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 98 MY E39):  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 LDP 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 (E36 shown). 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.

Fuel Tank Pressure Sensor (E36/Z3 Vehicles):  The pressure transducer mounted on the liquid/ vapor separator (fuel tank on Z3) provides the fuel tank pressure input to the ECM. This is used by the ECM to check the fuel storage and evaporative (purge) system for leaks.

The pressure sensor receives a power supply from the ECM and produces a varying voltage input (0-5v) to the ECM, representing the amount of pressure in the fuel tank/evaporative system.

If this input is defective, a fault code will be set and the "CHECK ENGINE" Light will illuminate when the OBD II criteria is achieved.

Active Carbon Canister:  As the hydrocarbon vapors enter the 3 liter 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 "breath". 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. The canister is located in the spare tire well (E36 shown).

Air Inlet Shut Off Valve (E36 VEHICLES):  A shut-off valve is installed on the intake or vent side of the charcoal canister as a component of the vacuum type Evaporative Fuel System Leak system.

Operation of the valve is controlled by the ECM. When the shut-off valve and purge valve are closed, the entire fuel tank/evaporative system is sealed from the atmosphere.

The shut-off valve is required to test the system for leaks. If this valve is defective, a fault code will be set and the "CHECK ENGINE" Light will illuminate when the OBD II criteria is achieved.

Evaporative Emission Valve:  This ECM controlled solenoid valve regulates the purge flow from the Active Carbon Canister into the intake manifold (located next to the HFM). 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 "CHECK ENGINE" 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.

LDP (Leak Diagnosis Pump - E39):  The LDP provides a means of testing the fuel/evaporative system for leaks. The pump is activated by ECM control. It pressurizes the fuel tank and evaporative system.

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 "CHECK ENGINE" Light will illuminate when the OBD II criteria is achieved.

Barometric Pressure Sensor (98 MY E39):  The 1998 528i with MS 41.1 requires an additional input signal for activation of the LDP function test.

The sensor is located on passenger side strut tower forward of ASC 5 Hydraulic Unit.

The LDP test function can occur during a long trip. If the test occurs while the vehicle is driven down a very steep grade just prior to the LDP test, the barometric pressure sensor provides a rapid change (varied DC voltage signal) informing the ECM to postpone the test function.

If this component/circuits are defective, a fault code will be set and the "CHECK ENGINE" Light will illuminate when the OBD II criteria is achieved.