Fuel, Ignition and Engine Management Systems
Fuel delivery and control
Candidates must compare carburettor and electronic fuel injection, and understand the sensors and actuators used in engine management. Key sensors include mass air flow (MAF), manifold absolute pressure (MAP), throttle position (TPS), coolant temperature (ECT), oxygen (O2), knock, crankshaft position (CKP) and camshaft position (CMP). The ECU uses these signals to control injector pulse width and ignition timing. Closed-loop operation uses the O2 sensor to trim fuel toward stoichiometric ratio, about 14.7:1 for petrol. Ignition systems use coils, spark plugs, and a triggering signal; timing advance depends on speed and load. Catalytic converters reduce hydrocarbons, carbon monoxide and oxides of nitrogen.
Traps in questions
A common trap is thinking a higher O2 sensor voltage means a lean mixture: for a zirconia sensor, high voltage means rich. Another trap is confusing knock sensor function (retards timing) with a detonation cause. Questions may ask why a MAF fault causes poor running: the ECU miscalculates air mass and injector pulse width. Do not assume a spark plug gap is adjustable on all plugs; iridium types are pre-gapped. Also remember that disconnecting a battery may erase adaptive learning and stored codes.
Rule of thumb
Diagnose fuel and ignition by checking air, fuel, and spark in a logical order before replacing parts.
Worked example: an engine runs rich, and the O2 sensor reads about 0.8 V instead of switching. The high voltage indicates a rich exhaust; check fuel pressure, injector leakage and air intake restrictions.