Cooling and lubrication systems
Cooling and lubrication systems
The exam tests coolant flow, pressure cap, thermostat, fan control, oil pump, filtration and oil pressure diagnosis.
Core rules
- The system is pressurised; a 1.1 bar cap raises the boiling point by about 25°C at sea level. Test the cap and system with a pressure tester, not by squeezing hoses.
- Thermostat opening temperature is stamped; test it in water with a thermometer. It should begin opening near rating and be fully open about 10°C later.
- Use a 50/50 coolant mix and silicate-free organic acid technology where specified. Electrolysis check: a voltmeter between coolant and battery negative should read under 0.3 V.
- Oil grade, such as 15W-40 API CK-4 for many diesels, must match ambient conditions and the OEM specification. Low oil pressure at idle but normal at speed suggests a worn pump or excessive clearance; low everywhere suggests gauge, relief valve or pickup blockage.
- The PCV and breather maintain crankcase pressure; a clogged PCV causes seals to leak.
Worked example
An engine overheats at idle but cools at highway speed. At speed, airflow is high; at idle, cooling depends on the fan. Check the fan clutch or electric fan relay. If the fan runs only with A/C on, the coolant temperature sensor or fan control module is suspect. A collapsed lower hose can also cause idle-only overheating because it closes at low flow.
Traps
- Opening the radiator cap hot.
- Removing the thermostat to fix overheating, which slows warm-up and can set codes.
- Using plain water, causing corrosion and a false temperature gauge reading.
- Overfilling oil and diagnosing a rear main seal leak when the PCV is blocked.