The functioning of the high-pressure fuel pump is monitored via the relevant higher-level engine control unit. Errors occurring in the system pressure are detected via pressure sensors and are stored in the control unit’s error memory. With a suitable diagnostic unit, the error codes and system parameters can be read out and used for further troubleshooting. A system-relevant fault is indicated by the engine warning lamp lighting up in the instrument cluster as a warning to the driver.
The following diagnostic information is presented using different vehicles as examples.
Before control unit diagnostics are undertaken, it is first advisable to carry out a visual inspection of the individual system components as part of initial troubleshooting activities. In this way, leaks in the fuel lines or defective plug connections on the sensors can be localised and eliminated in advance.
In this function the error codes stored in the error memory can be read out and deleted. In addition, information on the error code can be called up.
In our case study, the electrical plug connection on the fuel pressure control valve was disconnected and consequently the error code P2294 was stored in the error memory.
Error code P2294 - fuel pressure regulator / solenoid valve
In order to check and assess the system pressures, the parameters of the high and low-pressure sensors from the control unit should be looked at first.
And in order to narrow down possible causes of errors, the displayed actual values can be compared with the target values set by the vehicle manufacturer.
The high-pressure circuit is always tested after the low-pressure circuit has been tested and no errors detected.
The fuel pressure sensor for low pressure is installed in the feed line to the high pressure pump and sends its signal to the engine control unit.
As a result of such information, the engine control unit can calculate the required low pressure accordingly and forward the signal to the fuel pump control unit for demand-based activation of the pump.
In the case of a defective sensor, the pre-supply pump is controlled with a fixed value by the engine control unit and the pressure is raised accordingly. If the fuel pump control unit is defective, the pre-supply pump will not be activated.
The fuel pressure sensor for high pressure is screwed into the distributor pipe (rail) and sends its signal to the engine control unit. The data is evaluated in the engine control unit and the high pressure is adjusted accordingly via the high pressure control valve. In the event of a defective sensor, the control valve is activated by the control unit with a fixed substitute value.
If implausible values are read out from the control unit, additional checks can be carried out. The following tests were carried out on a Golf V 1.6 TSI as an example.
In such a case, the pressure gauge should be connected in the fuel supply line to the high-pressure pump. To do this, loosen the hose connection on the low-pressure nozzle of the high-pressure pump and clamp the pressure gauge in between. Then the engine should be allowed to run in idling mode. According to the manufacturer, the fuel pressure should be between 4.0 and 7.0 bar. The pressure can vary depending on the operating status and speed. However, the pressure should not drop below 4.0 bar when in idling mode.
Connect the pressure gauge as described in the low-pressure test and start the engine until operating pressure is reached. Switch off engine. Close the hose pipes to the high-pressure pump. This can be done with a clamping tongs or a shut-off valve on the pressure gauge. The pressure should still be approx. 3.0 bar after about 10 minutes.
Loosen the fuel supply line on the high-pressure pump, close it and place it in a suitable measuring cup. Disconnect the electrical connection on the pre-feed pump. Set up a separate power supply with a cable, switch and fuse to the vehicle battery. Switch on the pump until the pressure is built up. Open the clamp or stopcock/shut-off valve and start measuring. After 30 seconds, approx. 600 ml should be reached. The battery voltage should not fall below 10.0 volts during the test.
If the target value is not reached
The target value for the fuel system high pressure is 40-120 bar, depending on the operating status. There is no provision for a manual pressure test with a pressure gauge.
