The Evaporative Emission (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 (located in the engine compartment, in the rear of vehicle near luggage compartment area or along the frame rail) for storage until the vapors are purged to the engine for burning.

There are three types of Evaporative Emission (EVAP) systems:

• The Vapor Management Flow system.
• The Evaporative Emission (EVAP) Running Loss system.
• The On-Board Refueling Vapor Recovery (ORVR) Evaporative Emission (EVAP) system.

The Vapor Management Flow system consists of a fuel tank, fuel filler cap, fuel vapor vent valve, EVAP canister, EVAP canister purge valve, intake air tube assembly, powertrain control module (PCM) and connecting wires and fuel vapor hoses. This system is being used on flexible fuel vehicles only. Operation of the system is as follows: (Figure 85)

1. The Vapor Management Flow system uses inputs from upon the engine coolant temperature (ECT) sensor, the intake air temperature (IAT) sensor, the mass air flow (MAF) sensor and the vehicle speed sensor (VSS) to provide information about engine operating conditions to the PCM. The conditions necessary to activate the Vapor Management Flow system is that the engine must be warm, stable, running at a moderate load and rpm, at open or part throttle position, and in close loop fuel control. The PCM deactivates the fuel vapor management flow during idle or whenever a failure is detected in the EVAP canister purge valve or fuel vapor management flow required input.

2. The PCM calculates the difference between an idle speed air control value requested at a high purge flow duty cycle and at a no purge flow duty cycle. If the difference is below the calibrated threshold result, flow is inferred not to be occurring properly, and the PCM indicates an evaporative emission system malfunction with a Diagnostic Trouble Code (DTC).

3. The PCM outputs a variable duty cycle signal (between 0% and 100%) to the solenoid on the EVAP canister purge valve.
   
   
   
   Figure 86: Vapor Management Flow System Operation (Refer to the On-Board Diagnostics II Overview System for icon definitions.)

The EVAP canister purge valve (Figure 87) is the part of the vapor management flow and EVAP running loss systems that is controlled by the PCM. This valve controls the flow of vapors (purging) from the EVAP canister to the intake manifold during various engine operating modes. The EVAP canister purge valve is a normally closed valve.

The fuel vapor vent valve assembly (Figure 94) for the Vapor Management Flow system functions in the same manner as in the EVAP Running Loss system.

The EVAP Running Loss system (Figure 86) consists of a fuel tank, fuel filler cap, fuel tank mounted or in-line fuel vapor control valve, fuel vapor vent valve, EVAP canister, fuel tank mounted or fuel pump mounted or in-line fuel tank pressure (FTP) sensor, EVAP canister purge valve, intake manifold hose assembly, canister vent (CV) solenoid, powertrain control module (PCM) and connecting wires and fuel vapor hoses.

1. The EVAP Running Loss system uses inputs from the engine coolant temperature (ECT) sensor, the intake air temperature (IAT) sensor, the mass air flow (MAF) sensor, the vehicle speed sensor (VSS) and the fuel tank pressure (FTP) sensor to provide information about engine operating conditions to the PCM. The fuel level input (FLI) and FTP sensor signals to the PCM are used by the PCM to determine activation of the EVAP Monitor based on presence of vapor generation or fuel sloshing.

2. The PCM calculates a variable duty cycle based on the desired amount of purge vapor flow to the intake manifold for a given engine condition. The PCM can then output the duty cycle to the solenoid on the EVAP canister purge valve. The PCM uses the EVAP Running Loss system inputs to evacuate the system using the EVAP canister purge valve, seals the EVAP Running Loss system from atmosphere using the CV solenoid, and uses the FTP sensor to observe total vacuum lost for a period of time.

3. The canister vent (CV) solenoid seals the EVAP Running Loss system to atmosphere during the EVAP Running Loss Monitor test.

4. The PCM outputs a variable duty cycle signal (between 0% and 100%) to the solenoid on the EVAP canister purge valve.

5. The fuel tank pressure (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. The fuel tank mounted fuel vapor vent valve assembly, fuel tank mounted fuel vapor control valve (or remote fuel vapor control valve) are used in the EVAP Running Loss system to control the flow of fuel vapor entering the engine. All of these valves also prevent fuel tank overfilling during refueling operation and prevent liquid fuel from entering the EVAP canister and the EVAP canister purge valve under any vehicle altitude, handling or rollover condition. The liquid/vapor fuel discriminator is part of the fuel vapor control valve assembly on Escort/Tracer applications.

