Fuel Pressure Regulator and Holding Pressure, Checking
Fuel Pressure Regulator And Holding Pressure, Checking
Conditions
- Fuel pump relay OK
- Fuel pump OK
- Fuel filter OK
- Battery voltage at least 11 V
- Vehicles with automatic transmission: Selector lever set to P or N
NOTE: The fuel pressure regulator controls the fuel pressure as a function of intake manifold pressure.
WARNING: Fuel system is pressurized. Wrap a cloth around the joint before disconnecting hoses or opening the test connection (to measure fuel pressure). Then dissipate the pressure by carefully unfastening the bolted joint.
- Briefly open tank cap (pressure dissipation).
NOTE: Connection of fuel supply pipe (for connection of pressure gauge) is located beneath bottom part of air cleaner housing.
- Remove engine cover and air cleaner housing.
NOTE: The filter elements and the two mass air flow sensors are also removed in the process. The mass air flow sensors remain on the bottom part of the air cleaner housing.
CAUTION: The two intake connections to the turbochargers are now unprotected. To avoid serious turbocharger damage, make sure no items (such as cut cable ties, tools or cloths) fall into the intake connections.
- Cover screw connection -2- with a cloth (fuel pressure).
- Unfasten screw connection -2- and use a cloth to catch fuel which escapes.
- Mount pressure gauge V.A.G 1318 with adapters V.A.G 1318/7, 1318/8 (pressure hose) and 1318/15 between supply pipe and fuel rail.
- Open pressure gauge cut-off valve. Lever points in direction of flow.
NOTE: Cut-off valve of pressure gauge must be open (lever in direction of flow).
- Start engine and let it idle.
NOTE:
- The engine is operated without mass air flow sensors in this condition. This is not a problem for the short period of time involved. However, faults are stored in the engine control module. Erase these on completion of repair work.
- The greatest possible care must be taken to prevent objects falling into the intake connections to the turbochargers.
- Measure fuel pressure.
Specification: approx. 3.5 bar
- Disconnect vacuum hose from fuel pressure regulator and seal off pipe.
NOTE: Replace pressure regulator if fuel emerges at vacuum connection of pressure regulator when performing the following pressure test.
Fuel pressure must increase to approx. 4.0 bar.
If reading does not match specification:
- Try replacing pressure regulator and repeat pressure test.
- If reading still does not match specification, check fuel pump/supply pipe for damage (e.g. crushing) and replace if necessary.
If reading matches specification:
- Reconnect vacuum hose. Replace clamps and ensure proper attachment.
- Switch off ignition.
- Check freedom from leaks and holding pressure by way of pressure drop on pressure gauge.
After 10 minutes pressure must still be at least 3 bar.
If holding pressure drops below 3.0 bar:
- Start engine and let it idle.
- Switch off ignition once pressure has built up. Simultaneously, close cut-off valve of pressure gauge V.A.G 1318 (lever at right angles to direction of flow).
- Observe pressure drop on gauge.
Possible faults if pressure decreases again:
- Leak in pressure gauge unions downstream of cut-off valve
- Fuel pressure regulator faulty
- Injectors leaking
Possible faults if pressure does not decrease:
- Leak in screw connection between pressure gauge and fuel supply pipe
- Leak in supply pipe at fuel tank
- Non-return valve in fuel pump leaking.
NOTE:
- After closing cut-off valve, pressure gauge indicates pressure downstream of cut-off valve "on intake manifold end" (in direction of injectors, fuel pressure regulator). If pressure now drops upstream of cut-off valve (in direction of tank), this cannot be detected by the gauge.
- To remove pressure gauge, close cut-off valve, unfasten screw connection at adapter V.A.G 1318/15 and drain off excess fuel into collector by opening cut-off valve.