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

ENGINE 157.9 in MODEL 212, 218 as of model year 2014 with CODE 494 (US version) 

ENGINE 278.9 in MODEL 207, 212, 218 as of model year 2014 with CODE 494 (US version) 

Schematic diagram 

Fig 1: Purging System With Leak Test Function Diagram

Courtesy of MERCEDES-BENZ USA

Function requirements for system purging with leak test 

• 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 enabled
• Charge air temperature
• 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 sensor (B4/1) (model 207, 212), left fuel indicator fuel tank fuel level sensor (B4/1) (model 218)
• No error on the right fuel level indicator sensor (B4/2) (model 207, 212), right fuel indicator fuel tank fuel level sensor (B4/2) (model 218)
• No fault in the purge control valve
• Battery voltage > 11 V

Purge system with leak test, general points 

The legislator requires that no fuel vapors reaches the open air.

The leak test for the fuel evaporation control system must take place in three stages (function chain) and recognizes the following leak:

• Gross leak (a leak the same or greater than 3 mm in dia.)
• Fine leak (a leak the same or greater than 1 mm in dia.)
• Very fine leak (a leak the same or greater than 0.5 mm in dia.)

Function sequence for purge system with leak test 

The function sequence is subdivided into the following steps:

• Gross leak check function sequence 
• Fine leak check function sequence 
• Very fine leak check function sequence 

Gross leak check function sequence 

The activated charcoal canister shutoff valve is closed and the purge switchover valve is opened. The vacuum in the intake manifold reaches the fuel tank.

As a result, vacuum builds up in the fuel tank, and is measured by the fuel tank pressure sensor (B4/3).

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

The test will be interrupted and the error "major 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) 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 filled into the tank. Otherwise only the error "gross leak" is saved in the fault memory.

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. The fuel tank pressure sensor may be faulty to the extent that the signal remains constant and plausible (sensor sticking).

Fine leak check function sequence 

A vacuum of about -6 mbar is built up 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.

Very fine leak check function sequence 

If no "fine leak" has been detected, purging is briefly enabled and a vacuum of about -17 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 "micro 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 minor and micro leaks is aborted if an excessive lean correction of the lambda control takes place during the buildup of the vacuum.

The function of the purge control valve is checked at the same time by the activation. If the closed switchover valve jams, the "major 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 and chassis CAN 2.