7. The EVAP Running Loss system, including all the fuel vapor hoses, can be checked when a leak is detected by the PCM. This can be done by pressurizing the system using Rotunda Evaporative Emission Tester kit 134-00056 or equivalent and the leak (frequency) detector included with the kit.
   
   
   
   Figure 87: Evaporative Emission Running Loss System (Refer to the On-Board Diagnostics II System Overview for icon definitions.)

The EVAP canister purge valve (Figure 87) and (Figure 88) is the part of the EVAP running loss system that is controlled by the PCM. This valve controls the flow of vapors (purging) from the EVAP canister to the intake manifold during various engine operating modes. The EVAP canister purge valve is normally closed valve.

The fuel tank pressure (FTP) sensor (Figure 89) or inline fuel tank pressure (FTP) sensor (Figure 90) is used to measure the fuel tank pressure during the EVAP monitor test on vehicles equipped with the Running Loss-type system. Also, it is used to control excessive fuel tank pressure by forcing the system to purge.

During the EVAP Running Loss System test monitor, the canister vent (CV) solenoid (Figure 91) seals the EVAP canister from atmospheric pressure. This allows the EVAP canister purge valve to obtain the target vacuum in the fuel tank during the monitor run.

The fuel vapor control valve (Figure 92) is in series with the fuel vapor vent valve and the EVAP canister. This valve is referred to as a grade vent valve on some applications. The function of this fuel vapor control valve is to close the flow of liquid fuel to the EVAP canister purge valve or EVAP canister during refueling and to prevent the collection of liquid fuel in the fuel vapor hoses by overfilling the fuel tank.

The fuel vapor vent valve assembly that is mounted on the top of the fuel tank is used to control the flow of fuel vapor entering the fuel tank vapor delivery line to the EVAP canister. This valve is referred to as a fill limiting vent valve on some applications. The head valve portion of the assembly prevents the fuel tank from overfilling during refueling operations. The assembly also has a spring supported float which prevents liquid fuel from entering the fuel tank vapor delivery line under any vehicle severe handling or a vehicle rollover condition. In the vertical position, the open bottom float will lift and shut off the orifice. In the rollover position, the spring will push the float closed when the rollover angle permits liquid gasoline to reach the orifice. In the upside down position, the weight of the open bottom float and the spring force will close the orifice. Two types of fuel vapor vent valves are used that can be serviceable. They are: (1) O-ring type (Figure 93) and (2) rubber grommet type (Figure 94). The Escort/Tracer, Contour/Mystique, LS6/LS8, Ranger, Expedition/Navigator and heavy duty F-Series (F250, F350) have fuel vapor vent valves that are an integral part of the plastic fuel tank and cannot be repaired separately.

The fuel vapor separator assembly (Figure 95) is used to prevent dust and debris from entering the EVAP canister and canister vent (CV) solenoid from the atmosphere causing partial contamination or complete blockage of the EVAP canister-CV solenoid assembly.

The fuel filler cap (Figure 96) is used to prevent fuel spill and close the evaporative emission/fuel system to atmosphere.

The liquid/vapor fuel discriminator (Figure 97) is a device that separates the liquid and vapor state of fuel at the fuel tank vent and allows only the fuel vapor to move through the EVAP Running Loss system with the liquid fuel remaining in the fuel tank. This is used on Escort/Tracer (2V) and late-year introduction Contour/Mystique.

The basic elements forming the ORVR system (Figure 98) and (Figure 99) and (Figure 100) operation are as follows when fuel is dispensed:

1. The fuel filler pipe forms a seal to prevent vapors from escaping the fuel tank, while liquid is entering the fuel tank (liquid in the one inch diameter tube blocks vapors from rushing back up the fuel filler pipe).

2. A fuel vapor control valve controls the flow of vapors out of the fuel tank (valve closes when liquid level reaches a height associated with the fuel tank usable capacity). This valve accomplishes the following:
   1. Limits the total amount of fuel that can be dispensed into the fuel tank.
   2. Prevents liquid gasoline from exiting the fuel tank when submerged (and also when tipped well beyond a horizontal plane as part of the vehicle roll-over protection in road accidents).
   3. Minimizes vapor flow resistance during anticipated refueling conditions.

3. Fuel vapor tubing connects the fuel vapor control valve to the EVAP canister. This routes the fuel tank vapors (displaced by the incoming liquid) to the EVAP canister.

4. A check valve in the bottom of the fuel filler pipe prevents liquid from rushing back up the fuel filler pipe during the liquid flow variations associated with the filler nozzle shut-off.

