Fuel, Lubrication, and Cooling Systems
Domain 6 tests fuel delivery, lubricant specification and the cooling circuit - the systems whose neglect causes most engine failures.
Fuel system
Petrol engines use a carburettor (venturi, float chamber, jets) or MPFI with ECU-pulsed injectors; octane number measures resistance to knock. Diesel uses a high-pressure pump and injectors, up to about 2000 bar in common-rail systems; cetane number measures ignition quality. Water in diesel wrecks the pump, so the filter has a water trap and must be bled after replacement.
Lubrication
The oil pump (gear or rotor type) draws oil from the sump through a strainer, pushes it through a full-flow filter, then to main bearings, big ends, camshaft and rockers; a relief valve caps pressure. In SAE 15W-40, 15W is the cold (winter) grade and 40 the hot grade; API or ACEA letters give the service category. Low oil pressure needs the gauge and sender checked, not simply thicker oil.
Cooling
The water pump circulates coolant through block, head, radiator and heater; the thermostat stays closed until about 80–90 °C; the pressure cap raises the boiling point and the fan cuts in when hot.
Worked example
Making 5 litres of 50 : 50 coolant needs 2.5 L ethylene glycol plus 2.5 L clean water. Such a mix freezes near −37 °C and boils near 106–107 °C at atmospheric pressure; with a 0.9 bar cap the boiling point rises to roughly 125 °C, giving a safety margin on a hot day.
Traps that cost marks
Opening the radiator cap on a hot engine; topping up with water only and diluting the mix; overfilling engine oil; confusing the W grade with hot viscosity; ignoring a slow coolant loss.