An engine should be properly diagnosed before it is disassembled for two reasons. First, to determine that a repair is really necessary. And, secondly to diagnosis the exact location of the problem while the engine is still intact.
Inspect the piston, piston rings, and crankshaft. normally, rod bearing inspection requires at least the removal of the oil pan. see the service manual for the procedure to remove the oil pan on your vehicle.
Detonation can affect the rod bearing. if the rod bearing falls out as the piston and rod are removed from the cylinder, this means the bearing has lost its crush. check for detonation damage, especially on the upper rod bearing.
Excessive clearance at a connecting rod journal commonly results in a rod knock.
- The result of worn or loose connecting rod bearings, this noise is heard at idle as well as at speeds over 35 mph.
- Depending on how badly the bearings are worn, the noise can range from a light tap to a heavy knock or pound.
- During a cylinder power balance test, the intensity of the knock will diminish or disappear altogether as the offending cylinder's spark plug is grounded out. This condition is sometimes accompanied by low oil pressure, especially at idle.
- Shorting out the spark plug of the affected cylinder can lessen the noise, unless the bearing is extremely worn. In this case, shorting out the plug will have no effect.
- Rod-bearing noise is caused by a worn bearing or crankpin, a misaligned rod, or lack or lubrication, resulting in worn bearings.
- To correct it, service or replace the crankshaft, realign or replace the connecting rods, and replace the bearings.
An engine with high mileage can have worn connecting rod bearings. Results of this are:
- Low oil pressure when the engine idles (with normal pressure off idle).
- Oil consumption when excessive oil leaks out between the rod journals and bearings. The oil is thrown onto the cylinder walls at high speed.
Effect of excessive bearing clearance on oil throw-off.
- The oil rings do not have enough capacity to return all of this oil to the crankcase. Whatever oil enters the combustion chamber will be burned with the air/fuel mixture.