to operate properly. A stuck thermostat can either cause
engine overheating or overcooling.
If a thermostat is stuck closed, coolant will not
circulate through the radiator. As a result, overheating
could make the coolant boil.
When a thermostat is stuck open, too much coolant
may circulate through the radiator and the engine may
not reach proper operating temperature. The engine
may run poorly for extended periods in cold weather.
Engine efficiency (power, fuel mileage, and
driveability) will be reduced.
The procedure for thermostat replacement is as
To remove the thermostat, drain the coolant and
remove the upper radiator hose from the engine.
Remove the retaining cap screws holding the
thermostat housing to the engine. Tap the
housing free with a rubber hammer. Lift off the
housing and thermostat.
Scrape all of the old gasket material off the
thermostat housing and sealing surface of the
Make sure that the housing is not warped. Place
it on a flat surface and check the gaps between
the housing and the surface. If warped, file the
surface flat. This action will prevent coolant
Make sure the temperature rating is correct.
Then place the thermostat into the engine.
Normally, the pointed end on the thermostat
should face the radiator hose.
chamber should face the inside of the engine.
Position the new gasket with approved sealer.
Start the cap screws by hand. Then torque them
to the manufacturer's specifications in an
alternating pattern DO NOT overtighten the
housing bolts or warpage and or breakage may
result. Most housings are made of soft aluminum
or "pot metal."
Old radiator hoses and heater hoses are frequent
causes of cooling system problems. Hoses should be
checked periodically for leakage and general condition.
The leakage may often be corrected by tightening or
replacing hose clamps. After a few years of use, hoses
deteriorate. They may become soft and mushy, or hard
and brittle. Deteriorated hoses should be replaced to
preclude future troubles. Cooling system pressure can
rupture the hoses and result in coolant loss.
Inspect the radiator and heater hoses for cracks,
bulges, cuts, or any other sign of deterioration. Squeeze
the hoses to check whether they are hardened or
softened and faulty. Flex or bend heater hoses and
watch for signs of surface cracks. If any problem is
detected, the affected hose should be replaced.
However, where spiral spring stiffeners are used to
control the tendency to collapse, such test will not work
and the hose must be removed for inspection.
Fan and Belt
One of the easiest and quickest checks to the
cooling system is the fan and fan belt. Check the fan for
bent blades, cracks, and other problems. A bent or
distorted fan on one with a loose blade should be
replaced. Where the fan is just loose on its mounting,
tightening is in order.
Fan belts, or drive belts, should be checked for wear
and tension. Most wear occurs on the underside of the
belt. To check a V-belt, twist the belt with your fingers.
Check for small cracks, grease, glazing, and tears or
splits. Small cracks will enlarge as the belt is flexed.
Grease rots the rubber and makes the side slick so that
the belt slips easily. A high-pitched squeal results from
slippage. Large tears or splits in a belt allow it to be
tossed from the pulley. On vehicles with a set of two
belts, replace both if one is worn and requires
Use a belt tension gauge to check and adjust the fan
belt tension. When you do not have a gauge or if space
does not allow use of a gauge, you can make a quick
check of belt tension. Press down on the free span of the
belt, a point midway between the alternator or generator
pulley and the fan pulley. Measure the amount of
deflection. When free span is less than 12 inches
between pulleys, belt deflection should be 1/8 to 1/4
inch. When free span is longer than 12 inches, belt
deflection should be 1/4 to 1/2 inch.
A slipping belt can cause overheating and a run-
down battery. These troubles result because a slipping
belt cannot drive the water pump and alternator fast
enough for normal operation. Sometimes a belt will slip
and make noise even after it is adjusted to the proper
tension. Several types of belt dressing are available
which can be applied to both sides of the belt to prevent
this problem. Belt dressing helps to eliminate noise and
increase belt friction.