DIAGNOSIS AND TESTING
COOLING SYSTEM DIAGNOSIS
CONDITION
POSSIBLE CAUSES
CORRECTION
TEMPERATURE GAUGE READING IS
INCONSISTENT (FLUCTUATES,
CYCLES OR IS ERRATIC).
1. During cold weather operation with
the heater blower in the high position,
the gauge reading may drop slightly.
Fluctuation is also influenced by loads,
outside temperature, and cycling of the
air conditioning system.
1. A normal condition. No correction is
necessary.
2. Temperature gauge or engine
sending unit sensor defective or
shorted. Also, corroded or loose wiring
in this circuit.
2. Check operation of gauge and repair
if necessary. Refer to Group 8E,
Instrument Panel and Gauges.
3. Gauge reading rise when vehicle is
brought to a stop after heavy use
(engine still running).
3. A normal condition. No correction is
necessary. Gauge reading should return
to normal range after vehicle is driven.
4. Gauge reading high after restarting a
warmed-up (hot) engine.
4. A normal condition. No correction is
necessary. The gauge should return to
normal range after a few minutes of
engine operation.
5. Coolant level low (air will build up in
the cooling system causing the
thermostat to open late).
5. Check and correct coolant leaks.
Refer to Testing Cooling System for
Leaks in this group.
6. Cylinder head gasket leaking
allowing exhaust gas to enter cooling
system causing thermostat to open late.
6. (a) Check for cylinder head gasket
leaks with a commercially available
block leak tester. Repair as necessary.
(b) Check for coolant in engine oil.
Inspect for white steam emitting from
exhaust system. Repair as necessary.
7. Water pump impeller loose on shaft.
7. Check water pump and replace as
necessary. Refer to Water Pump
Removal in this group.
8. Air leak on the suction side of water
pump allows air to build up in cooling
system causing thermostat to open late.
8. Locate leak and repair as necessary.
PRESSURE CAP IS BLOWING OFF
STEAM AND/OR COOLANT.
TEMPERATURE GAUGE READING
MAY BE ABOVE NORMAL BUT NOT
HIGH. COOLANT LEVEL MAY BE
HIGH IN COOLANT RESERVE/
OVERFLOW TANK.
1. Pressure relief valve in radiator cap
is defective, or was not properly seated.
1. Check condition of radiator cap and
cap seal. Refer to Pressure Cap in this
group. Replace cap as necessary.
2. Incorrect cap was installed.
2. Replace cap as necessary.
3. Incorrect coolant mixture.
3. Make sure a 50% by volume mixture
of coolant is used.
COOLANT LOSS TO THE GROUND
WITHOUT PRESSURE CAP
BLOWOFF. GAUGE IS READING HIGH
OR HOT.
1. Coolant leaks in radiator, cooling
system hoses, water pump or engine.
1. Pressure test and repair as
necessary. Refer to Testing Cooling
System for Leaks in this group.
7 - 8
COOLING
LH