System Diagram

Courtesy of FORD MOTOR COMPANY

EVAP Leak Check Monitor 

The EVAP leak check monitor is an on board strategy designed to detect a leak in the EVAP system as small as 0.508 mm (0.020 inch). The EVAP leak check monitor is executed at ignition OFF if all conditions have been met on the previous drive cycle.

The correct function of the individual components of the EVAP system, and the ability of the EVAP system to flow fuel vapor to the engine, are also monitored.

For HEV and PHEV vehicles, the EVAP system is a sealed system by the use of the normally closed fuel tank isolation valve. If the natural vapor pressure or vacuum generation in the sealed fuel tank is sufficient the PCM commands the EVAP leak detection control module to check the fuel tank isolation valve, the EVAP purge valve, the switching valve in the EVAP leak detection control module and the lines between those components. The fuel tank pressure (FTP) sensor monitors the sealed portion of the EVAP system between the fuel tank isolation valve and the capless fuel filler pipe to determine the total pressure or vacuum in the fuel tank. If the target pressure or vacuum has been reached the fuel tank side of the fuel tank isolation valve is considered to have no leak. If the sealed portion of the EVAP system does not generate enough pressure or vacuum between the fuel tank isolation valve and the capless fuel filler pipe then the PCM will open the fuel tank isolation valve and will turn ON the EVAP leak detection control module vacuum pump to apply vacuum to the entire EVAP system.

The EVAP leak check monitor is initiated at ignition OFF if the conditions to run the monitor have been met during the preceding drive cycle.

The EVAP leak check monitor is executed by using the individual components of the enhanced EVAP system as follows:

• For HEV and PHEV vehicles, the FTP sensor is used to determine if the target pressure or vacuum necessary to carry out the leak check on the sealed fuel tank has been reached.
• For HEV and PHEV vehicles, the fuel tank isolation valve blocks the flow of vapors from the fuel tank to the EVAP canister. The fuel tank isolation valve is normally closed to prevent the canister from becoming saturated in a situation where the engine does not run during a drive cycle, causing hydrocarbons (HC) to be released into the atmosphere. The fuel tank isolation valve will automatically open to relieve excess pressure or vacuum in the fuel tank if the fuel tank pressure or vacuum reaches a calibrated value. The PCM will open the fuel tank isolation valve only if the sealed section of the EVAP system does not generate enough pressure or vacuum to complete the EVAP leak check monitor or for refueling.
• For HEV vehicles, the EVAP leak detection control module is used to seal the EVAP system from the atmosphere and draw a vacuum on the entire EVAP system for leak check monitoring.
  • A vacuum pump inside the EVAP leak detection control module draws a vacuum on the EVAP system as needed.
  • If the initial target vacuum cannot be reached, a leak is detected and a DTC will set.
  • The EVAP leak detection control module internal components are tested during the EVAP leak check monitor.
  • A DTC will be set if a concern is detected with the internal EVAP leak detection control module components.

• For HEV vehicles, the EVAP purge valve is used to control the purge flow from the EVAP canister during engine running conditions.
• For HEV vehicles, the EVAP canister is used to collect fuel vapors during normal engine operations and refueling.
• For PHEV vehicles, the EVAP canister is used on the sealed EVAP system to collect fuel vapors during refueling only.
• A valve inside the fuel vapor tube assembly prevents liquid fuel from entering the EVAP canister and the EVAP purge valve under any vehicle altitude, handling, or rollover condition.