Here's a detailed explanation of what might have happened:
Worn Camshaft Lobes or Rocker Arms:
- Worn camshaft lobes or rocker arms can cause excessive clearance between the camshaft and the valve lifters or between the rocker arms and the valve stems.
- This increased clearance produces a characteristic tapping or knocking noise as the valves open and close.
Reason for Noise Disappearance:
- Disconnecting the HT lead for the fourth cylinder essentially disabled that cylinder from firing.
- With no combustion happening in that cylinder, the noise associated with the worn camshaft lobes or rocker arms disappears.
- This further supports the valve train issue as the root cause.
Additional Observations:
- If the noise varies with engine speed, this is a strong indicator of a worn camshaft or rocker arms.
- Visual inspection of the camshaft and rocker arms can reveal excessive wear or damage, which can confirm this issue.
- Other symptoms that might accompany this problem include reduced engine power and increased exhaust smoke.
Recommendation:
- It is recommended to consult with a trusted mechanic or automotive professional to diagnose the exact problem and perform any necessary repairs or replacements of worn-out components in the valve train of the affected cylinder.
