CEI Evaporative Emission Control Systems 2 — Questions and Answers
Question 1: During a smoke machine test of an EVAP system, where is the smoke typically introduced?
- Into the intake manifold vacuum port
- Into the EVAP service port or vent solenoid port (Correct answer)
- Into the exhaust pipe upstream of the catalytic converter
- Into the PCV valve inlet hose
Correct answer: Into the EVAP service port or vent solenoid port
Smoke is introduced into the EVAP system through the service port or vent solenoid port with the purge valve closed, allowing the technician to pressurize the sealed system and visually locate any leaks.
Question 2: What is the function of the canister vent valve (vent solenoid) in an EVAP system?
- It opens to allow fresh air into the canister during purge and closes during leak tests (Correct answer)
- It prevents fuel liquid from entering the charcoal canister
- It regulates canister temperature to improve adsorption
- It opens the fuel filler neck vent during refueling
Correct answer: It opens to allow fresh air into the canister during purge and closes during leak tests
The canister vent valve allows fresh air into the canister during purge operations (so vapors are swept into the engine) and closes during PCM-commanded EVAP leak tests to seal the system.
Question 3: A P0446 diagnostic trouble code is most associated with which EVAP system component?
- Purge flow solenoid valve
- Fuel tank pressure sensor
- Canister vent control valve circuit (Correct answer)
- Fuel cap sealing ring
Correct answer: Canister vent control valve circuit
P0446 is defined as 'Evaporative Emission Control System Vent Control Circuit Malfunction,' indicating an electrical or functional fault in the canister vent valve circuit.
Question 4: What constitutes a 'gross leak' in OBD II EVAP system terminology?
- Any leak smaller than a 0.010-inch orifice equivalent
- A leak equal to or larger than approximately 0.080-inch orifice equivalent (Correct answer)
- Vapors leaking past the purge valve into the intake at idle
- A cracked charcoal canister body
Correct answer: A leak equal to or larger than approximately 0.080-inch orifice equivalent
A gross leak in OBD II terms is typically equivalent to an orifice of 0.080 inches or larger; a missing or loose fuel cap is a common cause and triggers a P0455 code.
Question 5: Which of the following BEST describes a non-intrusive OBD II EVAP test?
- A technician manually pressurizes the fuel tank with nitrogen
- The PCM commands tests using natural driving conditions without external equipment (Correct answer)
- A smoke machine is connected to the EVAP service port
- The fuel cap is removed and replaced to verify its seal
Correct answer: The PCM commands tests using natural driving conditions without external equipment
Non-intrusive EVAP tests use the vehicle's own PCM and existing components—such as the FTP sensor, purge valve, and vent valve—to conduct leak checks without any shop equipment connected to the vehicle.
Question 6: What effect does a saturated (fuel-soaked) charcoal canister have on vehicle emissions?
- It reduces NOx emissions by absorbing combustion byproducts
- It causes excessive hydrocarbon emissions and can damage the purge valve or engine (Correct answer)
- It improves fuel economy by returning liquid fuel to the tank
- It has no measurable effect on tailpipe emissions
Correct answer: It causes excessive hydrocarbon emissions and can damage the purge valve or engine
A saturated canister can allow liquid fuel to reach and damage the purge solenoid, flood the engine, and allow fuel vapors to escape to the atmosphere, significantly increasing hydrocarbon emissions.
Question 7: Which driving cycle condition is most likely to allow an EVAP large leak monitor to complete?
- Sustained highway driving above 70 mph
- Cold start followed by a warm-up period with moderate city driving (Correct answer)
- Aggressive acceleration from a stop repeated multiple times
- Idling for 20 or more minutes in park
Correct answer: Cold start followed by a warm-up period with moderate city driving
Many EVAP monitors require a cold start (engine at ambient temperature after extended soak) followed by warm-up, since the PCM uses natural fuel tank pressure/vacuum changes during heat soak to detect leaks.
CEI Evaporative Emission Control Systems 2
During a smoke machine test of an EVAP system, where is the smoke typically introduced?