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Engine valve fault finding

Posted by Ashley Wilson on

Valve Failure Analysis

Valve Face Burning

Result: Valve face burnt and torched.
Cause: Excessive localized heat in the valve head, distortion and seat leakage (poor seating).
Contributing Factors:
a) Lack of stem to guide clearance.
b) Worn valve guide and/or misalignment of valve stem and guide.
c) Pre-ignition (lean air-fuel mixture, incorrect fuel).
d) Improper compression ratio
e) Defective cooling system
f) Incorrect lash adjustment
g) Excessive carbon build up on valve
Result: Wide areas of valve face burnt but not torched through.
Cause: Excessive accumulated heat on the valve head, concentrated on the margin.
Contributing Factors:
a) Worn valve guide.
b) Excess material removed during a previous re-facing operation.
c) Poor valve seating.
Result: A hole burnt through the back of the valve head into the underhead radius just behind the valve face. This is more common on hard faced valves.
Cause: Excessive localized heat in the valve head, distortion and seat leakage (poor seating)
It starts as a radial rim crack or thermal fatigue, then the base material burns through behind the hardened face.
Contributing Factors:
a) Engine overload.
b) Fuel system problems (lean air-fuel mixture).
c) Pre-ignition.
d) Incorrect fuel.
e) Poor valve seating.

Valve face Pitting

Result: Pitted valve and valve seat faces
Cause: Solid particles pressed between valve face and valve seat
Contributing Factors:
a) Excessive oil consumption (through piston rings, valves guides and valve stem seals)
b) Abnormal combustion.
c) Long idle periods.
d) Thermostat malfunction (bellow normal engine temperature).

Corrosion

Result: 1) Corrosion on the valve underhead between the neck and the valve face. 
2)
 Valve stem necking.
Cause: Erosion and corrosion caused by exhaust gases.
Contributing Factors:
a) Use of inadequate valve material.
b) Excessive engine overload conditions (Overheating).
c) Incorrect fuel.
d) Lean air-fuel mixture.

Distortion

Result: Cupping or tuliping of the valve head.
Cause: Very high seating forces. Excessive combustion temperature and pressure.
Contributing Factors:
a) Engine overload.
b) Pre-ignition.
c) Detonation.
d) Excessive valve spring pressure.
e) Improper compression ratio.
f) Improper air-fuel mixture.
g) Difference in angle between the valve seat and the valve face.

Seizure or Scuffing

Result: Stem seizure or scuffing
Cause: High temperature caused by friction due to lack of clearance and/or lubrication
Contributing Factors:
a) Poor stem to guide clearance
b) Insufficient lubrication.
c) Valve underhead, neck, and stem carbon build up.
d) Engine running at a low speed or overloaded.
e) Valve stem bent (Possible collision with the piston).
f) Incorrect valve stem seals.
g) Falta de alineación entre vástago, guía y asiento de la tapa.           *

Wear

Result: Valve stem and guide wear
Cause: Excessive stem to guide clearance. Signs of high temperature and stem/guide seizure.
Contributing Factors:
a) Poor to stem to guide clearance (oil film breakdown)
b) Excessive stem to guide clearance (insufficient heat dissipation)
c) Poor stem lubrication
d) Wrong valve stem seals
e) Incorrect rocker arm geometry
f) Restricted exhaust flow                *
Result: a) Guide out-of-round due to wear
b) Valve stem uneven wear.
c) Valve tip wear
Cause: High temperature caused by friction and misalignment of the valve stem.
Contributing Factors:
a) Worn rocker arm pivot
b) Improper valve tip grinding
c) Guide/seat misalignment
d) Excessive lifter bore clearance
e) Too much spring tension
f) Insufficient lubrication          *
Result: Valve facing grooving
Cause:
a) Improper fuel .
b) Incorrect Springs
Contributing Factors:
a) Weak valve springs.
b) Valve seat/guide misalignment .
c) LPG or propane gas used as fuel.
Result: Valve wear in the keeper groove area (single or multi-groove).
Cause: Use of worn or improper keepers
Contributing Factors:
a) Worn keepers
b) Too much spring tension or defective springs
c) Worn valve retainers
d) Insufficient lubrication
e) High speed seating generating excess friction between the retainer, keepers, and valve.
Important:
These are the most common examples of valve related failures
Since the least common result of valve failure is the valve itself, it is always necessary to determine the cause of failure prior to replacement of the damaged valve(s).

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