John Deere Maintenance Monday: Gaging blow-by
Looking to buy a new tractor for your farm? They might be advertised as having "minimal blow-by," and you should know what's acceptable for blow-by on a diesel engine.
Radio interview source: Richard Kershner, John Deere Product Manger, John Deere
In this edition of Maintenance Monday, we're answering a question from Larry in Montana. He is looking to purchase a used 70-to-100-horsepower tractor for his farm. He has looked at several are advertised as having "minimal blow-by". He is wondering what is acceptable for blow-by on a diesel engine.
John Deere Product Manager Richard Kershner says blow-by is excessive leakage of combustion gasses past the piston and piston rings. The engine may be hard to start but runs fine once it gets going. Blow-by usually means there is significant wear in the piston or rings. You can tell there is a problem if there is excessive oil consumption and you're adding engine oil every 100-hours or so. However,
Kershner says there is a certain amount of blow-by that is normal in a diesel engine.
"You will get some compression leakage and some combustion gasses leak past a cylinder piston and piston rings," he says. "I can't say what an acceptable amount is right off the top of my head, a customer will need to talk to their dealer and maybe they can pull it up in a technical manual to give them an idea."
Kershner says the best way to gauge blow-by is with a compression test. If Larry has the know-how and proper equipment, he can do the test himself. But most people rely on their dealer for engine diagnostics.
"Compression tests are fairly easy to do from a dealer perspective," says Kershner. "You're basically just tapping into your cylinders, putting the compression gauge in and testing compression to see what that rating is."
If the compression test shows too much blow-by, Kershner says there is no way to repair it. You're looking at purchasing a new or remanufactured engine.
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