Fuel and Emission Systems Flashcards

What is the function of the fuel tank pressure sensor in the EVAP system?

Answer: To monitor tank pressure changes during EVAP leak testing and purge operations

The fuel tank pressure sensor reports pressure and vacuum levels in the fuel tank to the ECM, which uses this data to monitor EVAP system integrity during leak tests and to control purge valve operation.

A vehicle fails an emissions test for high NOx levels. What is the most likely cause?

Answer: A malfunctioning EGR system or excessive combustion temperatures

High NOx emissions are caused by excessive combustion temperatures. A non-functioning EGR system fails to lower peak temperatures, and lean conditions or cooling system problems can also contribute to high NOx.

What is the purpose of the canister vent valve in the EVAP system?

Answer: To seal the charcoal canister from atmosphere during EVAP leak testing

The canister vent valve is normally open to allow fresh air to flow through the charcoal canister during purge. During EVAP leak testing, it closes to seal the system so the ECM can detect leaks.

What causes surging at idle on a fuel-injected engine?

Answer: A vacuum leak causing an unmetered air entry into the intake manifold

A vacuum leak allows unmeasured air into the intake, creating a lean condition the ECM tries to correct by adding fuel. This constant correction cycle causes the RPM to rise and fall (surge).

What is the function of the oxygen sensor heater circuit?

Answer: To bring the oxygen sensor to operating temperature (approximately 315°C) quickly for faster closed-loop operation

The internal heater brings the zirconia sensing element to its minimum operating temperature quickly, allowing the ECM to enter closed-loop fuel control sooner and reducing cold-start emissions.

A technician notices fuel odour from a vehicle. A smoke test reveals vapour escaping from the EVAP charcoal canister. What is the likely cause?

Answer: A saturated charcoal canister caused by liquid fuel flooding from overfilling or a faulty purge valve

A saturated canister occurs when liquid fuel reaches the canister, typically from overfilling the tank or a purge valve stuck open that floods the canister. The charcoal loses its ability to adsorb vapours once saturated with liquid fuel.