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Definition of Engine Overspeed
Engine overspeed is the speed in revolutions per minute (rpm) that exceeds the maximum governed speed plus governor droop.
At a sufficiently high degree of overspeed, damage to engine components will occur. Base engine component failures associated with overspeed generally result from cam gear walking, valve-to-piston contact, or a no-follow condition in the valve train. Fuel system component failures associated with overspeed generally result in flyweight damage.
Valve-to-piston contact can occur when a piston is at (or near) top dead center (TDC). At high degrees of overspeed, the intake and exhaust valves do not close completely. This condition is commonly referred to as “valve floating.” Valve floating results in engine damage when a piston strikes an open valve. Valve-to-piston contact results in bent or broken push tubes, tappets, springs, valves, piston damage, cylinder head damage, and the possibility of extensive progressive damage to the other cylinders, cylinder heads, turbocharger, and other overhead components.
No-follow is induced by rapid changes in valve acceleration, resulting in impact loading between the components in the valve train. Impact loading can bend or break push tubes, cause surface distress on camshaft lobes and tappet faces, and cause excessive valve collet wear.
Causes of Engine Overspeed
Possible causes for engine overspeed are as follows:
Improper gear shift
Excessive vehicle speed down hills in the wrong gear
Excessive amounts of combustible vapors in the intake system
Fuel pump tampering
Diagnosing Engine Overspeed Failures
Common signs of engine overspeed are as follows:
Bent or broken push tubes
Worn valve collets
Cam gear walk-off
Occasional larger valve lash
When diagnosing a possible overspeed failure, always check for these common signs in addition to other component failures. If a component shows damage as described in the sections on Overspeed Symptoms, but the
Engine overspeed is the speed in revolutions per minute (rpm) that exceeds the maximum governed speed plus governor droop.
At a sufficiently high degree of overspeed, damage to engine components will occur. Base engine component failures associated with overspeed generally result from cam gear walking, valve-to-piston contact, or a no-follow condition in the valve train. Fuel system component failures associated with overspeed generally result in flyweight damage.
Valve-to-piston contact can occur when a piston is at (or near) top dead center (TDC). At high degrees of overspeed, the intake and exhaust valves do not close completely. This condition is commonly referred to as “valve floating.” Valve floating results in engine damage when a piston strikes an open valve. Valve-to-piston contact results in bent or broken push tubes, tappets, springs, valves, piston damage, cylinder head damage, and the possibility of extensive progressive damage to the other cylinders, cylinder heads, turbocharger, and other overhead components.
No-follow is induced by rapid changes in valve acceleration, resulting in impact loading between the components in the valve train. Impact loading can bend or break push tubes, cause surface distress on camshaft lobes and tappet faces, and cause excessive valve collet wear.
Causes of Engine Overspeed
Possible causes for engine overspeed are as follows:
Improper gear shift
Excessive vehicle speed down hills in the wrong gear
Excessive amounts of combustible vapors in the intake system
Fuel pump tampering
Diagnosing Engine Overspeed Failures
Common signs of engine overspeed are as follows:
Bent or broken push tubes
Worn valve collets
Cam gear walk-off
Occasional larger valve lash
When diagnosing a possible overspeed failure, always check for these common signs in addition to other component failures. If a component shows damage as described in the sections on Overspeed Symptoms, but the