Yanmar Tractor Engine Blow-By

How much blow-by is too much for a Yanmar tractor and what causes it? 

So you have noticed a fair amount of smoke blowing out of a hose on the side of your engine. That hose is your crankcase vent tube and that smoke is blow-by. Blow-by is simply the compression that leaks past the piston rings during combustion. All diesel engines will have blow by. Even when brand new. 

So when is blow-by considered to be a problem? The short answer- when the engine is hard to start. When too much compression leaks past the piston rings then there might not be enough compression developed to ignite the diesel fuel. Diesels rely on extreme cylinder pressures to ignite the fuel instead of spark plugs like gasoline engines. If your engine can not develop enough pressure then there will be no ignition. You can test how much compression you have with a Yanmar Diesel Compression Tester. A Yanmar engine will need a minimum of 400psi. They come new from the factory with 500-600psi. Read more about compression here

So what causes excess blow-by? Typically blow-by is caused by the piston rings being worn down by a clogged and leaking air filter. That fine dust gets sucked into the engine and acts as a very efficient sandpaper. Blowby can also happen after overheating when the piston rings get so hot that they lose their spring tension and no longer seal in the cylinder. Another common cause of blow-by is using starting fluid. 

So how do we fix it?  There are some additives that you can put in your fuel or oil that claim to help blow-by. I have not personally tried them but it might be worth a shot. The only other real fix is going to be to replace your piston rings. This is not a terribly complicated job but it will require the help of a Yanmar service manual to get step-by-step instructions, torques, as well as wear limits to be sure your piston or sleeves do not need replaced as well. 

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