Troubleshoot a Main Bearing Engine Noise
Main-bearing noise is caused by worn main bearings that are oversized or loose on the crankshaft, or lack oil pressure. The main bearings can become worn from age or mileage. If diagnosed at the first occurrence and the engine is shut down shortly thereafter, new bearings can be installed and the problem solved. If oil pressure is the cause, it is possible to replace the oil pump or correct the oil-pressure problem and still save the crank. If the engine is driven for any amount of time with bearing noise, the crankshaft journals will be harmed, thus requiring a new or machined crankshaft to accomplish a repair.
Instructions
1. Check the oil dipstick and make sure that the vehicle has enough oil.
2. Look in the valve cover, through the oil filler cap, and see if there is an inordinate amount of sludge. This won't be necessary if you are familiar with the servicing of the vehicle since new and the oil has been changed on a regular basis. Many times the oil changes were not done on a regular basis and sludge has built up so thick under the valve cover that the oil will not return readily to the oil pan. There may be three quarts of oil or more under the valve covers because the drain holes are stopped up.
3. Check the engine oil for water infiltration. A blown head gasket can allow water into the oil, effectively reducing its viscosity or lubricating properties.
4. Start the engine and watch the oil-pressure gauge. There should be at least ten pounds of oil pressure at idle with a warm engine. If the vehicle has no oil gauge, install a remote oil gauge in place of the oil-pressure sender. Let the engine warm up completely and observe the oil pressure. If the oil pressure drops when the engine gets warm, it's probable that an oil-pressure problem exists. Check the oil filter for age. If it is new, the oil pump could be worn.
5. Listen to the engine while it's running. Grab the throttle linkage and snap it open a small amount. No need to race the engine--just quickly raise the rpm to approximately 2,000 and release. If the engine has a hard, solid knock every time the throttle is snapped open and goes away when the throttle is released, or knocks continuously, the bearings are definitely bad.
6. Raise and support the vehicle on jack stands. Drain the engine oil into the drip pan. Remove the oil pan, using the appropriate socket. Remove the bolts in the front main bearing and remove the main bearing cap.
7. Wipe the journal and the bearing in the cap and inspect them for excessive wear. Look at the thickness of the bearing in the cap and see if it has an uneven thickness or any big gouges. Look at the crankshaft journal. There should be no grooves. It may have small scratches but as long as you cannot feel them with a fingernail they are usually okay. To be sure, check the clearance with plastigauge by wiping all the oil off the journal and the bearing in the lower cap.
8. Place the piece of plastigauge on the bearing width-wise and reinstall the bearing cap. Tighten it to 30 foot-pounds of torque and then remove it. Tightening the cap flattens the plastigauge. Compare the thickness of the plastigauge to the paper it came in. There is a scale on the side of the paper. Hold the plastigauge up to the paper gauge and match it with the scale. The acceptable clearance is .001 to .003 of an inch, no more. If the clearance is greater the bearings must be replaced. If the crankshaft is scored, it needs to be replaced as well. If there is an issue with the oil pressure, that must be dealt with as well.
Tags: Bearing Engine, Bearing Engine Noise, Engine Noise, journal bearing, Main Bearing Engine
