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Tuffy

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Tuffy
What causes engine crankcase blow by? What is normal crankcase pressure and how do you measure it? I have an Onan 20 hp and just can't seem to find the problem. I have good compression 120 # in the cyls and I have cleaded the breather mechanism. Does the breather mechanism ever need to be replaced? The engine does not smoke, just lots of blow by even backing up into the air cleaner. Ideas? Thanks, Steve

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ReedS
Steve, crankcase blow is most commonly caused by worn rings and/or worn cylinder bores. When rings and/or blocks are worn they leak cylinder pressure past the rings (which act as seals) into the crankcase thus the term blowby. You could have good oil control rings keeping oil from entering the combustion chamber but have poor compression rings allowing leakage. I'm not familiar with onans so I cant help much with specifics. All automotive engines have a pcv (positive crankcase ventilation) system which allows crankcase fumes to be drawn into the intake manifold to be reburned. Engines when the rings and cyliners wear will create enough crankcase pressure to overwhelm the pcv system with oil which can and does sometimes does get into the air cleaner. Also(in vehicles) if the pcv system get plugged up crankcase pressure will push blow by through the breather into the air filter housing. Usually the breather filter is the intake for the pcv system. Maybe someone else more familiar with these engines can be of more help here. Good luck Reeds

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Al
Steve, The air cooled engines have a crankcase breather which is a check valve. When the pistons come down they create pressure in the crankcase, this valve lets the pressure out. When the pistons go up they create a vacuum in the crankcase. The check valve doesn't let any air in as a result the crankcase has a vacuum in it. Any leak in the crankcase letting air in drops the vacuum in the crankcase. When the vac.[vaccuum] drops on the down stroke when it gets low enough, you blow out the breather tube, typically into the air cleaner, oil and oily vapor into the air cleaner. In addition to any external leaks, gaskets, dipstick seals, crankshaft seals, or a bad breather valve cause the vac. to be low, resulting in excessive oil consumption from burning the oil and vapors that are fed to the carb. Crankcase vac. is measured with a water manometer. In most single cyl and opposed twins 12 to 20 inches of water is pretty normal at high speed. A manometer is a U shaped tube maybe 36" tall, it would be filled with water to bring the level of water in each tube to about 18". The source of vacuum is then connected to 1 of the legs of the tube. Caution: Put a valve in this line, if you don't, when the engine starts the first downstroke will blow all of the water out of the u tube. Most common connection is a rubber cone inserted in the dipstick tube with a hose barb for a hose connected to the manometer. Any port to the crankcase is suitable, if the engine has a vacuum fuel pump, the pulse line can be teed and used. When the engine is running, open the valve to the manometer SLOWLY and measure the water column. If the water goes up 6" in one and 6" down in the other the difference is 12" which would be good in most cases. The manometer can be made with flexible plastic tubing hung from the ends of the tubing creating a U. V twins typically run lower vac than other engines because as one piston is coming down the other is going up during part of the stroke. When you have combustion gases going by the rings into the crankcase the vac is reduced with BAD blowby, the crankcase can have positive pressure and will blow most of the oil out of the crankcase in a short time. On an OHV engine a leaking head gasket into the push rod or oil return passage will do the same thing. When we see a Briggs Vangard blowing oil into the air filter, the crankcase vac is the first thing we do. Next we do a compression check. The comprssion is often good, as the leakage is not bad enough to show on a compression measurement. If we see very low vac, we check for dipstick o-ring and any other obvious leaks. If none, pull the head [s] and you will see the leakage. This is very common on the 28 series engines and the 5 or 6 year old v twins that have the steel head gaskets. You can see the leakage when you get them apart. Heads need to be checked for flat, over .002 resurface. We have a guage calibrated in inches of water and use it all the time. [About $60.00] Crankcase vacuum is one of the most useful and seldom used checks in these engines. In the last 3 or 4 years all of the engine companies have STRESSED the need to do it, along with the cylinder leakdown tests in service schools. There, you asked what time it was and you got told how to build a watch, and IT may not give accurate time. Its free, value accordingly. Good luck, Al Eden

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powerking_one
Al, This was an excellent response to this subject. I dare say your shop is probably one out of 200 around the country that actually measure and check for proper crankcase breathing by using empirical methods. Hats off to ya, Tom(PK)

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Tuffy
In reading Al's reply again (nice post!!) I have to think are the Onan engines opposite ( I think) and does one piston go down while the other goes up? Or do they both go up (and down) at the same time (probably). Does anyone know? Anyone torn one down who would know? thanks again, Steve

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Al
Steve, Yes we use a vacuum guage, it is a sensitive one calibrated in inches of water instead of inches of mercury as most are. Almost all twin cylinder engines are alternate firing and the pistons both go up and down at the same time. This cancels the vibration and makes them smooth. That is why they use a coil with tow wires. They fire both plugs each revolution and the cylinder at the end of the exhaust and beginning of the intake stroke gets/has a wasted spark which has no effect. Small watch this tiime. Good luck. Al Eden

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Tuffy
"I now see" said the blind man: Since they both go up and down at the same time that is why there is significat vacuum (up stroke) and pressure (down stroke) in the crankcase. Thus one must insure there is not excess vacuum 'cause that will in intself induce more of a pressure problem on the down stroke. Thanks again for all the great info. Steve

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HubbardRA
I like Al's explanation of the measurement techniques. It was very informative. Piston rings seal because of differential pressure. In other words they need the high cylinder pressure during compression on one side versus a crankcase vacuum on the other. If the crankcase becomes pressurized or has a much lower vacuum, the rings will start leaking and produce even more blow-by and allow oil to be forced past the rings into the cylinder during the power stroke when the piston is going downward. I have seen a plugged crankcase vent cause an engine in otherwise perfect condition to smoke excessively. When the vent was fixed the smoking completely stopped. Rod H.

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BigSix
Kent: Al's article was great, and I wanted to suggest it be enshrined/easily retreivable somewhere. How about a "Tools You Can Build" section? Al, from your answer re: substituting an automotive-type vacuum gauge for this test, I gather the latter is not sensitive enough, as Mercury weighs more than water. However, is it that a vacuum gauge won't register at all, or is there some rough ratio one can use to convert the reading from a vacuum gauge to the manometer's meaurement? Peter

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CarlH
Rod, Your reply prompted my fuller understanding of my experience with the 16 HP Kohler on my AC 716H. The SECOND time I rebuilt it I noticed that various pieces were missing in the crankcase vent assembly. Sometime after the rebuild and re-installation/breakin of the engine, I got the missing pieces and installed them. It made a *TREMENDOUS* improvement in the fuel consumption of the tractor. The thermodynamic efficiency of an internal combustion engine is highly correlated with piston ring seal and effective compression ratio.

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JimS
In regard to piston travel and engine balance in a two-cylinder engine, it depends on whether it is an opposed or an in-line configuration(pistons side-by side). In an opposed engine like an Onan, the pistons move in opposite directions relative to each other but are both in downstroke or upstroke relative to their respective cylinder. One cylinder fires on each 360 degrees of crankshaft rotation. In an in-line engine like an old John Deere farm tractor, the pistons move opposite to each other and are opposite on upstroke and downstroke as well. This type engine has an uneven firing order. For example, cylinder 1 fires at 0 and cylinder 2 fires at 180 degrees of crankshaft shaft rotation. The crank must rotate thru 540 degrees before cylinder 1 fires again.

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