ASE A1 Engine RepairCylinder Head and Valve TrainMedium
A technician is diagnosing an overhead cam (OHC) engine with a persistent ticking noise coming from the cylinder head. The noise increases with engine speed. A stethoscope confirms the noise originates from a specific valve train component. Further inspection reveals excessive clearance between a camshaft lobe and its corresponding cam follower (or rocker arm). What is the MOST likely cause?
- AA worn camshaft lobe.
- BA worn valve guide.
- CA bent valve stem.
- DA broken valve spring.
Show answer & explanationAnswer & explanation
Correct answer: A. A worn camshaft lobe.
Excessive clearance between a camshaft lobe and cam follower, manifesting as a ticking noise that increases with engine speed, is a classic symptom of a worn camshaft lobe. The lobe has lost material, creating extra space and leading to impact noise.
Why the other options are wrong
- B. A worn valve guide would cause oil consumption and possibly a valve wobble, but not typically excessive lobe-to-follower clearance or a distinct ticking noise from that specific interface.
- C. A bent valve stem would cause the valve to stick or not seat properly, leading to misfires or severe damage, but not increased lobe-to-follower clearance.
- D. A broken valve spring would cause the valve to not return to its seat, leading to a severe misfire, loss of compression, and potentially valve-to-piston contact, not just a ticking from excessive lobe clearance.
Worn Camshaft Lobe
A condition where the surface of a camshaft lobe wears down, reducing its profile and thus the amount of valve lift and duration, leading to excessive valve lash or clearance.
- Causes ticking noise from valve train.
- Reduces valve lift and engine performance.
- Can be diagnosed by measuring lobe lift or inspecting visually.
Memory trick: Tick-tock, cam's worn, engine's stock.