Evaporative Emissions

The control of the evaporative fuel vapors (Hydrocarbons) from the fuel tank is important for the overall reduction in vehicle emissions. The evaporative system has been combined with the ventilation of the fuel tank, which allows the tank to breath (equalization). The overall operation provides:

• An inlet vent, to an otherwise "sealed" fuel tank, for the entry of air to replace the fuel consumed during engine operation. 
• An outlet vent with a storage canister to "trap and hold" fuel vapors that are produced by the expansion/evaporation of fuel in the tank, when the vehicle is stationary. 

The canister is then "purged" using the engine vacuum to draw the fuel vapors into the combustion chamber. This "cleans" the canister allowing for additional storage. Like any other form of combustible fuel, the introduction of these vapors on a running engine must be controlled.

The ECM controls the Evaporative Emission Valve which regulates purging of evaporative vapors. The evaporative system must be monitored for correct purge operation and Leak Detection.

On-Board Refueling Vapor Recovery (ORVR - DM TL Equipped Vehicles):  The ORVR system recovers and stores hydrocarbon fuel vapor that was previously released during refueling. Non ORVR vehicles vent fuel vapors from the tank venting line back to the filler neck and in many states reclaimed by a vacuum receiver on the filling station's fuel pump nozzle.

When refueling an ORVR equipped vehicle, the pressure of the fuel entering the tank forces the hydrocarbon vapors through the larger tank vent line to the liquid/ vapor separator, through the rollover valve and into the charcoal canister. The HC is stored in the charcoal canister, and the system can then "breath" through the DM TL and the air filter. The vent Line to the filler neck is smaller, but still necessary for checking the filler cap/neck during Evaporative Leak Testing.

Liquid/Vapor Separator:  Fuel vapors are routed from the fuel tank filler neck through a hose to the Liquid/Vapor Separator. The vapors cool when exiting the fuel tank, the condensates separate and drain back to the fuel tank through a return hose. The remaining vapors exit the Liquid/ Vapor Separator to the Active Carbon Canister.

Active Carbon Canister:  As the hydrocarbon vapors enter the canister, they will be absorbed by the active carbon. The remaining air will be vented to the atmosphere through the end of the canister allowing the fuel tank to "breathe". When the engine is running, the canister is then "purged" using intake manifold vacuum to draw air through the canister which extracts the hydrocarbon vapors into the combustion chamber.

Evaporative Emission Valve:  This ECM controlled solenoid valve regulates the purge flow from the Active Carbon Canister into the intake manifold. The ECM Relay provides operating voltage, and the ECM controls the valve by regulating the ground circuit. The valve is powered open and closed by an internal spring.

If the Evaporative Emission Valve circuit is defective, a fault code will be set and the "Malfunction Indicator Light" will illuminate when the OBD II criteria is achieved.

If the valve is "mechanically" defective, a driveability complaint could be encountered and a mixture related fault code will be set.

MS42 - Evaporative System Leak Diagnosis with Leakage Diagnosis Pump (LDP):  The LDP provides a means of testing the fuel/evaporative system for leaks. The pump is activated by the ECM and pressurizes the fuel tank and evaporative system.

The LDP and charcoal canister (combination) assembly is located under the right rear trunk floor on the E46 (for example). This system is capable of detecting a leak as small as 0.5 mm. 

The LDP is a unitized component that contains the following:

• Vacuum chamber
• Pneumatic pump chamber
• DME activated vacuum solenoid
• Reed switch providing a switched voltage feedback signal to the ECM

The LDP assembly is only replaceable as a complete unitized component, however, it is separate from the charcoal canister.

The upper chamber contains an integrated reed switch that produces a switched high/low voltage signal that is monitored by the ECM.

The switch is opened by the magnetic interruption of the metal rod connected to the diaphragm when in the top dead center position.

The repetitive up/down stroke is confirmation to the ECM that the valve is functioning and the basis for determining if a leak is present in the system.

The ECM monitors the length of time it takes for the reed switch to open, which is opposed by pressure under the diaphragm in the lower chamber. If this component/circuits are defective, a fault code will be set and the "Malfunction Indicator Light" will illuminate when the OBD II criteria is achieved.

MS43 - Evaporative Leakage Detection (DM TL):  This component ensures accurate fuel system leak detection for leaks as small as 0.5 mm  by slightly pressurizing the fuel tank and evaporative components. The DM TL pump contains an integral DC motor which is activated directly by the ECM. The ECM monitors the pump motor operating current as the measurement for detecting leaks.

The pump also contains an ECM controlled change over valve that is energized closed during a Leak Diagnosis test. The change over valve is open during all other periods of operation allowing the fuel system to "breath" through the inlet filter. The DM TL is located under the luggage compartment floor with the Active Carbon Canister (E46 for example).

1. In its inactive state, filtered fresh air enters the evaporative system through the sprung open valve of the DM TL.
2. When the DME activates the DM TL for leak testing, it first activates only the pump motor. This pumps air through a restrictor orifice (0.5 mm) which causes the electric motor to draw a specific amperage value. This value is equivalent to the size of the restrictor.
3. The solenoid valve is then energized which seals the evaporative system and directs the pump output to pressurize the evaporative system.

• A large leak is detected in the evaporative system if the amperage value is not achieved.
• A small leak is detected if the same reference amperage is achieved.
• The system is sealed if the amperage value is higher than the reference amperage.