
tom
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From: suburban atlanta
Back to the Big-Six. If the PCV valve is clogged or the lines are full of muck, you will get blowby pressure inside the crankcase. To find out the condition of the engine, you should do a leakdown test. This will tell you if the rings are leaky, or the valves not seating well. Some blowby is normal, but if the ventilation system is not working, the small amount normally present can appear magnified. If it doesn't burn oil, and runs well, and gets reasonable gas mileage, live with it... If, your state requires emissions testing, you may be in trouble, but you could hook up the vent for testing purposes.
tom
tom
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