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In order to run reliably and smoothly under harsh conditions, the internal engine parts are manufactured to precise dimensions, assembled with precision clearances between moving parts, and lubricated by a pressurized oiling system.

Most unidentified engine noises result from clearances which have become too large due to worn or failed parts, lack of adequate lubrication, or both. The importance of lubrication cannot be over-emphasized. For best results, troubleshooting engine noises should only be done when the oil and filter have been recently changed and the oil level is correct.

High-pitched metallic tapping noises are caused by relatively small, lightweight parts and are most likely an indication of excessive clearances in the valve train. Valve train noise accompanied by burning oil (blue-gray smoke in the exhaust), particularly at startup or when decelerating from high RPM, is an indication of worn valve guides which can only be remedied by overhaul or replacement of the cylinder head. In a high-mileage engine, a light metallic rattle or chatter under acceleration, accompanied by increased oil consumption and smoking, may indicate severely worn or broken piston rings. Since this diagnosis means overhaul or replacement of the engine, the problem should be further investigated with a compression or cylinder leakage test. See 3.2 Diagnostic Testing.

Deep, metallic knocking sounds are caused by excessive clearances between heavier components. Closer analysis of the noise will often help identify the problem. Piston slap, caused by excessive piston skirt to cylinder wall clearance, is worse when the engine is cold and may be accompanied by increased oil consumption and reduced compression due to accelerated piston ring wear. A double knock, most pronounced at idle or low load, is due to excessive clearance at the piston pin and upper connecting rod bushing.

Crankshaft bearing problems produce a deep, hollow knock that is worst when the engine is warm. A noise that is very pronounced under load, perhaps louder during the transition from acceleration to coasting, is most likely caused by a damaged connecting rod bearing. Crankshaft main bearings produce a lower, dull knock. An intermittent knock, indicating excessive crankshaft end play, may be most apparent when depressing or releasing the clutch. These problems seldom occur as isolated failures. They are almost always an indication of the overall engine condition which can only be properly corrected by complete engine overhaul or replacement.

Rumbling or groaning from the engine compartment may not indicate engine problems at all, but rather a worn bearing or bushing in an engine-driven accessory. They include the coolant pump, alternator, and may include a power steering pump and air conditioning compressor. The air conditioning compressor is equipped with an electrically-switched clutch-type pulley, so a bad compressor will only be noisy when the air conditioning is on. To check other accessories, run the engine briefly with the drive belt disconnected and see if the noise has stopped. Once the drive belt is removed, turning the pulley and shaft by hand may also reveal a bad bearing or bushing. A properly functioning accessory should turn smoothly.

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