Automotive Service Excellence (ASE) A8 Engine PerformanceEmissions Control Systems Diagnosis and RepairHard
A technician is diagnosing a vehicle with a 'Check Engine' light and a DTC P0440 (Evaporative Emission System Malfunction). The technician connects a scan tool and commands the EVAP purge valve to 0% duty cycle (closed) and then to 100% duty cycle (open). During the 100% duty cycle command, the fuel tank pressure (FTP) sensor reading changes significantly. What does this indicate about the purge valve?
- AThe fuel tank pressure sensor is faulty.
- BThe purge valve is stuck open.
- CThe purge valve is stuck closed.
- DThe purge valve is operating correctly.
Show answer & explanationAnswer & explanation
Correct answer: D. The purge valve is operating correctly.
When the purge valve is commanded open (100% duty cycle), it should create a vacuum in the EVAP system, drawing vapors into the engine. A significant change in the Fuel Tank Pressure (FTP) sensor reading (indicating a vacuum being pulled) confirms that the purge valve is opening and allowing flow, thus operating correctly.
Why the other options are wrong
- A. If the FTP sensor were faulty, its reading might not change at all, or it could show erratic readings, but a significant change in response to a commanded purge valve opening suggests the sensor is working, and the valve is too.
- B. If the purge valve were stuck open, the FTP sensor would likely show a constant vacuum or not change significantly when commanded open, as it's already open.
- C. If the purge valve were stuck closed, the FTP sensor reading would not change significantly when commanded to 100% duty cycle, as no vacuum would be pulled.
EVAP Purge Valve Test
Testing the EVAP purge valve involves commanding it open and observing a corresponding change in fuel tank pressure sensor readings to confirm proper operation.
- Scan tool used to cycle purge valve.
- FTP sensor monitors vacuum/pressure changes.
- Change in FTP indicates valve is opening and flowing.
Memory trick: Command purge open, FTP drops: valve works, the system hops!