Purging Function - GF47.30-P-3015MRA

Engine 274.9 in model 205 (except 205.047/053/054/147/247/253) 

Engine 274.9 in model 253 (except 253.354/954) 

Courtesy of MERCEDES-BENZ USA

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

Purging function requirements, general points 

• Circuit 87M (Engine management ON)
• Engine start before at least 120 s
• Coolant temperature > 70°C
• Engine not in decel 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 signals:

• Purging pressure sensor (B4/4) (with CODE 494 (USA version))
• Coolant temperature sensor (B11/4)
• Charge air temperature sensor upstream of throttle valve (B17/7)
• Charge air temperature sensor downstream of throttle valve (B17/9)
• Pressure sensor downstream of throttle valve (B28/7), engine load
• Crankshaft Hall sensor (B70), engine rpm
• Oxygen sensor, sensor element downstream of catalytic converter (G3/1b1)
• Oxygen sensor, sensor element upstream of catalytic converter (G3/2b1)

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 flow to the activated charcoal canister where they are stored and drawn off into the charge air distributor when purging is enabled.

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

To control the purge quantity, the ME-SFI [ME] control unit actuates the purging switchover valve at the ground end with a pulse width modulated signal at a frequency of 10 to 30 Hz.

The purge quantity is determined by continuously opening and closing the purging switchover valve with on-periods of varying durations.

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.