Checking a high-pressure fuel pump

Here you will find useful information and valuable tips on all aspects of high-pressure fuel pumps.

Important safety note The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their day-to-day work. The information provided on this website is intended for use by suitably qualified personnel only.


High-pressure fuel pump: Background information

Since the introduction of common rail systems in diesel engines, high-pressure fuel pumps have become an indispensable part of the fuel processing system. With the introduction of direct petrol injection, high-pressure pumps are also used in petrol engines.


The fuel system of a modern direct petrol injection engine is made up of a low-pressure system with a high-pressure circuit. In the low-pressure circuit, the fuel is sucked from the tank by an electrical fuel pump and then conveyed to the high-pressure pump.


The system pressure in the low-pressure circuit is regulated by the engine control unit as required and can be up to 6.0 bar, depending on the system. In the high-pressure circuit, the fuel is pumped by the high-pressure pump via high-pressure fuel lines into the fuel distribution pipe (rail), where it reaches the relevant cylinders via the connected electrical high-pressure injection valves. The fuel pressure in the high-pressure circuit is monitored by the engine control unit and regulated from 50 to 350 bar in accordance with the appropriate system configuration.

Design and function of the high-pressure fuel pump: Functionality

Depending on the vehicle manufacturer and engine concept, different types of high-pressure pumps can be installed. A distinction is made here between radial piston pumps, axial piston pumps or in-line pumps, which can be run with one or several pump elements. As a result of the different drive concepts, the pump can be lubricated either by the fuel or by the engine oil.


Regardless of the design, the high-pressure fuel pump has the task of compressing the fuel provided by the pre-feed pump to the fuel pressure required for the injection valves and of then making it available in the fuel distribution pipe (rail). Since the high-pressure pump is driven mechanically via the camshafts, the pump's delivery rate is proportional to the engine speed.


The fuel pressure is monitored by the engine control unit via a pressure sensor and regulated via a flow control valve installed in the pump. This fuel pressure regulator is attached directly to the high-pressure pump. It measures out the supply to the high-pressure pump and thus regulates its performance.


This demand-based control means that only the high pressure that is actually needed for the current operating situation is generated in the pump.

System integration in the vehicle

The high-pressure system of direct petrol injection is made up essentially of the following:

  • High-pressure pump,
  • Pressure control device,
  • Distribution pipe (rail),
  • High-pressure fuel line,
  • Fuel pressure sensor and
  • High-pressure injection valves.

Effects and causes of defective, high-pressure fuel pumps: Causes of failure

Severe mechanical strain, high fuel pressures, lack of lubricant and temperature differences promote wear and can lead to a defect in the high-pressure pump over time.

The following effects may indicate a defective high-pressure pump

  • Erratic engine running
  • No power in the upper speed range
  • Poor starting behaviour
  • Engine stops – engine warning light comes on
  • Oil dilution

Causes of failure of the high-pressure pump can be as follows

  • External mechanical damage
  • Internal mechanical damage to the drive, roller tappet or pump element
  • Leaks - fuel loss
  • Inspection service intervals/maintenance intervals not observed
  • Poor lubrication - oil dilution or poor oil quality
  • Contamination in the low-pressure system

A defective high-pressure pump must always be replaced!


Manufacturer repairs are not provided for.


Before installing a new pump, the pump drive, roller tappets and camshaft are all to be checked for damage and proper functioning. Defective parts have to be replaced.

Testing and fault diagnosis for high-pressure fuel pumps: Instructions

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.

Visual inspection

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.

Reading the error memory

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

  • Break in the circuit
  • Signal missing / no communication
  • Error permanent

Reading off parameters

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.

Checking fuel low pressure

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.


  • Pre-supply pressure in normal operation approx. 4.0 bar
  • Pre-supply pressure for cold or hot start approx. 7.0 bar


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.

Checking fuel high pressure

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.


The control unit diagnostics have been illustrated using the mega macs 77 diagnostic unit as an example. The respective test depth and variety of functions can be set out differently depending on the vehicle manufacturer and these are dependent on the relevant system configuration of the control unit.

Further testing options for the high-pressure fuel pump: Instructions

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.

Checking fuel pressure in the low-pressure system with the manometer - the pressure gauge

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.

Checking holding pressure of fuel pressure

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.

Checking fuel quantity of the pre-feed pump

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

  • Check fuel lines for kinks or blockages
  • Check fuel filter and replace if necessary
  • Check tank ventilation
  • Check fuel delivery unit and replace if necessary

Testing the high pressure system

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.

Repair and installation instructions for high-pressure fuel pumps: Important information

Safety notice!

The fuel system is either under pressure or under high pressure. 

  • There is a risk of injury from leaking fuel.
  • Before repairing the fuel system, make sure to bring down the fuel pressure. 
  • Wear safety goggles and protective clothing.

If the high-pressure fuel pump is to be replaced as part of the repair, it is imperative that you observe the safety advice and the repair instructions of both the product manufacturers and of the vehicle ones.


  • Only trained specialists are permitted to carry out repair work on a high-pressure fuel system.
  • If necessary, please use the installation or removal tools specified by the vehicle manufacturer
  • Before replacement, the electrical connection of the pre-feed pump must be disconnected from the vehicle electrical system or the battery must be disconnected.
  • The fuel system must be checked for contamination and, if necessary, flushed out and cleaned before the installing of a new high-pressure or low-pressure pump. Existing fuel filters are to be replaced.
  • Mechanical components such as the pump drive and camshaft must be checked for damage and proper functioning before installation takes place. Defective parts have to be replaced.
  • After installing the high-pressure pump, the fuel system must be bled and checked for leaks. 
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