ASE A1 Engine RepairLubrication and Cooling SystemsHard
A vehicle is brought into the shop with a loud 'knocking' noise that increases with engine RPM, especially under load. The oil pressure gauge reads significantly lower than normal. Which of the following is the most likely cause?
- AA worn camshaft lobe
- BWorn main or connecting rod bearings
- CA failed oil pump
- DA collapsed hydraulic lifter
Show answer & explanationAnswer & explanation
Correct answer: B. Worn main or connecting rod bearings
A loud knocking noise that increases with RPM and low oil pressure are classic symptoms of worn main or connecting rod bearings. These bearings rely on a film of pressurized oil for lubrication, and when worn, they create excessive clearance, causing a metallic knocking sound and reducing oil pressure as oil escapes the clearances.
Why the other options are wrong
- A. A worn camshaft lobe would typically cause a ticking or tapping noise and misfires, not a heavy knock with low oil pressure.
- C. A failed oil pump would cause extremely low or no oil pressure, but the knocking noise would be a secondary symptom of subsequent bearing damage, not necessarily the primary cause of the knock itself initially.
- D. A collapsed hydraulic lifter would cause a ticking or tapping noise from the valvetrain, not a deep knocking sound and low oil pressure.
Worn Engine Bearings
Worn main or connecting rod bearings result in excessive clearance between moving parts, leading to a loud metallic knocking sound, especially under load, and a significant drop in engine oil pressure due to increased oil leakage.
- Loud, deep knocking noise (increases with RPM, under load)
- Low engine oil pressure
- Often requires engine disassembly for repair
Memory trick: Low pressure, loud knock? Bearings are on the rock!