Lesson 5/Learning Event 2
Engine Overheats
Let's assume that you are trying to locate the cause for an engine overheating in an M151 1/4-ton truck. As
you check the truck out, do not be too hasty in deciding that the cooling system is at fault. There are many
things other than the cooling system that may cause the engine to overheat.
Outside the cooling system, incorrect ignition timing is a common cause of engine overheating. Therefore, this
possibility should be checked out and the timing corrected if needed.
Another cause for overheating, other than the cooling system, is a clogged muffler or exhaust outlet pipes.
Recall that the exhaust carries heat away from the engine. If the muffler or pipes are restricted so they fail to
carry exhaust and heat away from the engine as they should, replace the clogged parts.
Still another cause for overheating, outside the cooling system, is excessive friction in the power train. This
could be anything, such as binding gearboxes or brakes, which will put an excessive load on the engine. In this
case, the binding must be located and corrected.
If the overheating is actually caused by the cooling system, one common cause is low coolant level. When the
coolant is low, refill the radiator and inspect the system thoroughly for leaks. All leaks must be corrected.
Another common trouble that causes overheating is loose or worn fan belts. Always adjust or replace loose or
worn fan belts.
A thermostat that fails to open will cause the engine to overheat very quickly. If the thermostat is suspected of
being faulty, remove and test it.
Anything that blocks or reduces airflow through the radiator core will cause overheating.
Therefore, you
should always check for and remove any obstructions that may restrict the flow of cooling air.
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