Lesson 3/Learning Event 1
The connecting rod knock is caused by excessive clearance between the connecting rod bearing inserts and the
bearing surface of the crankshaft throw. It makes a loud, sharp knock while the engine is running at a constant
speed without a load. You can detect the faulty connecting rod bearing by disconnecting and connecting the
spark plug leads one at a time. When you disconnect the lead from the cylinder with the loose rod bearing, the
knock will go away or at least change a great deal.
Loose main bearings on the crankshaft cause a heavy, dull, thud type noise which is usually worse with the
engine loaded. Loose connecting rod bearings or main bearings will usually cause low oil pressure. This low
pressure will be more noticeable at slow engine speeds. This is because the oil pressure leaks off past the loose
bearing, and at slow engine speeds, the oil pump turns too slow to pump enough oil to maintain the proper
pressure. At higher speeds, enough oil is pumped to overcome the leak and build up the oil pressure. If we
have a sharp knock or a dull thud sound and the oil pressure is low, the connecting rods or main bearings are
worn.
A piston slap usually sounds off only when the engine is on a pull. Like the connecting rod, disconnecting the
spark plug wire to the affected cylinder will generally stop the knock. Pistons have been known to knock quite
loud when the engine is cold, then the noise completely disappears after the engine warms up.
Water pumps sometimes make noises that are hard to locate. It may make a grinding noise, may sound like
marbles rolling around in a tin can, or may be a high-pitch whistle. One way to find out if a noise is caused by
the water pump is to remove the water pump (fan) drive belt. But you must keep in mind that the same belt
may drive other parts, such as the generator, which could be the noise maker. If this is the case, lay your hand
on the components and feel for the vibration that is making the noise. If the parts are hard to reach, use a
metal bar or a stick for a sounding bar. To do this, place one end of the sounding bar against the suspected
noise maker and touch the opposite end of the bar to your ear.
If the operating clearance between the valve stem and its operating linkage is too great, there will be a tapping
noise each time the valve opens. The noise remains the same regardless of the load on the engine. But the
tapping will get faster and louder as engine speed increases. This noise can be easily stopped in a few minutes,
in most cases, by an adjustment. Can
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