System Pressure, Fuel Pressure Regulator and Residual Pressure
Fuel Pressure Regulator And Holding Pressure, Checking
NOTE: The fuel pressure regulator regulates the fuel pressure according to the intake manifold pressure. The pressure drop at the injectors remains constant for each engine speed and load range.
Special tools and equipment
- VAG 1318 with VAG 1318/6 and VAG 1318/7.
Test requirements:
- Fuel pump relay OK.
- Fuel pump OK.
- Fuel filter OK.
- Battery voltage at least 12.7 V
- Vehicles with automatic transmission: Selector lever in position P or N.
Test sequence
- Remove rear engine cover (arrows).
Checking system pressure
WARNING: The fuel system is under pressure! Place a cloth around the connection point before opening the system. Then carefully loosening the connection to relieve pressure.
- Briefly open fuel tank filler cap (to release pressure).
- Connect pressure gauge VAG 1318 to fuel supply line using adapters 1318/6 and 1318/7.
- Disconnect vacuum hose from pressure regulator to intake manifold at pressure regulator and close off hose.
NOTE: If fuel flows at the vacuum connection on the pressure regulator during the following pressure test, replace pressure regulator.
- Start engine and run at idling speed.
- Measure fuel pressure.
- Specified value: 3.8 to 4.2 bar
- If fuel pressure does not match specification, install a new fuel pressure regulator and repeat pressure test.
- If fuel pressure still does not match specification, check fuel pump and supply line for damage (e.g. crushing) and replace if necessary.
- If pressure is higher than specification, check return line for damage (e.g. crushing) and replace, if necessary.
NOTE: During the following test, do not leave the engine running too long with the vacuum hose disconnected, because the higher fuel pressure causes the fuel and air mixture to be rich. This could lead to the Oxygen sensor control limits to be exceeded. This would result in a DTC being recorded.
- Start engine and run at idling speed.
- Switch off electrical accessories (air conditioner, etc).
- Re-connect vacuum hose to pressure regulator and observe drop in pressure on pressure gauge.
- Fuel pressure must be lowered by approx. 0.5 bar when vacuum hose is attached.
Perform the following checks if there is no change in pressure as described above:
- Check vacuum hose for cracks and damage.
- Test vacuum hose for blockage at intake manifold. To do this, disconnect hose at fuel pressure regulator and blow into it.
- If there is no leak and no blockage in vacuum hose, install a new fuel pressure regulator.
Testing holding pressure
- Holding pressure 10 minutes after switching off engine (minimum pressure).
- Specified value for cold engine: approx. 2.2 bar
- Specified value with engine at operating temperature: approx. 3.0 bar
NOTE: The increase in temperature with the engine at operating temperature is caused by the expansion of the fuel is normal.
- Start engine and run at idling speed.
- Allow pressure to build up, then switch off ignition. At same time close cut off valve on pressure gauge VAG1318 (valve lever at right angles to direction of flow (arrow)).
- Observe pressure drop on pressure gauge.
If the pressure does not drop, the following DTCs are possible:
- Union between pressure gauge and fuel supply line leaking.
- Supply line leaking at fuel tank.
- Non-return valve in fuel pump leaking.
NOTE: When the shut off valve is closed the pressure gauge shows the pressure after the isolating valve "intake manifold end" (direction of injectors, fuel pressure regulator). If the pressure before the isolating valve (tank direction) should now fall, the equipment cannot recognize this.
If the pressure drops again, the following DTCs are possible:
- Fuel pressure regulator malfunction.
- Injectors leaking.
- Unions on pressure gauge after cut-off valve leaking.
NOTE: When disconnecting the pressure gauge, close the shut-off valve, unscrew the union on adapter VAG 1318/15, then open the shut-off valve to drain off excess fuel into a suitable container.