Fuel Pressure: Testing and Inspection

Fuel Pressure Regulator And Residual Fuel Pressure, Checking

Requirements
- Fuel pump relay OK
- Fuel pump OK
- Fuel filter OK
- Battery voltage at least 11 V


NOTE: The fuel pressure regulator regulates the fuel pressure according to intake manifold pressure. This ensures that the drop in pressure at the injectors remains the same throughout all engine speed and engine load ranges.


WARNING!
Fire hazard! The fuel system is pressurized Before loosening hose connections or opening the test connection (to measure fuel pressure), place a cloth around the connection Then release pressure by carefully pulling off the hose or cap.


Testing system pressure







- Connect pressure gauge VAG 1318 to supply pipe using adapters 1318/7,1318/8 (pressure hose) and 1318/15.
- Disconnect vacuum hose between fuel pressure regulator and intake manifold at fuel pressure regulator end; seal off end of hose.


NOTE: If fuel flows out at the vacuum connection on the fuel pressure regulator during the following pressure test, fit a new fuel pressure regulator.


- Start engine and run at idling speed.
- Measure fuel pressure.
Specification: approx. 3.8...4.2 bar
- If fuel pressure does not match specification, install a new fuel pressure regulator for test purposes and repeat pressure test.
- If fuel pressure still does not match specification, check fuel pump and supply pipe for damage (e.g. pinching) and replace if necessary.
- If pressure is higher than specification, check return pipe for damage (e.g. pinching) and renew if necessary.


NOTE: During the following test, do not allow the engine to run for too long with the vacuum hose disconnected, otherwise the increased fuel pressure will enrich the air/fuel mixture and may cause the lambda control limits to be exceeded. This would be stored as a fault in the fault memory.


- Start the engine and run at idling speed.
- Switch off electrical consumers (air conditioner etc.).
- Re-connect vacuum hose to fuel pressure regulator and observe drop in pressure on pressure gauge.
- When the vacuum hose is re-connected the fuel pressure should drop by around 0.5 bar.

If the pressure does not change as specified, carry out the following tests:
- Check vacuum hose for cracks and other damage.
- Test vacuum hose for blockages at intake manifold. To do this, disconnect hose at fuel pressure regulator and blow into it.

- If there are no leaks and no blockages in the vacuum hose, install new fuel pressure regulator.
- Replace hose clamp that secures vacuum hose to the fuel pressure regulator and make sure that new clamp is properly installed.

Checking residual fuel pressure
- Residual pressure 10 minutes after switching off engine (minimum pressure)
With engine cold: at least 2.2 bar
With engine at operating temperature: approx.: 3.0 bar


NOTE: A rise the pressure (due to fuel expansion) when the engine is at operating temperature is normal.


- Start the engine and run at idling speed.
- Switch ignition off after the pressure has built-up. At the same time close pressure gauge cut-off tap (lever at 90° to through flow direction).

If the pressure does not drop, the following faults are possible:
- Union between pressure gauge and fuel supply pipe leaking
- Supply pipe leaking at fuel tank
- Return valve in fuel pump leaking


NOTE: When the cut-off tap is closed the pressure gauge will indicate the pressure downstream of the cut-off tap, i.e. on the same side as the intake manifold, injectors and fuel pressure regulator. If there is then a drop in pressure upstream of the cut-off tap (same side as fuel tank), the change will not be registered by the pressure gauge. This is why the paragraph beginning of the pressure does not drop... " has been included above.


If the pressure drops again, the following malfunctions are possible:
- Fuel pressure regulator faulty
- Fuel Injectors leaking
- Unions on pressure gauge downstream of cut-off valve leaking


NOTE: When disconnecting the pressure gauge, close the cutoff valve, unscrew the union on adapter VAG 1318/15, then open the cut-off valve to drain off excess fuel into a suitable container.