Fuel System, Checking For Leaks

• The function of the leak detection system is based on the premise that the fuel system creates a natural vacuum when the fuel in the fuel tank becomes colder.
• The Fuel Tank Pressure Sensor -G400- -2- senses the vacuum in the bleed line from the EVAP canister -3-. The Fuel Tank Leak Detection Control Module -J909- -5- is attached directly to the wall of the fuel tank and monitors the temperature inside the fuel tank -4-. The system is considered to be closed if the measured vacuum is 2.5 mbar (0.036 psi). If there is no vacuum, then there is a leak in the fuel system, which will result in an entry in the engine control module DTC memory.
  Fig 1: Fuel System

  Courtesy of AUDI OF AMERICA, LLC

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Bleeder Line to Engine