Answer :
The Powertrain Control Module (PCM) on a Plug-in Hybrid Electric Vehicle (PHEV) is responsible for controlling various functions of the car's powertrain, including emissions systems like the Evaporative Emission Control System (EVAP). The EVAP system is designed to prevent fuel vapors from escaping into the atmosphere and consists of components such as a fuel tank, vapor lines, a carbon canister, and a purge valve.
To test for leaks in the EVAP system, the PCM typically uses a series of tests and checks based on pressure and vacuum conditions. Here’s how it usually works:
Sealing the System: The PCM initiates a test by sealing the system. This includes closing the vent valve to ensure that no external air enters the system.
Creating Vacuum or Pressure: The PCM then uses the canister purge valve to create a vacuum or, in some instances, applies pressure within the system. Sensors measure the rate at which the pressure or vacuum decays.
Monitoring with Sensor: The PCM uses a fuel tank pressure sensor to monitor the pressure or vacuum level within the EVAP system. It looks for changes that would indicate a leak.
Interpreting Results: If the pressure or vacuum decreases too rapidly, it could indicate a leak. The PCM will then store a diagnostic trouble code (DTC) which can be accessed by a technician during diagnostics.
The PCM can perform these tests during specific driving conditions or when the vehicle is off, known as a "key-off" test, depending on the design of the vehicle sometimes at different intervals.
Through this testing process, the PCM ensures that the EVAP system is functioning properly, thus helping to keep emissions within legal limits and protecting the environment.
