Fuel, Emission and Cooling Systems
What this domain tests
This domain covers fuel delivery, air-fuel mixture, emission control, and engine cooling. Questions deal with injectors, fuel pressure, stoichiometry, catalytic converters, EGR, BS-VI norms, DPF and SCR systems, thermostat operation, coolant and radiator caps.
Core rules
- Stoichiometric petrol mixture is about 14.7:1 by mass; lambda 1 means neither rich nor lean. The oxygen sensor closes the loop and the ECU trims injector pulse width, normally within about ±10%.
- Fuel trims near +20% indicate a lean fault, such as a vacuum leak; negative trims indicate rich running.
- Three-way catalysts oxidise CO and HC and reduce NOx only near stoichiometry, so a rich or lean misfire overheats and destroys the catalyst.
- Modern diesel engines use high-pressure common rail, EGR, DPF and SCR with AdBlue (urea solution) to meet BS-VI limits.
- Coolant is a 50/50 glycol mix. A pressure cap raises the boiling point to roughly 120 °C at 1.0 bar, which is why the cap must not be opened hot.
- A thermostat stuck open causes slow warm-up and poor heater output; stuck closed causes rapid overheating.
Worked example
An engine idles rough with a long-term fuel trim of +18% but runs normally at cruise. The ECU is adding fuel at idle because unmetered air enters after the airflow meter. Check intake hoses, the manifold gasket and the PCV system with a smoke test. A weak fuel pump would usually make trims positive under load too, so the idle-only pattern points to a vacuum leak rather than fuel supply.
Traps that cost marks
- Opening the radiator cap while hot.
- Running plain water instead of coolant, causing corrosion and a higher boiling point risk.
- Pouring AdBlue into the diesel tank.
- Removing a thermostat to cure overheating instead of finding the fault.