by Alain Hoffmann
They make an flapping, muffled, hollow noise inside. Not click or clack unless the piston pin is also worn out. Drive the car with very low rpm's in high gear and apply brakes and gas. If the noise now amplifies you have definite piston slap. On 4- and 6-cylinders you can run the stethoscope along the block (junk that garden hose and spend 10 bucks on a real one!) while listening carefully. V8's are much harder as the cylinders are much closer and noise crosses over between rows.
Want to nail it down any more before buying a new engine? Right. Pull the spark plugs or injectors of the suspected cylinder and put 2 ounces of engine oil inside. Refit injectors/plugs. Turn it over a few times (pull the coil wire or push it 30 yards if it's a diesel). Then start it up. If the sound is now gone or much reduced you got it. A word of warning: Don't overdo the oil dose and do this not in your garage. The oil will burn and make an immense blue cloud, pesting all the neighbourhood and upsetting your wife.
Severe slap means you need an rebuilt complete with new pistons and an overbore. But wait. If the noise just is audible when the engine is cold and vanishes when it warms up you have still some life left in. Take note of it and start saving for a later rebuilt in about half a year or so. If anybody asks about the noises just tell him it's an V8 diesel, an rare experimental model and worth at least 50 grand.
Piston wrist pins make very characteristic noises - an sharp double knock. The double comes from the pistons changing direction at TDC and BDC, hitting the pin a fraction of a second later. This noise gets louder when you pull the right coil wire. Several worn wrist pins make a noise like some mice throwing dice inside an empty beer can. In that case you lost - same procedure as above for repair.