Fuel Tank: Testing and Inspection

If a leak in the tank ventilation system is indicated by the "Check engine" indicator lamp in the instrument cluster lighting up, a leak test must be carried out.

First the fault memory of the DME control unit must be read out and a diagnosis conducted. If the sensors are OK, then begin the leak test.

The following conditions must be observed to obtain plausible measuring results:
- Contents of fuel tank: 50% ± 25%.
- Park the vehicle in the workshop at least 2 hours before the test so that the fuel temperature is approximately that of the workshop temperature (ideal fuel temperature approx. 10-20°C).
- Never refuel the vehicle directly prior to the leak test due to the strong emission of gas by the fresh fuel.

PROCEDURE:
- Remove rear left wheel.
- Remove left rear wheel housing trim.




- Remove air cleaner (2) of leak diagnosis pump from retaining bracket of activated carbon filter from below and seal with special tool 16 1 161.
- Use hose clamp (1).




- Remove tank filler cap and connect special tool 16 1 150 to tank filler neck.




- Screw in rear towing eye and hook special tool 16 1 150 into it.
- Connect special tool to workshop compressed air system (8 - 10 bar) with compressed air hose (1).
- Connect pressure sensor (Diagnosis and Information System) with measuring range of 0-3.5 bar.

CAUTION: Make sure that the pressure regulator on special tool 16 1 150 is set completely in the "-" direction and the non-return cock between the pressure regulator and the pressure sensor is closed.


- Select the "Measuring Technology" function on the Diagnosis and Information System (DIS). Set the measuring range in accordance with the pressure sensor to 0-3.5 bar.
- Open the non-return cock on special tool 16 1 150 and increase the pressure in the tank system by 0.050 bar with the pressure regulator.

CAUTION: Never increase the pressure by more than 0.100 bar, as otherwise damage to the fuel tank and tank ventilation system is possible.

- Close non-return cock.
- Wait for approx. 20 secs. for system to settle, read off pressure value then wait for 60 secs.
- Read off final pressure value and compare with starting pressure value. If the pressure drop is in the area between 0.008 and 0.12 bar, the measurement must be repeated to obtain a confirming result.
- If the pressure drop in 60 sec. is greater than 0.010 bar, the system is impermissibly leaky.

CONDUCTING LEAK TEST:

NOTE: Charge fuel tank and tank ventilation system with pressure.

During the leak test with a leak detector, be sure to follow the operating instructions of the equipment manufacturer.





CAUTION: Escaping fuel vapors; only work in well-ventilated rooms or use suitable exhaust extraction unit. Observe country-specific accident prevention and occupational safety regulations.




- The leak test can be carried out with a leak detector.
- Possible causes may be:
- Fuel filler cap leaky.
- Tank ventilation lines leaky (fuel tank-activated carbon canister-tank ventilation valve).
- Tank ventilation valve leaky (engine compartment).
- Tank sensor on fuel tank leaky.
- Activated carbon canister non-return valve leaky (spare wheel recess).