* Low oil pressure: When the oil is hot, its viscosity (thickness) decreases. If your oil pump is weak, or you have a significant oil leak, the pressure might drop too low to adequately lubricate the bearings and other critical parts when hot. This is a serious problem that can lead to catastrophic engine failure if ignored. Check your oil pressure gauge immediately.
* Worn bearings: Worn crankshaft or connecting rod bearings create clearances that allow metal-to-metal contact when the engine is hot and the components expand. This is a common cause of hot-oil knock.
* Insufficient oil clearance: If the engine has been rebuilt, improper machining or clearance specifications during the rebuild can lead to insufficient clearance between components when hot, resulting in knocking.
* Connecting rod knock: Similar to bearing wear, a damaged connecting rod can cause knocking, particularly when the oil is hot and the increased heat causes expansion.
* Piston slap: Excessive piston-to-cylinder wall clearance can cause a knocking sound, especially when the engine is warm and the components expand. This is more of a "slap" than a sharp knock.
* Wrist pin knock: Wear or damage to the wrist pin, which connects the piston to the connecting rod, can also create a knocking noise when the engine is hot.
* Cam issues: Though less likely to present as a hot-oil-only knock, a worn camshaft or lifter can contribute to noise that's more noticeable when the oil is thin.
Troubleshooting Steps:
1. Check your oil pressure: This is the most crucial first step. A low oil pressure reading when hot strongly indicates a serious lubrication problem.
2. Inspect the oil: Check the oil level and its condition. Is it milky (indicating coolant contamination)? Is it excessively dirty or sludgy?
3. Listen carefully: Try to pinpoint the location of the knock. Is it a consistent knock, or does it change with RPM? This can help determine the source.
4. Professional diagnosis: This situation requires professional attention. A mechanic can perform a compression test, a leak-down test, and inspect the bearings and other internal components to accurately diagnose the problem. Ignoring this issue could result in severe and costly engine damage.
Ignoring a knock, especially one that only appears when the oil is hot, is extremely risky. Have the engine inspected immediately to prevent more serious damage.
