Evaporative Emission (EVAP) Systems
EVAPORATIVE EMISSION (EVAP) SYSTEMSOverview
The EVAP system prevents fuel vapor build-up in the sealed fuel tank. Fuel vapors trapped in the sealed tank are vented through the vapor valve assembly on top of the tank. The vapors leave the valve assembly through a single vapor line and continue to the EVAP canister for storage until the vapors are purged to the engine for burning.
All applications required to meet on board diagnostics (OBD) regulations use the enhanced EVAP system. Some applications also incorporate an on-board refueling vapor recovery (ORVR) system. Refer to the Evaporative Emissions for vehicle specific information on the description and operation of the evaporative emission system.
Enhanced Evaporative Emission (EVAP) Natural Vacuum Leak Detection (NVLD) System - Fiesta
The enhanced EVAP NVLD system consists of the capless fuel tank filler pipe, EVAP canister, normally closed EVAP purge valve, fuel tank, fuel tank mounted vapor control valve, fuel vapor hoses, fuel vapor vent valve, intake manifold hose assembly, the NVLD module, and powertrain control module (PCM). The PCM and the NVLD module check the entire EVAP system, including all the fuel vapor hoses from the NVLD module to the intake manifold, for a leak when the calibrated conditions are met. For additional information on the EVAP system components, refer to Engine Control Components.
1. The PCM uses inputs from the engine coolant temperature (ECT) sensor, the fuel level input (FLI), the intake air temperature (IAT) sensor, the mass airflow (MAF) sensor, the NVLD ambient air temperature sensor, the vehicle speed sensor (VSS) and the NVLD module to determine conditions of the enhanced EVAP system. The PCM uses the combination of these signals to determine when to activate the EVAP leak check monitors.
2. The PCM uses inputs from the fuel level input (FLI), the NVLD ambient air temperature sensor, and the EVAP canister load to determine the desired amount of purge vapor flow to the intake manifold for a given engine condition. The PCM then outputs the required signal to the EVAP purge valve. The PCM uses the enhanced EVAP system inputs to evacuate the system using the EVAP purge valve.
3. The PCM outputs a duty cycle between 0% and 100% to control the EVAP purge valve.
4. The NVLD module vacuum switch and the NVLD relief valves seal the enhanced EVAP system from the atmosphere during the EVAP leak check monitors.
5. 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.
Enhanced Evaporative Emission (EVAP) System - All Others
The enhanced EVAP system consists of a fuel tank, fuel filler cap or capless fuel tank filler pipe, fuel tank mounted or inline fuel vapor control valve, fuel vapor vent valve, EVAP canister, fuel tank mounted or fuel pump mounted or inline fuel tank pressure (FTP) sensor, EVAP purge valve, EVAP check valve (turbocharged engines only), intake manifold hose assembly, EVAP canister vent valve, PCM and connecting wires, and fuel vapor hoses. The enhanced EVAP system, including all the fuel vapor hoses, can be checked when a leak is detected by the PCM. For additional information on the EVAP system components, refer to Engine Control Components.
1. The enhanced EVAP system uses inputs from the ECT sensor or cylinder head temperature (CHT) sensor, the IAT sensor, the MAF sensor, the VSS and the FTP sensor to provide information about engine operating conditions to the PCM. The PCM uses the FLI and FTP sensor signals to determine activation of the EVAP leak check monitor based on the presence of vapor generation or fuel sloshing.
2. The PCM determines the desired amount of purge vapor flow to the intake manifold for a given engine condition. The PCM then outputs the required signal to the EVAP purge valve. The PCM uses the enhanced EVAP system inputs to evacuate the system using the EVAP purge valve, seal the enhanced EVAP system from the atmosphere using the EVAP canister vent valve, and uses the FTP sensor to observe total vacuum lost for a period of time.
3. The EVAP canister vent valve seals the enhanced EVAP system to atmosphere during the EVAP leak check monitor.
4. For Flex 3.5L 4V, Fusion 3.5L, MKS 3.7L, MKT 3.7L, MKZ 3.5L, Taurus 3.5L 4V and Transit Connect the PCM outputs a variable current between 0 and 1,000 mA to control the EVAP purge valve. For all others, the PCM outputs a duty cycle between 0% and 100% to control the EVAP purge valve.
5. The FTP sensor monitors the fuel tank pressure during engine operation and continuously transmits an input signal to the PCM. During the EVAP monitor testing, the FTP sensor monitors the fuel tank pressure or vacuum bleed-up.
6. A valve inside the fuel tank-mounted 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.
7. On turbocharged engines, the EVAP check valve prevents boost pressure from entering the EVAP system.
8. On turbocharged engines, the turbocharger creates boost pressure in the intake manifold.
9. On turbocharged engines, an EVAP ejector (if equipped) is used to create a vacuum in the purge line during boost conditions. When in boost conditions a percentage of the boost pressure is applied to the EVAP ejector to create a vacuum. This vacuum draws purge vapors through the EVAP ejector into the intake air system upstream of the turbocharger.
