What's all the Noise About?

Various underhood noises can reveal a lot about the health of an engine … or more accurately, the lack of health.

Internal engine noise can take many forms and can be difficult to pinpoint and correct. The cause can be dirt in a hydraulic valve lifter, an out-of-adjustment valve tappet, excessive piston skirt-to-cylinder wall clearance or detonation/pre-ignition in the combustion chamber. Whatever the cause, engine noise is troubling and annoying to vehicle owners. And it usually represents a problem that must be corrected. Let’s take a look at the source and correction of many of the more common internal engine noises.

Bearing Noise

Connecting rod and crankshaft main bearing noise can be the result of many conditions, such as low oil level, low oil pressure, diluted oil, excessive bearing clearance, and worn or out-of-round bearing journals. The most common cause, though, is excessive bearing clearance.

Excessive clearance can result from bearing wear (this occurs if there is dirt in the engine that causes accelerated bearing wear), or journal-to-bearing clearance that was incorrectly established at the time the engine was assembled. This can happen from time-to-time when the engine is rebuilt.

The easiest method to check bearing-to-journal clearance is to use Plastigage. Let’s review the procedure for using Plastigage on rod bearings (the method is similar for main bearings).

1. Rotate the crankshaft until the connecting rod is at the bottom of its stroke.
2. Clean the oil film off of the surface to be checked.
3. Place a piece of Plastigage across the entire width of the bearing shell in the bearing cap about 1/4" off center and away from the oil hole.
4. Assemble the rod cap with the Plastigage in place and tighten the cap to the specified torque. Do not rotate the crankshaft.
5. Remove the bearing cap and compare the width of the flattened Plastigage with the scale provided with the package.
6. Locate the band nearest to the width of the Plastigage (see Figure 1). This indicates the oil clearance. Compare it to specifications to determine if it is excessive.

If the surface of the bearing is heavily scratched, that’s an indication of an excess amount of dirt in the engine. The dirt wore the bearings abnormally and increased the clearance, causing noise.

Piston Noise

A slapping sound that is sometimes heard in an engine can be caused by excessive piston-to-cylinder wall clearance. While this is uncommon for a factory engine, a rebuilt engine can have this noise if the cylinder bore was incorrectly finished.

Checking the piston skirt-to-cylinder wall clearance is pretty straightforward. First, measure the diameter of the piston skirt at a point 90 degrees from the piston pin bore and about halfway down the length of the skirt. Next, using a dial bore gauge as shown in Figure 2, measure the diameter of the cylinder about halfway down the bore. The skirt-to-wall clearance is the difference between the cylinder bore dimension and the piston skirt diameter measurement.

If the clearance is excessive, the piston slapping sound will be present. In order to correct this problem, the cylinder must be bored to the next oversize and an oversize piston installed.

Valve Train Noise

Valvetrain noise is usually traced back to the hydraulic valve lifters. If the oil level is too high or too low, air can be introduced into the lifter (high oil levels lead to foaming and low oil levels allow the oil pump to ingest air). Regardless of the oil level problem, the air allows the valve lifter to become soft, resulting in the valves making noise when seating. With these air problems, the noise problem is usually not confined to one lifter.

Valve lifter noise ranges from a light noise to a heavy click. This light noise is usually caused by excessive leak down around the plunger unit, or the plunger partially sticking in the lifter body cylinder. In either case, the problem can be remedied by replacing the valve lifter.

The loud, clicking noise is caused by dirt. Often times, the dirt might only be present in one or two lifters (it depends upon the dirt content in the oil). The dirt either prevents the lifter check valve from seating or causes the plunger to be stuck in the down position. In either case, the result is excessive clearance, or lash, in the valvetrain. The clicking noise is the contact between the rocker arm and valve stem.

This problem can easily be corrected by cleaning the engine (new oil and filter or engine disassembly) to remove all traces of dirt. Other valvetrain noise problems can be caused by worn valve guides and/or cocked valves springs. Quite often these conditions are mistaken for valve lifter noise. Check the valve springs and apply a side thrust force. If this doesn’t stop the noise, then inspect the rocker arm push rod sockets and push rod ends for wear. If these components are not worn, the noise is probably from the lifters.

Timing Chain Noise

V-type engines that use a metal timing chain can experience timing chain noise if the chain becomes excessively worn. What actually occurs is the worn chain is making contact with the inside wall of the timing chain cover. Though not a common problem, it does occur in engines with excessively high mileage.

To determine if the chain is worn, measure the amount of sprocket/chain movement by doing the following:

1. Position a scale next to the timing chain so that any movement can be measured (Figure 3).
2. Place a torque wrench and socket on the camshaft sprocket retaining bolt.
3. Apply force in the direction of crankshaft rotation to take up any slack.
4. Block the crankshaft to prevent rotation.
5. Holding the scale in place, apply force in the reverse direction and measure the movement of the sprocket/chain.
6. In general, if the movement exceeds 1/8", the chain is worn. Replace the chain and the sprockets.

Detonation/Pre-Ignition Noise

The last engine noise we’ll discuss is the result of abnormal ignition and not a mechanical problem within the engine. Often times, the noise is heard as a knock or ping, particularly under heavy acceleration. The sources of the problem are many and include hotspots in the combustion chamber from carbon build-up, improperly set ignition timing (advanced too much) and the use of low-octane fuel.

In modern automotive engines, knock sensors detect this abnormal ignition and automatically retard the ignition timing to eliminate the problem. On older engines, however, no such technology exists to remedy the situation.

Always be sure that the ignition timing is set correctly. If that doesn’t solve the problem, try using premium grade gasoline, and maybe even an octane booster. If the engine has excessively high mileage, Mopar Combustion Chamber Cleaner, P/N 04318001AB, can be used to remove or reduce these deposits. If this product doesn’t work, you’re probably looking at an engine teardown.