Engine Lubrication and Fuel System Servicing
Domain 4 tests lubrication and fuel delivery: choosing and changing oil correctly, reading oil pressure, and diagnosing fuel supply on gasoline injection and diesel systems.
Core rules
- Oil grade: the first number (e.g. 15W) is cold-flow behaviour, the second (40) is viscosity at operating temperature; follow the API/ACEA category and the OEM specification, which can be stricter than the label.
- Change the filter with the oil; wet the gasket, make sure the old gasket is not stuck to the block, and torque the drain plug with a new washer.
- Oil pressure: compare a mechanical gauge reading with the dash gauge before condemning the pump. Low pressure can be a faulty sender, a stuck relief valve, a blocked pickup screen, diluted oil or worn bearings.
- Petrol injection: measure rail pressure with the regulator vacuum hose removed; check for pressure drop after shut-off (leaking injector or check valve); injector resistance is typically 12–16 Ω.
- Diesel: keep the system surgically clean, drain the water separator, bleed the low-pressure side after running dry, and never crack a high-pressure common-rail line with the engine running.
- Air filter restriction and a dirty MAF sensor skew mixture before the pump is at fault.
Worked example
Specification is 3.0 bar key-on, engine off; the gauge shows 2.2 bar and the engine cranks long before starting. Reasoning: low rest pressure means the pump cannot deliver or pressure is bleeding away. Clamp the return line briefly: if pressure jumps to 3.0 bar the regulator is dumping pressure; if it stays at 2.2 bar the restriction is upstream - filter, pickup sock or pump. Pressure that falls after ten minutes at key-off points instead to a leaking check valve or injector.
Traps
- Overfilling oil for safety, which aerates it.
- Using a generic oil on a turbocharged or DPF-equipped engine.
- Leaving the old filter gasket in place, causing a double-gasket leak.
- Assuming low oil pressure always means a worn pump.