EVAP System, Checking for Leaks

Function 

The leak detection system recognizes leaks in the complete fuel system, including the fuel tank, EVAP canister and the Evaporative Emission (EVAP) Canister Purge Regulator Valve N80.

On the mechanical side, the leak detection system consists mainly of a vacuum-driven diaphragm pump equipped with a reed contact on the actuation rod of the pump diaphragm. The pump is supplied with vacuum by the engine intake stroke.

• When leak detection is activated, the diaphragm pump pumps the fuel system up to 20 to 30 mbar. The pump diaphragm is then moved out so that the reed contact at the operating rod remains in the "open" position.
• During the leak diagnosis, the reed contact is monitored to check if it remains open within 10 Sec. This indicates the system is sealed.
• If the pressure falls (indicating a leak), the diaphragm moves back and the reed contact closes.
• If the reed contact closes within 10 Sec. of leak detection time, a further test is performed: In this case the diaphragm pump pumps four times again. The Engine Control Module (ECM) J623 measures the time for each of the four pumps until the reed contact is closed again. From that, the control module can recognize a "small leak" (greater than 1 mm in diameter) or a "large leak.".

Leak diagnosis is activated automatically shortly following every engine start. If a malfunction is determined, an entry is made to the Diagnostic Trouble Code (DTC) memory. The Malfunction Indicator Lamp (MIL) K83 in the instrument cluster lights up if the malfunction is recognized for two subsequent starts.

Special tools, testers and auxiliary items required 

• Smoke tester.
• EVAP and Fuel Supply System Vacuum hose and line routing diagram.

Leak checking 

• Using a Smoke tester , check the Evaporative Emission (EVAP) canister system for leaks.

If a leak is detected:

• Check the fuel filler cap seal for damage and for proper installation. Replace if necessary.
• Check the seals under the locking flanges and the union nuts are properly tightened on the fuel tank.
• Check all hose connections of the fuel supply system and replace or repair any leaking lines.
• Check all hose connections of the EVAP system and replace or repair any leaking lines.

• Repair or replace any damaged component.

If no leaks are found in the EVAP and Fuel Supply System:

• Erase the DTC memory if a DTC was set. Refer to → Diagnostic Mode 04 - Reset/Delete Diagnostic Data .
• Perform a road test to verify repair.

If a DTC was set and does not return:

Repair complete, Generate readiness code. Refer to → Readiness Code .

• End diagnosis.

If the DTC does return and no leaks are found in the EVAP and Fuel Supply System:

• Check the DTC memory for any DTC codes pertaining to the EVAP and Fuel Supply Systems. If a DTC is stored, perform the diagnostic test procedure for the suspected component. Refer to → Diagnostic Mode 03 - Interrogating Fault Memory .