Evaporative Emission System Monitor

The Evaporative (EVAP) emissions system monitor is an on-board strategy designed to test the proper operation of the EVAP system by checking the function of its components and ability to flow fuel vapor (hydrocarbons) to the engine. In addition, the monitor detects leaks equal to or greater than 0.040 inch by performing a vacuum check of the complete evaporative system. See Fig 1. The monitor relies upon the Canister Vent (CV) solenoid to seal the entire evaporative system from atmosphere and the Vapor Management Valve (VMV) to pull engine vacuum on the fuel tank. Then with system sealed and vacuum maintained, the monitor uses the Fuel Tank Pressure (FTP) sensor to observe the rate at which the vacuum is lost during a period of system vacuum bleed-up.

The monitor is enabled only after the following conditions are first satisfied:

• In closed-loop fuel control, during a period of 75% or greater duty-cycle purge.
• Fuel vapor in system of 0.02 pound per minute or less.
• Vehicle speed between 40 and 70 MPH.
• Engine load between 0.20 and 0.70 percent.
• Intake air temperature between 40 and 110° F.
• Fuel tank pressure between -15 and +1 inches of water.

1. The Canister Vent (CV) solenoid is a normally open solenoid used to control evaporative flow between the carbon canister and atmosphere.
2. The Vapor Management Valve (VMV) is a normally closed solenoid used to control the flow of fuel vapors into the engine.
3. Fuel Tank Pressure (FTP) sensor is used strictly by the EVAP monitor to provide fuel system pressure information to the Powertrain Control Module (PCM).
4. The EVAP flow and leakage check begins by closing the CV solenoid and opening the VMV a calibrated amount. If a target vacuum is not sensed by the FTP sensor within a given time, then a leak or flow fault exists. If the target vacuum is reached, then both solenoids are closed in order to hold the vacuum for a calibrated period of time. If the vacuum bleeds up above a fault threshold within that period time, the EVAP monitor test fails. DTCs are set after 3 unsuccessful attempts to hold vacuum. The DTCs associated with a minor or gross EVAP leak are DTCs P0442 and P0455. The DTC associated with the EVAP system unable to bleed up fuel tank vacuum is DTC P1450. All other EVAP component DTCs are tested as part of the Comprehensive Component Monitor (CCM).
5. The Malfunction Indicator Light (MIL) is illuminated after one of the above tests fail on 2 consecutive drive cycles.

Fig 1: Identifying Evaporative Emissions System Monitor Components

Courtesy of FORD MOTOR CO.