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Checking vacuum system > < Safety precautions when working on the ignition system

A4 Mk3

Reducing pressure in high-pressure section of injection system
Reducing pressure in high-pressure section of injection system
  WARNING
The fuel system operates at extremely high pressure. This can cause injury.
t  The injection system consists of a high-pressure section (maximum approx. 140 bar) and a low-pressure section (approx. 7 bar).
t  Prior to opening the high-pressure section (e.g. when removing the high-pressure pump, fuel rail, injectors, fuel lines or fuel pressure sender -G247-), the fuel pressure in the high-pressure section must be reduced to a residual pressure of approx. 7 bar. The procedure is described below.
Reducing fuel pressure in high-pressure section
–  Connect a → Vehicle diagnostic tester.
–  Start engine and run at idling speed.
–  Select „Engine electronics“ in vehicle self-diagnosis.
–  Then select „Measured values“.
–  Select „Fuel pressure“ from the list.
   
–  Pry off cover -1- on left side/right side of dash panel using removal wedge -3409--arrow- or similar.
  A70-10665
–  With engine idling, pull out fuse for fuel pump control unit -J538- → Current flow diagrams, Electrical fault finding and Fitting locations.
  Note
Removing the fuse will interrupt the voltage supply »terminal 30« for the fuel pump control unit -J538-.
–  Observe fuel pressure displayed on diagnostic system.
l  The fuel pressure will decrease very quickly because the mechanical high-pressure pump is no longer being supplied with fuel by the fuel system pressurisation pump -G6-.
–  Switch off ignition as soon as fuel pressure has dropped to approx. 8 bar.
  Note
Fuel pressure must not fall below 6 bar, otherwise the engine will stall (this could damage the catalytic converter).
  WARNING
There is a risk of injury: avoid skin contact with fuel.
t  The fuel lines are still filled with fuel, however the fuel is no longer under high pressure. Wear safety goggles and protective clothing when opening the fuel system.
t  Before opening the high-pressure section, wrap a cloth around the connection.
–  The high-pressure system must be opened »immediately« after reducing the fuel pressure; wrap a clean cloth around the connection. Catch the escaping fuel.
  Note
The pressure will increase again due to the effect of residual heat if the high-pressure system is not opened immediately.
Additional steps required
–  Refit fuse.
–  Erase event memory and generate readiness code in engine control unit in „Guided Functions“ mode.
  A97-10558

Checking vacuum system > < Safety precautions when working on the ignition system