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

A3 Mk2

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. 120 bar) and a low-pressure section (approx. 6 bar).
t  Prior to opening the high-pressure section (e.g. when removing the high-pressure pump, fuel rail, injectors, fuel pipes or fuel pressure sender -G247-), the fuel pressure in the high-pressure section must be reduced to a specified level. 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 function read „Measured values“.
–  Select measured value block 140.
–  With engine idling the fuel pressure is displayed in zone 3.
t  Specification: between 35 and 45 bar
   
–  Pry off cover on left side of dash panel using front-end hook -3370--arrow-.
  A97-0808
–  With engine idling, pull out fuse for fuel pump control unit -J538- in fuse carrier in dash panel (driver's side) → Current flow diagrams, Electrical fault finding and Fitting locations.
–  Observe fuel pressure displayed on tester.
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
t  The pressure will increase again due to the effect of residual heat if the high-pressure system is not opened immediately.
t  The ignition must not be switched on again from this point on as this would increase the pressure again.
Additional steps required
–  Refit fuse.
–  Erase event memory and generate readiness code in engine control unit in „Guided Functions“ mode.
  A97-0972

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