Fuel Pressure: Testing and Inspection

TEST CONDITIONS
^ Fuse 17 OK
^ Fuel filter OK
^ Battery fully charged (12 volts minimum)

CHECKING SYSTEM PRESSURE

Fuel Pressure Gauge Connection:

- Connect VAG 1318 pressure tester between fuel supply line and fuel rail.
- Set lever on pressure tester to the "open" position.
- Disconnect vacuum hose from fuel pressure regulator to intake manifold at fuel pressure regulator, and plug hose.

NOTE:
Replace the fuel pressure regulator if fuel runs out of the vacuum connection during the pressure test that follows.

- Activate fuel pump relay using ECM output Diagnostic Test Mode.
^ Fuel pump must run. Testing and Inspection

If NOT:
- Check fuel pump.

If fuel pump runs:
- Check fuel pressure on gauge.
Specification: 3.8-4.2 bar (55-7O psi)

If the specified pressure is not obtained:
- Replace fuel pressure regulator and repeat pressure test. If the specified pressure is still not obtained:
- Check fuel pump and fuel supply line for damage (e.g. crushed or pinched sections) and replace if necessary.

If the specified pressure is exceeded:
- Check fuel return line for damage (e.g. crushed or pinched sections) and replace if necessary.

CAUTION!
Excessive fuel pressure may rupture the fuel pressure regulator diaphragm. This may cause fuel to enter the engine via the vacuum hose.

NOTE:
During subsequent testing, the engine should not run for an unnecessarily long time with the vacuum hose disconnected. The increased fuel pressure will cause mixture enrichment, which in turn may cause Oxygen Sensor (02S) control limits to be exceeded, and a malfunction to be registered and stored.

- Start engine and let idle (this automatically interrupts output diagnostic test mode).
- Switch electrical consumers (A/C, etc.) off.
- Connect vacuum hose to fuel pressure regulator and observe pressure drop on pressure gauge.
^ Fuel pressure should drop by about 0.5 bar (7 psi) after connecting the vacuum hose.

If NOT, perform the following tests:
- Check vacuum hose for leaks (cracks, damage).
- Check passage of vacuum connection on intake manifold by disconnecting hose on fuel pressure regulator and blowing through it.

If there is no leak and the vacuum connection is not blocked:
- Replace fuel pressure regulator.

CHECKING RESIDUAL PRESSURE

Residual pressure 10 minutes after switching off the engine:
^ Cold engine: 2.2 bar (32 psi) minimum
^ Warm engine: 3.0 bar (44 psi) minimum

NOTE:
Higher pressure for a warm engine is normal and is the result of thermal expansion of the fuel.

If residual pressure is not OK, perform following tests:
- Check pressure tester connections for leaks.
- Check fuel lines for leaks.
- Check fuel pump check valve.
- Check fuel injectors for leaks.

If there are no leaks and the fuel pump check valve is OK:
- Replace fuel pressure regulator.
- Repeat residual pressure test.