Standard Models (Non Bi-Fuel)

EVAPORATIVE EMISSION SYSTEMS

Overview
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 for storage until the vapors are purged to the engine for burning.

All applications required to meet OBD-II regulations, utilize the Enhanced Evaporative Emission (EVAP) System. Some applications also incorporate an On Board Refueling Vapor Recovery (ORVR) System.

Enhanced Evaporative Emission (EVAP) System

Enhanced Evaporative Emission System:

The Enhanced EVAP system (Figure 100) 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 Enhanced EVAP system uses inputs from the engine coolant temperature (ECT) sensor or cylinder head temperature (CHT) 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 leak check Monitor based on 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 can then output the required signal to the EVAP canister purge valve. The PCM uses the Enhanced EVAP system inputs to evacuate the system using the EVAP canister purge valve, seals the Enhanced EVAP 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 Enhanced EVAP system to atmosphere during the EVAP leak check Monitor.
4. The PCM outputs a variable duty cycle signal (between 0% and 100%) to the solenoid on the EVAP canister purge valve. On applications with Electronic EVAP Canister Purge Valve, the PCM outputs a variable current (between 0 mA and 1000 mA).
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 Enhanced EVAP 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.
7. The Enhanced EVAP system, including all the fuel vapor hoses, can be checked when a leak is detected by the PCM. Refer to the appropriate Vehicle System for information on leak detection tools and procedures.

Hardware

EVAP Canister Purge Valve

EVAP Canister Purge Valve:

Electronic EVAP Canister Purge Valve:

The EVAP canister purge valve (Figure 101) is part of the Enhanced EVAP 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 electronic EVAP canister purge valve (Figure 102) controls the flow of vapors electronically by way of a solenoid thereby, eliminating the need for an electronic vacuum regulator and vacuum diaphragm. The PCM outputs a signal between 0% and 100% duty cycle to control the EVAP canister purge valve. On applications with Electronic EVAP canister purge valve, the PCM outputs a signal between 0 mA and 1000 mA to control the solenoid.

Fuel Tank Pressure Sensor

Fuel Tank Pressure (FTP) Sensor:

In-Line Fuel Tank Pressure (FTP) Sensor:

The fuel tank pressure (FTP) sensor (Figure 103) or inline fuel tank pressure (FTP) sensor (Figure 104) is used to measure the fuel tank pressure during the EVAP Leak Check Monitor.

Canister Vent Solenoid

Typical Canister Vent (CV) Solenoid:

During the EVAP Leak Check Monitor, the canister vent (CV) solenoid (Figure 105) 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 EVAP Leak Check Monitor.

EVAPORATIVE EMISSIONS

The evaporative emission (EVAP) system:
- prevents hydrocarbon emissions from reaching the atmosphere.
- stores fuel vapors in the EVAP canisters that are generated during vehicle operation or hot soak until they can be consumed by the engine during normal engine operation.
- routes the stored fuel vapors to the engine during engine operation.
- is controlled by the powertrain control module (PCM) which, using various sensor inputs, calculates the desired amount of purge flow. The PCM regulates the purge flow, induced by the application of intake manifold vacuum, by varying the duty cycle applied to the EVAP canister purge valve.
- has an EVAP test port for test purposes.

The fuel vapors are routed:
- from the fuel tank through the fuel vapor vent valve.
- to the EVAP canisters through a vapor line.
- to the engine when the EVAP canister purge valve is opened by the PCM.

The fuel tank pressure (FTP) sensor:
- monitors the pressure levels in the fuel tank.
- communicates the pressure reading to the PCM during the OBD II leak test.

The evaporative emissions (EVAP) canisters:
- are located under the vehicle along the left rocker panel.
- contain activated carbon.
- store fuel vapors.

The fuel tank filler cap:
- relieves system vacuum below 3.8 kPa (15.26 inches H2O).

The canister vent solenoid:
- is normally open.
- seals the evaporative emissions system for the inspection and maintenance (I/M 240) test and OBD II leak and pressure tests.
- is mounted to the evaporative emissions canister.

The evaporative emission (EVAP) canister purge valve:
- is normally closed.
- regulates the purging of the EVAP canisters.
- is controlled by the PCM.

The evaporative emission (EVAP) system test port:
- is a part of the EVAP canister purge valve.
- is used to connect the Evaporative Emissions System Leak Tester to the EVAP system.

