Fuel and Emission Systems Flashcards

What happens when the purge valve in the EVAP system is stuck open?

Answer: Excess fuel vapours are continuously drawn into the intake, causing a rich condition especially at idle

A stuck-open purge valve continuously draws fuel vapours from the canister into the intake manifold, enriching the mixture beyond what the ECM expects, causing rich running, rough idle, and possible stalling.

What is the function of the upstream oxygen sensor in a closed-loop fuel system?

Answer: To provide exhaust oxygen content feedback to the ECM for real-time fuel mixture adjustment

The upstream (pre-catalyst) O2 sensor measures remaining oxygen in the exhaust. The ECM uses this feedback to continuously adjust fuel injector pulse width, maintaining the mixture near the stoichiometric ratio.

What does a returnless fuel system use to regulate fuel pressure?

Answer: An ECM-controlled fuel pump module that adjusts pump output to maintain target pressure

Returnless fuel systems regulate pressure by controlling the fuel pump speed through a driver module commanded by the ECM. There is no return line to the tank, reducing emissions, fuel temperature, and system complexity.

A diesel vehicle enters limp mode with reduced power. The DPF warning light is on. What is the most likely cause?

Answer: An excessively soot-loaded DPF that failed to regenerate, causing high exhaust back pressure

When the DPF becomes excessively loaded with soot and passive/active regeneration has failed, the ECM limits engine power to prevent damage from excessive exhaust back pressure and triggers limp mode.

What is the purpose of the positive crankcase ventilation (PCV) system in relation to emissions?

Answer: To prevent crankcase blow-by gases from escaping to the atmosphere by routing them into the intake for combustion

The PCV system is an emissions device that routes crankcase blow-by gases (unburned hydrocarbons that leak past the piston rings) back into the intake manifold to be burned in the combustion process.

A technician is performing a fuel injector balance test. What is this test measuring?

Answer: The pressure drop when each injector is individually pulsed for a set duration, indicating relative flow rates

An injector balance test energizes each injector individually for a set number of pulses and measures the fuel rail pressure drop. Equal drops indicate balanced flow; a smaller drop indicates a restricted or weak injector.