Fuel, lubrication and cooling systems
Fuel, lubrication and cooling systems
This domain tests fuel delivery, air-fuel mixture, lubrication, and temperature control. It covers petrol injection, diesel common rail, oil specification, filters, coolant, thermostat, radiator, and fan operation.
Core rules and concepts
- Fuel trims: positive long-term trim means the ECU is adding fuel, often from unmetered air or low fuel pressure; negative trim means it is reducing fuel.
- Diesel common rail: verify low-pressure supply and filter first; high-pressure work needs clean areas and correct torque.
- Engine oil: viscosity and API or ACEA service category must match the engine and climate. API CK-4 is a diesel category; do not substitute a petrol-only oil.
- Oil pressure warning: stop the engine, check level, and verify pressure with a gauge; do not assume the sensor is faulty.
- Cooling: use a 50/50 coolant-water mix; the pressure cap raises boiling point; the thermostat controls minimum temperature; check fan clutch or electric fan operation.
- Overheat diagnosis: coolant level, leaks, belt, thermostat, radiator airflow, pressure test, and combustion gas test.
Worked example
An engine overheats at idle but cools at road speed. That points to airflow: fan clutch or electric fan. Check fan command, relay, and coolant temperature sensor. If the fan works, pressure-test the system and check the thermostat. Combustion gases in coolant indicate a head gasket fault.
Traps that cost marks
Opening a hot pressure cap; using straight water; fitting the wrong oil category; overfilling the cooling system; and ignoring fuel dilution of engine oil.