The evaporative emission (EVAP) dust separator:
- is attached to the rear of the EVAP canister bracket.
- prevents suspended dust and dirt particles from entering the evaporative emission system.
- is repaired as a separate item.

The evaporative emission system monitor:
- is a self-test strategy within the powertrain control module (PCM) which tests the integrity of the EVAP system.
- monitors the EVAP system for leaks.
- monitors electronic EVAP components for irrationally high or low voltages.
- monitors for correct EVAP system operation.
- uses negative and positive leak test methods to test and activate the EVAP system.

The EVAP Running Loss System leak test:
- utilizes intake manifold vacuum to test the system and involves several stages.

PRINCIPLES OF OPERATION

Evaporative Emission System:

Evaporative Emission (EVAP) Canister Purge Valve
The EVAP canister purge valve is controlled by the powertrain control module (PCM). The EVAP canister purge valve controls the flow of fuel vapors from the EVAP canisters to the engine intake manifold during various engine operating modes. The EVAP canister purge valve is normally closed.

Evaporative Emission (EVAP) Canisters
Fuel vapors from the fuel tank are stored in the EVAP canisters. When the engine is running, the vapors are purged from the EVAP canisters for combustion.

Canister Vent Solenoid
During the Evaporative Emission Running Loss System Monitor Test, Evaporative Emissions Repair Verification Drive Cycle, and the Evaporative Emission System Leak Test, the canister vent solenoid is closed to allow either a vacuum to be drawn on the fuel tank or to hold a specified pressure in the system. The canister vent solenoid is normally open.

Fuel Tank Pressure (FTP) Sensor
The fuel tank pressure (FTP) sensor is used to measure the fuel tank pressure during the Evaporative Emissions Monitor Test. It is also used to control excessive fuel tank pressures by forcing the EVAP system to purge. The fuel tank pressure sensor is mounted in the main vapor line.

Fuel Vapor Vent (FVV) Valve Assembly
The fuel vapor vent (FVV) valve assembly is mounted on the top of the fuel tank. It is used to control the flow of fuel vapors entering the EVAP system. The head portion of the assembly prevents the fuel tank from overfilling during refueling. The assembly also has a spring float, which prevents liquid fuel from entering the vapor delivery system under severe handling or vehicle rollover conditions. In the upright position, the open bottom of the float will lift and shut off the orifice. Under severe handling conditions, the spring will push the float closed when angles allow liquid fuel to reach the orifice. In a rollover condition, the weight of the open bottom float and spring pressure will close the orifice.

Fuel Filler Pipe Check Valve
The fuel filler pipe check valve is an integral part of the fuel tank or the fuel filler pipe. It is intended to prevent liquid fuel from re-entering the fuel filler pipe from the fuel tank after refueling.

Fuel Filler Cap
The fuel filler cap is used to prevent fuel spill and to close the EVAP system to the atmosphere.

Fill Limit Vent Valve (FLVV)
The fill limit vent valve is mounted on top of the fuel tank. It has a main orifice which allows fuel vapor to route to the carbon canisters during refueling. This orifice is controlled by a float, when the tank is full the flow of vapor is stopped and prevents the tank from overfilling.

Evaporative Emission (EVAP) System Monitor
When a fault occurs, the EVAP system monitor is reset to NO and a diagnostic trouble code (DTC) is set in the PCM memory. After the DTC is repaired, the vehicle drive cycle must be completed to reset the monitor in preparation for inspection and maintenance testing.

EVAP Running Loss System Leak Test
To start the testing, conditions of stable purging and vehicle speed must be satisfied. During the first stage, the EVAP canister vent solenoid is closed, while the EVAP canister purge valve remains open, applying and building vacuum in the system as indicated by the FTP sensor. This phase checks for major leaks in the EVAP system.

In the second stage, the EVAP canister purge valve closes and the system looks for minimal decay rate in the vacuum, indicating the absence of any small EVAP system leaks.

The last stage is entered only if stage two of the leak test has failed and checks whether the failed test was due to excess vapor generation. It monitors fuel vapor generation rate. Initially, the canister vent solenoid is opened to equalize EVAP system pressure to atmosphere. Then the canister vent solenoid is closed, allowing pressure to build if vapor generation is present in sufficient quantity. If the rate of generation is found to be too high, the EVAP running loss system leak test is aborted. If not, then a small leak is diagnosed.