Causes of Blowby in Detroit Diesel Engines (Semi Trucks)

Blowby—combustion gases leaking past piston rings into the crankcase—is typically caused by wear or damage to the cylinder sealing components:

• Worn or broken piston rings
• Incorrectly installed piston rings (gaps aligned)
• Scored or worn cylinder liners
• Cracked pistons
• Worn valve guides
• Bad valve stem seals
• Excessively worn or poorly seating valve seats

Excessive blowby increases crankcase pressure, oil consumption, and emissions. A compression test and leak-down test can pinpoint the source. Regular oil analysis helps detect early wear.