Fuel, Lubrication and Cooling Systems
What the exam tests
This domain covers petrol and diesel fuel delivery, air-fuel ratio, engine oil and filters, lubrication circuits, coolant flow, thermostat operation and overheating diagnosis.
Core rules
- Petrol stoichiometric air-fuel ratio is about 14.7:1 by mass; rich mixtures increase power but raise emissions and carbon deposits. Diesel uses compression ignition and direct injection at high pressure.
- Engine oil viscosity is shown by SAE grades such as 15W-40; the API or manufacturer specification must also be met. Oil lubricates, cools, cleans and seals.
- The cooling system includes radiator, water pump, thermostat, fan, hoses and pressure cap. Coolant raises the boiling point and prevents corrosion; plain water is not a substitute.
- Never open a hot radiator cap. Allow the system to cool and use the correct coolant mix.
Worked example
An engine overheats and the radiator fan runs, but the top hose stays cool while the engine is hot. Reasoning: a thermostat stuck closed stops coolant circulation to the radiator. Check coolant level, temperature gauge and hose temperature. Replace the thermostat and bleed the system. If the top hose were hot but the radiator cold, a blocked radiator or faulty fan would be more likely.
Traps that cost marks
Using the wrong oil viscosity, overfilling coolant without allowance for expansion, leaving air locks, ignoring a weak radiator cap, and diagnosing overheating as a head-gasket fault before checking the thermostat.