Yanmar 4BY2 - Engine Performance Problems

Vibration - Drive Engaged
Problem / Symptom Cause Action
Rough at All Speeds Engine and propeller shaft mis-
aligned
See your authorized Yanmar deal-
er or distributor
Leaking cylinder head gasket
Bent propeller shaft
Rough at Higher Speeds Bent propeller See your authorized Yanmar deal-
er or distributor
Slipping clutch / clutch dog
Incorrect injection pressure
Injection timing is incorrect
Engine Knocks
Problem / Symptom Cause Action
Excess Fuel Injected Defective fuel injector See your authorized Yanmar dealer
or distributor
High fuel injection pressure
Noise Changes with Engine Load Incorrect or poor quality fuel Drain and refill tank with proper fuel
Worn crankshaft / bearings See your authorized Yanmar dealer
or distributor
Broken piston / rings
Low Power Output
Problem / Symptom Cause Action
Miscellaneous Clogged air filter Replace air filter
Leaking cylinder head gasket See your authorized Yanmar deal-
er or distributor
Damaged turbocharger
Incorrect propeller
Excessive exhaust back pres-
sure
Fuel Plugged fuel filter(s) Replace as necessary
Faulty fuel supply pump See your authorized Yanmar deal-
er or distributor
Low Fuel Injection Pressure Defective fuel pressure regula-
tor / sensor
See your authorized Yanmar deal-
er or distributor
Worn high-pressure fuel pump
Injection timing is incorrect
Low RPM at Wide Open Throttle Propeller pitch too great See your authorized Yanmar deal-
er or distributor
Engine overheated
Damaged turbocharger
TROUBLESHOOTING
62 BY Series Operation Manual
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