Purging function - GF47.30-P-3015MM

ENGINE 276.9 in MODEL 204, 207, 212 (except 212.095), 218 

Schematic diagram of fuel tank aeration and ventilation (except code 494 (USA version)) 

Fig 1: Fuel Tank Aeration And Ventilation - Schematic Diagram

Courtesy of MERCEDES-BENZ USA

Purging function requirements, general points 

• Circuit 87M (engine timing ON)
• Engine started for at least 60 to 80 s
• Coolant temperature > 40°C
• Engine not in deceleration mode
• Engine not in the lambda adaptation phase

Purging, general points 

When the fuel tank is being ventilated, no fuel vapors should escape into the outside air.

The fuel vapors are stored in the activated charcoal canister and burnt later in the engine.

In order to enable the purging, the ME-SFI [ME] control unit (N3/10) reads in the following sensors:

• Intake air temperature sensor (B2/5b1) (model 204, 207, 212), intake air temperature sensor (B2/5b1) (model 218) (for code 920 (Gasoline direct injection with stratified charge))
• Coolant temperature sensor (B11/4) (model 204, 207, 212), coolant temperature sensor (B11/4) (model 218)
• Intake air temperature sensor (B17) (model 204, 207, 212), intake air temperature sensor (B17) (model 218) (except code 920 (Gasoline direct injection with stratified charge))
• Intake manifold intake air temperature sensor (B17/1)
• Pressure sensor downstream of throttle valve (B28/7), engine load
• Crankshaft Hall sensor (B70), engine RPM
• Left and right oxygen sensors upstream of catalytic converter (G3/3, G3/4)

Function sequence for purging 

Ventilation of the fuel tank (operational ventilation) takes place over the aeration/vent valves, the aeration/vent lines and the activated charcoal canister.

The fuel vapors here flow to the activated charcoal canister, where they are stored and from where they are extracted into the intake manifold when purging is enabled.

When the engine is running, the fuel vapors stored in the activated charcoal canister are drawn off through the purging switchover valve and combusted in the engine.

The purging switchover valve is actuated by the ME-SFI [ME] control unit at the ground end to regulate the purging quantity by means of an electric signal between 0 and 500 mA.

The purging quantity is determined through regulation of the opening cross-section of the purging switchover valve.

The idle speed control prevents the purging from causing a change in RPM when the engine is at idle. The fuel/air mixture is leaned out depending on the level of fuel vapors load in the activated charcoal canister.