ASE A1 Engine RepairCylinder Head and Valve TrainEasy
A technician is diagnosing an engine that exhibits a consistent knocking noise originating from the cylinder head area, especially noticeable after a cold start and diminishing slightly as the engine warms up. Upon inspection, the technician identifies excessive clearance between the rocker arm and the valve stem tip on one cylinder. Which of the following is the most likely cause of this condition?
- AWorn valve guide
- BWorn camshaft lobe
- CStretched pushrod
- DWorn rocker arm pivot
Show answer & explanationAnswer & explanation
Correct answer: D. Worn rocker arm pivot
Excessive clearance between the rocker arm and valve stem tip, leading to a knocking noise, is most commonly caused by wear at the rocker arm pivot, which allows the rocker arm to move more than intended.
Why the other options are wrong
- A. Worn valve guides primarily cause excessive oil consumption and valve stem wobble, not direct excessive clearance between the rocker arm and valve stem tip.
- B. A worn camshaft lobe would typically result in reduced valve lift and duration, causing a misfire or reduced power, not necessarily excessive rocker arm to valve stem clearance.
- C. A stretched pushrod would decrease the effective length, reducing valve lift and potentially causing insufficient lash, not excessive clearance.
Rocker Arm Pivot Wear
Wear in the pivot point of a rocker arm can lead to excessive clearance between the rocker arm and the valve stem, resulting in noise and improper valve operation.
- Causes a knocking noise, especially when cold.
- Results in excessive valve lash.
- Affects valve timing and lift.
Memory trick: Knocking sounds from the valve train often point to wear in the pivot points or excessive clearance.