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Question on Blowby

L

lrobry01

Member
Main cause's of blowby on gas tractor engines ? No water in oil at all, No oil in Radiator. Starts/runs super but have blowby blue/white color Advise on ways to correct issue.
 
valve guide seals and piston rings come to mind, run a dry and wet compression check, that will likely tell you which one it is, sounds like an overhaul is in order
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Depends on what you think the definition of "blow-by" is. It's supposed to mean the condition when exhaust gasses escape (i.e. "blow by") past the piston during combustion. That means smoke out your vent tube, not the exhaust pipe. The only cause is wear or damage on the cylinder wall, piston, rings, etc.
 
If you're talking about excessive smoke/oil mist blowing out the crankcase vent tube, most likely it's worn rings.

Did this happen over a long period of time, progressively getting worse? If so, time for a rebuild.

If it happened suddenly there is a chance it could be from only one cylinder if something damaged that that cylinder, and a chance it could be a blown head gasket sending combustion gasses into the crankcase. If so, it could be repaired short of a complete rebuild.

Run a compression test, or better yet a cylinder leak down test (remove the rocker assembly, adapt an air line to the spark plug hole, pressure up the cylinders one at a time, see where the air goes). If only one cylinder shows excessive leakage, investigate that one.
 
Contrary to what has been stated....
Valve guides and valve seals do NOT cause blowby. There is only pressure present when the valves are CLOSED. The guides and seals CAN and DO contribute to oil consumption.
Pistons, rings, and cylinder wall scoring are all possible causes. Pistons can be cracked or have porosity. Rings can be worn or lose their tension. Cylinder walls can be scored or cracked. All of these items have pressure against them during combustion. Valve seals do not.
 
How the heck do valve-stem seals get combustion pressure against them? Valve stem seals leak either due to gravity, or from suction during the intake stroke (intake valves open) - NOT during the compression stroke.
 
I assume when you say "blowby", you are referring to a fog coming from the crankcase ventilation breather tube or cap.
The amount of blowby observed depends on the crankcase ventilation system. An example would be the Farmall Cub. The early Cubs and Farmalls in general had a tube from the crankcase to the air cleaner to recycle the fumes to be burned. The later model numbered series Cub LoBoys do not have the tube. Consequently, the 154, 184 & 185 LoBoy owner's become concerned about the blowby from the breather cap.
Depending on what you have for crankcase ventilation, a reasonable amount of blowby may be expected. If the engine doesn't use an excessive amount of oil, I wouldn't worry about it.
 
Worn rings. It probably cost me a grand on an IH 504 diesel that I sold at auction. Ground was dry and dusty, and the tool driving it through the sale area had it tached up to about 2000 RPM- so the blowby was raising such a dust cloud you could hardly see the tractor. Needless to say, it didn't bring much. Had a loader, and ran fine. Somebody got a deal.
 
I agree that the likely cause will be worn rings.I have to disagree that valves and valve seals do not cause blowby. If a valve is worn at the seats ,burnt or bent they will leak by.Then if the stems are worn excessivly you will get pressure to the crankcase.This is the reason you should never install new rings without doing a valve job.
 
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