Purging Function - GF47.30-P-3015MML
Engine 157.9, 276.8, 278.9 in model 217, 222
Schematic diagram of fuel tank aeration and ventilation (shown except code 494 (USA version))
Purging function requirements, general points
- Circuit 87M (Engine management 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 signals:
- Coolant temperature sensor (B11/4), coolant temperature
- Charge air temperature sensor (B17/8), charge air temperature
- Pressure sensor downstream of throttle valve (B28/7), engine load
- Crankshaft Hall sensor (B70), engine rpm
- Oxygen sensor elements at left and right upstream of catalytic converter (G3/3b1, G3/4b1), oxygen and nitrogen oxide content
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.
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.
Function sequence for purging
The purging is described in the following points:
- Function sequence for neutral purging
- Function sequence for wide open throttle purging
Function sequence for neutral purging
In idle mode, the fuel vapors stored in the activated charcoal canister are drawn off by the existing vacuum, over the purging switchover valve, the partial load operation check valve and the partial load operation bleed line behind the throttle valve and are burned in the engine.
Function sequence for wide open throttle purging
Charging induces an overpressure behind the throttle valve in wide open throttle operation which prevents any extraction of the fuel vapors through the idle operation vent line. The idle operation check valve prevents any pressure buildup towards the activated charcoal canister.
If an overpressure occurs in wide open throttle operation, the fuel vapors are extracted over the wide open throttle operation check valve and the wide open throttle vent line. The vacuum required for this is generated by a Venturi nozzle. The fuel vapors are then routed over the wide open throttle vent line, ATL and the charge air cooler to the charge air distributor.
To check whether the wide open throttle purging can be enabled, the purging switchover valve is opened suddenly three times in succession. Pressure peaks of > 120 mbar must occur here. The purging pressure sensor (B4/4) (for code 494 USA version) records the given pressure conditions in the wide open throttle bleed line, and forwards a signal to the ME-SFI [ME] control unit.
Schematic diagram of the wide open throttle purging
| Electrical function schematic for purge control | PE47.30-P-2052-97SED | ||
| Overview of system components for gasoline injection and ignition system with direct injection | Engine 157.9, 278.9 in model 217, 222 | GF07.70-P-9998MML | |
| Engine 276.8 in model 217, 222 | GF07.70-P-9998MMT |