Car Care Tip: What's that sound under the hood?

Diagnosing engine noises can be the most difficult thing an automotive technician can do.

Some engine noises are normal. Others indicate serious problems that require prompt attention. Regardless, unusual engine noises should be checked by a tech immediately to avoid major engine damage.

Certain engine noises can be the result of bad pistons or pistons rings.

Most common engines have 4, 6, or 8 pistons which move up and down in the cylinders. On the upper side of the piston is the combustion chamber where the petrol and air mix before ignited. On the other side is the crankcase, which is full of oil. Pistons have rings which serve to keep the oil out of the combustion chamber and the fuel and air out of the oil.

A loose piston pin creates a metallic, double-knocking sound. Piston slap produces a muffled, hollow, knocking noise. Although a slight amount of piston slap is normal in many engines, loud, continuous piston slap means that the engine needs service.

Symptoms of bad pistons include reduced oil control and dark blue smoke being released out of the exhaust. This can be identified by the need to "top off" the oil level regularly between oil changes. Most cars that "burn oil" and have to have a quart added every 1,000 miles are burning it because the engine is old and the piston rings no longer seal things properly.

When inspecting pistons, your tech will check the top and underside of the pistons for damage and cracks, inspect the piston skirt for wear, and look for such problems as galling in the pin bores, excessive piston skirt wear, uneven wear around the rings, and oil on the top of the piston.

If you hear a light knocking or pounding sound coming from under the hood when the engine speed is constant, you could have a bad connecting rod.

The connecting rod connects the piston to the crankshaft. It can rotate at both ends so that its angle can change as the piston moves and the crankshaft rotates. The rod is under tremendous stress from the reciprocating load represented by the piston, actually stretching and relaxing with every rotation, and the load increases rapidly with increasing engine speed. Extreme wear is indicated by low pressure and high oil consumption.

Rod failure is one of the most common causes of catastrophic engine failure. Techs will inspect connecting rods for bending, twisting, and cracking. Bent or twisted rods should be straightened or replaced. Some connecting rods can be reconditioned and reused. Others require replacement.

When trying to diagnose engine noises as a motorist, help your tech out by being as descriptive as possible. Take note to when the noise occurs, at what throttle position, and when the noise started occurring.

For more information on connecting rods and pistons, or if your looking to upgrade engine performance, visit www.carrilloind.com and www.jepistons.com.