Cooling System Diagnosis and Repair
What the candidate must know
The cooling system includes the radiator, water pump, thermostat, fan and fan clutch, hoses, coolant, pressure cap, and the engine's coolant passages. Know how a pressure cap raises boiling point, how a thermostat regulates temperature, how a fan clutch engages, and how to test for combustion gases in coolant (block tester), pressure-test the system, and check for a restricted radiator or heater core.
Traps in questions
- Overheating at speed points to a restricted radiator or a failing water pump; overheating at idle points to a fan clutch or fan problem. Questions often reverse these.
- No heat at idle but heat at speed suggests low coolant, air in the system, or a restricted heater core - not necessarily a thermostat.
- A pressure cap that does not hold pressure lowers the boiling point and causes overheating and coolant loss without an external leak. Test the cap, not just the system.
- Combustion gases in coolant (bubbles, hydrocarbon test positive) indicate a head gasket or cracked head, not a water pump.
- Thermostat stuck open causes overcooling and poor heater performance; stuck closed causes rapid overheating. Do not confuse the two.
- Coolant loss with no visible leak can be an internal leak (head gasket, oil cooler) or a cap/overflow problem.
Worked example
A truck overheats only on the highway. The fan clutch tests fine and the thermostat opens. A pressure test holds, but a flow test shows low radiator flow. The radiator is restricted - the speed-related symptom pointed there, not to the fan.
Rule of thumb
Speed-related overheating = radiator or pump; idle-related overheating = fan or airflow. Test the cap and check for combustion gases before assuming a simple leak.