Purging system with leak test function - GF47.30-P-3016MMP

ENGINE 276.8 in MODEL 207, 212 as of model year 2014 with CODE 494 (US version) 

ENGINE 276.8 in MODEL 218 as of model year 2015 with CODE 494 (US version) 

Schematic diagram 

Fig 1: Purging System With Leak Test - Schematic Diagram

Courtesy of MERCEDES-BENZ USA

Function requirements for system purging with leak test in general 

• Engine at idle
• Vehicle stationary
• Gear range "D" or "R"
• Blocking time after engine start elapsed (approx. 3 minutes) or self-adjustment of the mixture formation occurs
• Lambda control released
• Charge air temperature <72°C
• Coolant temperature for engine start <100°C
• Air pressure >780 hPa (which means that no test takes place above an altitude of about 2500 meters)
• Low loading of activated charcoal canister
• Fuel level in the fuel tank at least 30%
• No fault in the activated charcoal canister shutoff valve
• No error on the left fuel level indicator for fuel feed module (B4/1) (model 207, 212), left fuel level indicator fuel tank fuel level sensor (B4/1) (model 218)
• No error on the right fuel level indicator for fuel feed module (B4/2) (model 207, 212), right fuel level indicator fuel tank fuel level sensor (B4/2) (model 218)
• No fault in the purging switchover valve
• Battery voltage > 11 V

Purging system with leak test, general points 

The legislator requires that no fuel vapors reaches the open air. The leak test for the evaporative emission control system must take place in three stages (function chain) and recognizes the following leak:

• Gross leak (a leak the same or greater than a 2.5 mm dia.)
• Fine leak (a leak the same or greater than a 1 mm dia.)
• Very fine leak (a leak the same or greater than a 0.5 mm dia.)

Function sequence for purging system with leak test 

The function sequence is subdivided into the following steps:

• Function sequence for gross leak-fine leak check 
• Function sequence for gross leak-very fine leak check 

Function sequence for gross leak-fine leak check 

To check for a gross leak the activated charcoal canister shutoff valve closes and the purging switchover valve opens. The vacuum created in the intake air system reaches the fuel tank.

Vacuum then builds up in the fuel tank, and is measured by the fuel tank pressure sensor.

If no vacuum builds up in the fuel tank (approx. -6 mbar within approx. 12 s), there is a major leak (e.g. fuel tank cap open, hose line loose).

The test will be interrupted and the error "gross leak" stored in the fault memory.

If the ME-SFI [ME] control unit (N3/10) detects a gross leak in the system, chassis CAN 1 (CAN E1), front SAM control unit with fuse and relay module (N10/1 and chassis CAN 2 (CAN E2) are used to actuate the fuel reserve warning lamp (A1e4) in the instrument cluster (A1) (warning lamp flashes).

The additional message "CHECK FILLER CAP" appears on the multifunction display (A1p13) when >20 liters of fuel are put into the tank. Otherwise only the error "gross leak" is saved in the fault memory.

While checking for fine leaks, a vacuum of about -6 mbar is built up again and the system is made airproof. The vacuum is them measured for approx. 30 seconds. The vacuum must not reduce more rapidly than 0.3 to 0.5 mbar per second (depends on fuel level). If the vacuum drops more rapidly, there is a fine leak.

The test will be interrupted and the error "fine leak" is stored.

If no leak can be found, although a "gross leak" fault is indicated (if the leak diameter is more than 3 mm, there should be a loud hiss), replace the fuel tank pressure sensor.

Function sequence for gross leak-very fine leak check 

If no "fine leak" has been detected, purging is briefly enabled and a vacuum of about -6 mbar is then built up again. In a closed system, the vacuum must not drop faster than 0.10 to 0.15 mbar per second (depending on fuel level). If the vacuum builds up more rapidly, the "very fine leak" fault is stored.

The reduction in vacuum depends on the fuel level in the fuel tank and on fuel outgassing. Fuel outgassing is measured shortly before fuel tank pressure sensor is checked, and taken into account for fault indication.

The activated charcoal canister shutoff valve is reopened after the tests.

The test for fine and very fine leaks is aborted if an excessive lean correction of the lambda control takes place during the buildup of the vacuum.

The function of the purging switchover valve is checked at the same time by the activation. If the closed switchover valve jams, the "gross leak" fault is stored.

If there are faults recognized then these are stored by the ME-SFI [ME] control unit in the fault memory and the engine diagnosis indicator lamp (A1e58) is actuated in the instrument cluster via chassis CAN 1, front SAM control unit with fuse and relay module and chassis CAN 2.