Between refueling events, the EVAP canister is purged with fresh air so that it may be used again to store vapors accumulated engine soaks or subsequent refueling events. The vapors drawn off of the carbon in the EVAP canister are consumed in the engine

The fuel filler pipe check valve (Figure 101) on ORVR systems for Taurus/Sable (2V), Crown Victoria/Grand Marquis and Town Car is located internal to the fuel filler pipe where the tube joins the fuel tank. The purpose of this check valve is to prevent liquid fuel from re-entering the fuel filler pipe from the fuel tank on refueling.

The fill limit valve assembly (Figure 102) on the Escort/Tracer and Contour/Mystique (late-year introduction) provides two functions. These functions are to control fuel tank volume and to prevent fuel from entering the vent tube in a roll-over condition. The fill limit valve assembly is a three piece device consisting of a vent tube, a vapor seal (which has an O-ring on both ends for leak protection) and a check valve which consists of a float with a spring assembly. The vent valve during vehicle driving (in upright position), controls fuel level by a float point based on the fuel density. In a roll-over situation, the spring closes the float and the vapor seal O-rings provide the additional seal to prevent fuel from entering the vent tube.

The fill limit vent valve assembly on the LS6/LS8 is welded on the top center of the fuel tank assembly and functions and appears the same as a fuel vapor control valve (Figure 104). It is connected to the EVAP canister by the EVAP canister tube. During refueling the fuel vapor passes through the valve, which by design prevents liquid from passing and is stored in the EVAP canister. It is a single stage shut-off valve. The 1.12 cm (44 inch) and the 0.25 cm (0.10 inch) openings shut off when the fuel height reaches the valve bottom. When the fuel tank is full no vapor passes through the fill limit vent valve. The small opening of 0.10 cm (0.040 inch) the fuel vapor vent valve is the only vent for relieving the pressure in the fuel tank. When the fuel level drops below the valve bottom the fuel tank vents through the opening in the fill limit vent valve as well as the fuel vapor vent valve. There is a pressure relief valve in the fill limit vent valve. It starts opening at 12.44 kPa (50 in-H₂O) pressure and is fully open at 19.90 kPa (80 in-H₂ 0) pressure in the event of a fuel tank excessive pressure condition.

The fuel filler pipe flapper valve (Figure 103) is located internally between bottom of the fuel filler pipe and the fuel tank. The purpose of this valve is to minimize fuel flow from backing up the fuel filler pipe. Also, this valve is not a positive seal to the fuel tank.

The fuel vapor control valve (fuel tank mounted) (Figure 104) is part of the On-Board Refueling Vapor Recovery (ORVR) system used on the Mustang, Taurus/Sable, Crown Victoria/Grand Marquis, LS6/LS8, Town Car and Continental for preventing liquid fuel from entering the EVAP canister and EVAP canister purge valve.

The fuel vapor vent valve (Figure 93) and (Figure 94), dependent of vehicle application), for the ORVR systems functions in the same manner as in the EVAP Running Loss system.

The liquid/vapor fuel discriminator (Figure 97) description for the ORVR system on the Escort/Tracer and Contour/Mystique is located in the EVAP Running Loss system hardware in this section.

The fuel vapor recovery system primary component on the Escort/Tracer and Contour/Mystique is a fixed orifice and a roll-over check valve contained in a T - connector assembly (Figure 105). The T - connector assembly is located between the liquid/vapor fuel discriminator inlet port (hose) and the fuel tank vent tube path. The orifice recirculates fuel vapor to the fuel filler neck/pipe during the refueling process to displace fresh air (along with vapor) pulled into the fuel filler neck/pipe by the refueling nozzle. The system vapor generation is reduced by replacing the fresh air with the recirculated vapor. The roll-over feature of the T- connector protects against fuel from passing back from the fuel tank through the fuel filler neck/pipe and down the fuel filler recirculation tube during a crash roll-over. This prevents fuel leakage in cases where fuel vapor hoses or EVAP components may be displaced during a high speed crash.

The fuel vapors from the fuel tank are stored in the EVAP canister. When the engine is running above idle, the vapors are purged from the EVAP canister into the engine for combustion. OBD II vehicle EVAP canisters are sometimes used in pairs depending on the size or number of fuel tanks. The OBD II vehicle applications and number of EVAP canisters used are shown in the following table:

 EVAPORATIVE EMISSION SYSTEM EVAP CANISTER CONFIGURATIONS