Cooling, Lubrication and Fuel Systems
Core concepts
Understand cooling system operation: radiator, thermostat, water pump, hoses, expansion tank and pressure cap. Know the purpose of coolant additives, the difference between types, and the risks of mixing them. Cover lubrication: oil grades, viscosity ratings, filters, oil pumps and the consequences of low oil pressure. Understand fuel systems, including pump, filter, injectors, fuel pressure testing and the difference between port and direct injection. Diagnose overheating, coolant loss, oil consumption, fuel smell and poor economy.
Traps
Questions often test whether a thermostat stuck open causes overheating (it causes overcooling) and whether opening a hot cooling system is safe (it is not). Watch for confusion between oil viscosity ratings and quality ratings, and for the assumption that a fuel smell always means a leaking injector.
Worked example
A vehicle overheats at idle but runs normally at speed. Suspect the cooling fan or its control circuit. Verify fan operation when hot, check the relay and fuse, and test the coolant temperature sensor signal. Confirm the thermostat opens by checking radiator hose temperature.
Rule of thumb
Never open a pressurised cooling system when hot. Allow it to cool fully first.