A pressure drop in the radiator can cause the
boiling point to lower causing the cooling system to
boil over. A cooling system pressure tester may be
used to check the cooling system pressure as well
as the pressure cap relief valve. If the cap fails the
test, check the rubber seal on the cap as well as
the operation of the pressure relief valve.
Incorrect fan, fan or shroud not in correct position:
An incorrect fan, or a fan or shroud in a wrong
position will cause a reduction or a loss of air flow
through the radiator. The fan must be large enough
to send air through most of the area of the radiator
Loose fan or water pump belts will cause a
reduction in air and coolant flow. Tighten the belts
according to the Belt Tension Charts.
Defective hoses with leaks can normally be seen.
Hoses that have no visual leaks can collapse (pull
together) during operation and cause a restriction in
the flow of coolant. Hoses become soft and/or get
cracks after a period of time. The inside of a hose
can deteriorate, and the loose particles of the hose
can cause a restriction of the coolant flow.
Air inlet restriction:
Restriction of the air coming into the engine can
cause high cylinder temperatures and more than
normal amount of heat to pass to the cooling
system. Check for restriction with a water
manometer or a vacuum gauge (which measures in
inches of water). Connect the gauge to the engine
air inlet between the air cleaner and the inlet to the
turbocharger. With the gauge installed, run the
engine at full load rpm and check the restriction.
Maximum restriction of the air inlet is 635 mm (25
in) of water. If the indication is higher than
foreign material from the filter element, or install a
new filter element and check for the restriction
again. If the indication is still too high, there must
be a restriction in the inlet piping.
Restriction in the exhaust system can cause high
cylinder temperatures and more than normal amount of
heat to pass to the cooling system. To check if there is
an exhaust restriction, make a visual inspection of the
exhaust system. Check for damage to piping or for a
defective muffler. If no damage is found, check the
exhaust system for back pressure from the exhaust
(pressure difference measurement between exhaust
outlet and atmosphere). The back pressure must not
be more than 1016 mm (40 in) of water. Check the
system by removing the exhaust pipes from the
exhaust manifolds. With the exhaust pipes removed,
start and run the engine to see if the problem is
A restriction of the shunt line from the radiator top
tank to the engine water pump inlet, or a shunt line
not installed correctly, will cause a reduction in
water pump efficiency. The result will be low
coolant flow and overheating.
Water temperature regulator:
A water temperature regulator that does not open,
or only opens part of the way, can cause above
normal heating. To test the water temperature
regulator, see the Testing and Adjusting Section of
the Service Manual.
Defective water pump:
A water pump with a loose or damaged impeller
does not pump enough coolant for correct engine
cooling. Remove the water pump and check for
damage to the impeller. If the impeller has no
damage, check the impeller clearance. The
clearance between the impeller and the housing is
0.75 0.25 mm (0.030 to 0.010 in).
Air flow through engine compartment:
The air flow through the radar comes out of the
engine compartment. Make sure the filters, air
conditioners and similar items are not installed in a
way which prevents free flow of air into and out of
the engine compartment.
Restriction of air flow through the air to air
aftercooler (if equipped). Check for debris or
deposits which would prevent the free flow of air
through the aftercooler.
High outside temperature:
When outside temperatures are too high for the
rating of the cooling system, there is not enough
temperature difference between the outside air and