CEI Evaporative Emission Control Systems 1 — Questions and Answers
Question 1: What is the primary purpose of an evaporative emission control (EVAP) system?
- To cool fuel before it enters the engine
- To prevent fuel vapors from escaping into the atmosphere (Correct answer)
- To increase fuel efficiency by recirculating exhaust gases
- To regulate fuel pressure in the fuel rail
Correct answer: To prevent fuel vapors from escaping into the atmosphere
The EVAP system captures and stores fuel vapors from the fuel tank and fuel system, preventing them from venting directly into the atmosphere and contributing to hydrocarbon emissions.
Question 2: Which component in the EVAP system stores fuel vapors before they are purged into the engine?
- Fuel pressure regulator
- PCV valve
- Charcoal (activated carbon) canister (Correct answer)
- Throttle body
Correct answer: Charcoal (activated carbon) canister
The activated charcoal (carbon) canister adsorbs and temporarily stores fuel vapors from the fuel tank until conditions allow them to be drawn into the engine and burned.
Question 3: What is the function of the purge valve (purge solenoid) in an EVAP system?
- It vents the charcoal canister to atmosphere when vapor pressure is high
- It seals the fuel tank during refueling
- It controls airflow from the canister to the intake manifold (Correct answer)
- It regulates fuel delivery pressure from the pump
Correct answer: It controls airflow from the canister to the intake manifold
The purge valve, controlled by the PCM, opens to allow stored fuel vapors in the charcoal canister to be drawn into the intake manifold and burned during normal engine operation.
Question 4: A vehicle sets a P0442 diagnostic trouble code. What does this indicate?
- EVAP system large leak detected
- EVAP system small leak detected (Correct answer)
- Purge valve stuck open
- Charcoal canister saturated
Correct answer: EVAP system small leak detected
P0442 is defined as 'Evaporative Emission Control System Leak Detected (small leak),' indicating the EVAP system cannot maintain pressure, typically caused by a leak equivalent to a 0.020-inch orifice.
Question 5: What does the fuel tank pressure (FTP) sensor measure in an EVAP system?
- Fuel rail injection pressure
- Pressure or vacuum inside the fuel tank (Correct answer)
- Barometric pressure for altitude compensation
- Canister purge flow rate
Correct answer: Pressure or vacuum inside the fuel tank
The fuel tank pressure sensor monitors the pressure (positive) or vacuum (negative) inside the fuel tank, providing the PCM with data needed to perform EVAP leak detection tests.
Question 6: Which of the following conditions is typically required before the OBD II EVAP monitor will run?
- Engine coolant temperature above 220°F
- Fuel level between 15% and 85% (Correct answer)
- Vehicle speed above 60 mph for 10 minutes
- Engine oil pressure below 20 psi
Correct answer: Fuel level between 15% and 85%
Most OBD II EVAP monitors require a fuel level between approximately 15% and 85% to ensure there is enough vapor to detect leaks while avoiding inaccurate readings from an overfull or nearly empty tank.
Question 7: What does a P0455 diagnostic trouble code indicate?
- EVAP system small leak detected
- EVAP system gross (large) leak detected (Correct answer)
- Vent valve circuit malfunction
- Purge flow insufficient during self-test
Correct answer: EVAP system gross (large) leak detected
P0455 indicates an 'Evaporative Emission Control System Leak Detected (gross/large),' meaning the EVAP system has a significant leak, often as large as a missing or loose fuel cap.
CEI Evaporative Emission Control Systems 1
What is the primary purpose of an evaporative emission control (EVAP) system?