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Tractor Talk Discussion Forum

What is blow-by ?

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KYfarm

01-12-2006 10:57:52




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Technically what is happening when a diesel motor has "blow-by" and what are the signs that show that this is happening? Thanks.




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CRAIG LOEWER

01-15-2006 02:06:34




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 Re: What is blow-by ? in reply to KYfarm, 01-12-2006 10:57:52  
I have an old hatz z 788,and was experiencing blowby out of the crank case breather tube. it was blowing exhaust and some engine oil. I removed the head, and discovered that the rings on one of the 2 cylinders where in line with each other, the grooves in the rings were not staggered.I placed ring grooves 45 degrees from each other and put engine back together. fired it up and no blue smoke or engine blowby.

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clueless

01-13-2006 10:10:32




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 Re: What is blow-by ? in reply to KYfarm, 01-12-2006 10:57:52  
Technically speaking:in good-shape engines the pistons/rings fit well in the cylinders, sealing fuel-explosion power atop the piston(thereby driving it down with maximum power) and lube oil out of the combustion chamber; when pistons/rings/cyl walls wear space is left where fuel-explosion "blows-by"(down past the piston into the lower engine)resulting in less engine power and oil getting up into the combustion chamber. Sorry if this sounds pedantic.

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Leroy

01-13-2006 06:00:32




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 Re: What is blow-by ? in reply to KYfarm, 01-12-2006 10:57:52  
You don't have to be limited to a diesel to have blowby, all internal combustion engines either have it or will have it when they get a little wear



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Didnt Know

01-12-2006 20:19:31




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 Re: What is blow-by ? in reply to KYfarm, 01-12-2006 10:57:52  
Cripes. I always though Blow By was when you were sweezing yer cheeks to keep it in but it just happened to Blow By.
Usually followed by the " Not me look" As others cough and look at the bottom of shoes to make sure they didnt drag in a bit of dog dooo on the way in.



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Bill(Wis)

01-12-2006 19:34:33




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 Re: What is blow-by ? in reply to KYfarm, 01-12-2006 10:57:52  
Blowby usually refers to partially burned diesel fuel getting by the rings into the crankcase. There is probably not a diesel out there that doesn't do that to some degree due to relatively high compression ratios in diesel engines. That's basically what causes the crankcase oil to blacken. When it gets really bad it is usually accompanied by hard starting, power loss, etc. There is another type of blowby which happens when crankcase oil is sucked up into the intake. Probably caused by overfilling of crankcase or foaming and then delivered through crankcase ventilation tube to air cleaner (PCV) system. That, in theory, can cause the infamous "runaway diesel" phenonomen where the engine can reach very high RPMs totally uncommanded. Better refer to the real experts for anymore info.

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Unimog

01-12-2006 13:49:29




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 Re: What is blow-by ? in reply to KYfarm, 01-12-2006 10:57:52  
Blow by is what happens when I take my Unimog out
on the street. Everybody blows right by me. I got
passed by a Divco milk truck the other day, it was
embarassing to say the least.



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old

01-12-2006 11:22:13




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 Re: What is blow-by ? in reply to KYfarm, 01-12-2006 10:57:52  
Blow by is from bad rings most of the time, can be valves also but not going there at this time. One way to tell if you have blow by it to remove the oil filled cap and or oil breather and hold you hoand over it. If you get a lot of blow, push of your hand away from the oil luiller tube breather then you probably have blow by and may need new rings/rebuild of the engine. Simple way theres a lot more to it then just that.

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