310S Fuel and Emission Systems 3 — Questions and Answers
Question 1: What is the function of the fuel tank pressure sensor in the EVAP system?
- To regulate fuel pump speed
- To monitor tank pressure changes during EVAP leak testing and purge operations (Correct answer)
- To measure fuel level in the tank
- To prevent fuel tank collapse during rapid fuel consumption
Correct 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.
Question 2: A vehicle fails an emissions test for high NOx levels. What is the most likely cause?
- A rich fuel mixture
- A malfunctioning EGR system or excessive combustion temperatures (Correct answer)
- A clogged catalytic converter
- A leaking EVAP system
Correct 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.
Question 3: What is the purpose of the canister vent valve in the EVAP system?
- To release fuel vapours to the atmosphere during refueling
- To seal the charcoal canister from atmosphere during EVAP leak testing (Correct answer)
- To purge fuel vapours into the intake manifold
- To regulate fuel tank pressure during driving
Correct 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.
Question 4: What causes surging at idle on a fuel-injected engine?
- An oversized battery
- A vacuum leak causing an unmetered air entry into the intake manifold (Correct answer)
- Excessive fuel pressure
- A dirty air filter
Correct 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).
Question 5: What is the function of the oxygen sensor heater circuit?
- To warm the exhaust manifold faster
- To bring the oxygen sensor to operating temperature (approximately 315°C) quickly for faster closed-loop operation (Correct answer)
- To prevent the sensor from freezing in cold climates
- To increase sensor output voltage
Correct 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.
Question 6: A technician notices fuel odour from a vehicle. A smoke test reveals vapour escaping from the EVAP charcoal canister. What is the likely cause?
- Normal canister venting
- A saturated charcoal canister caused by liquid fuel flooding from overfilling or a faulty purge valve (Correct answer)
- A cracked fuel tank
- A missing gas cap
Correct 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.
310S Fuel and Emission Systems 3
What is the function of the fuel tank pressure sensor in the EVAP system?