Internal Engine Noises
It occurs at every power stroke, so it sounds off in time with the valve train. Piston slap is a dull, hollow sound. It's difficult to hear over the normally loud VW engine components and not easy to isolate. In fact, there is a better chance of hearing it while driving than when stationary. If you think you hear piston slap, remove and replace the spark plug wire to each cylinder one at a time. When you get to the affected cylinder, the noise will greatly diminish. Disabling the cylinder will reduce piston slap because combustion loads no longer exist. Reconnecting the plug lead will restore the noise. Remember to ground the plug lead when disabling a cylinder.
The major internal engine noise associated with air-cooled VWs is rod knock. Rod knock describes the knocking sound made by a connecting rod when there is excessive clearance between it and the crankshaft.
When rod-bearing clearance is large, oil cannot fill the gap between the rod bearing and crankshaft. Metal-to-metal contact starts as the rod bearing is slammed against the crankshaft. If one rod is bad, it will occur as one steady knocking beat. On a long journey as the engine heats up, the rod knock may be initially heard as a light tap, developing into a medium rap and finally finishing as a knock. At the stage of a rod is knocking, serious damage has been done to the crankshaft and rod. A full overhaul of the engine is required. Continued driving may spin the bearing.
A spun bearing is one that is rotating relative to the rod and the crankshaft. The bearing will no longer be clamped by the connecting rod. When the bearing spins, it covers the oil holes in the crankshaft. Oil ceases to flow to that rod and it rapidly overheats. Many times, the rod will weld itself to the crankshaft, seize and break. The rod, crankcase, and camshaft are likely to follow. Usually metal particles from the rod have been pumped through the engine with the oil, and all other precision clearances will have been destroyed by the passing metal.
When you hear a rod knock, STOP! Rebuild the engine while there is still enough to rebuild. Rods also wear with accumulated mileage. Then they wear in sets, so you will hear a castanet like rattling with old, tired bearings. This sound is often heard during a cold start before oil pressure builds.
Sounding similar to, but deeper than, worn rod bearings are bad main bearings. Main bearings knock for the same reasons as rods, excessive oil clearance, but under different conditions.
Bad main bearings can
also be heard while going uphill, accelerating or during other periods of
high engine load. VW main bearings are quite large and strong for the horsepower
of the engines. Therefore, they rarely fail, wear out, or make noise.
The rod bearings are much more suspect. If knocking or pounding is heard from the engine, it may be a rod is about to fail.