Ford 2003 Mustang Workshop Manual page 1846

If coolant is low, add specified coolant mixture only.
If the engine coolant tests too weak, add straight engine coolant until the readings
are within acceptable levels.
If the engine coolant tests strong, remove some of the engine coolant and add
water until the readings are within acceptable levels.
5. If the concern remains after the inspection, determine the symptom(s) and GO to
Chart
Symptom Chart
Symptom Chart
Condition
Loss of coolant
The engine overheats
The engine does not reach
normal operating temperature
Pinpoint Tests
PINPOINT TEST A: LOSS OF COOLANT
A1 CHECK THE ENGINE COOLANT LEVEL
WARNING: Never remove the pressure relief cap
while the engine is operating or when the cooling system is
hot. Failure to follow these instructions can result in
damage to the cooling system or engine or personal injury.
To avoid having scalding hot coolant or steam blow out of
the degas bottle when removing the pressure relief cap,
wait until the engine is cooled, then wrap a thick cloth
around the pressure relief cap and turn it slowly. Step back
Possible Sources
Radiator.
Coolant pump seal.
Radiator hoses.
Heater hoses.
Heater core.
Engine gaskets.
Degas bottle or
coolant expansion
tank.
Thermostat.
Coolant pump.
Internal engine coolant
leak.
Radiator.
Cooling fan.
Pressure relief cap or
radiator cap.
Thermostat.
Test Step
Symptom
Action
Go To
Pinpoint Test
A .
Go To
Pinpoint Test
B .
Go To
Pinpoint Test
C .
Result / Action to Take
Yes
GO to A2 .
No
REFILL the engine
coolant as necessary.
GO to A2 .