Purging function

P47.30-2179-09

Fuel tank aeration and ventilation shown
(except code (494) USA version)

45 Fuel filler neck

45/2 Float with flap

58 Fluid reservoir

75 Fuel tank

Y58/1 Purge control valve

A Air admission line and vent line

B Purge line

75/3 Vent and breather pipe

76 Vent valve (for engine 275.954/81)

77 Activated charcoal canister

Purging function requirements, general

 Circuit 87M ON (engine control ON)

 Engine started for at least 45 s

 Coolant temperature > 40°C

 Engine not in deceleration mode

 Engine not in the lambda adaptation phase

Purging, general

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.

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

- Coolant temperature sensor (/)B11/4

- Charge air temperature sensor (B17/8) and pressure sensor downstream of throttle valve actuator (B28/7), engine load

- LH and RH O2 sensors upstream of catalytic converter (G3/3, G3/4), oxygen sensor signals

- Crankshaft position sensor (L5), engine speed

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Purging function sequence

The fuel tank is ventilated through the aeration/ventilation pipe, the vent valve (for engine 275.954/81) and the activated charcoal canister.

To ventilate the fuel tank, the vent valve (for engine 275.954/81) opens at an overpressure of about 30 to 50 mbar.

The fuel vapors flow to the activated charcoal canister, where they are stored and then drawn 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 purge control valve (Y58/1) 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 (PWM signal) (with a frequency of 10 Hz).

The purge quantity is determined by continuously opening and closing the purge control valve with on-times of varying duration.

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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 according to the level of fuel vapor charge in the activated charcoal canister.