Test and Diagnostic Equipment for Fuel Systems

Frequently Asked Questions About Fuel Systems and Their Diagnosis

Modern fuel systems consist of numerous components for fuel delivery, pressure regulation, injection, return, and tank ventilation. Depending on the vehicle and injection system, very different pressures and test procedures may be required. Here, we answer frequently asked questions about the professional diagnosis of fuel systems in automotive repair shops.

• What tests can be performed on a fuel system?

  The required test depends on the vehicle, the fuel injection system, and the specific fault symptoms.

  Typical diagnostic procedures include, among others:

  • Measurement of fuel pre-delivery pressure
  • Pressure and vacuum testing in the low-pressure range
  • Common-rail high-pressure testing
  • Injector return flow rate testing
  • Return pressure measurement for compatible common-rail injectors
  • Testing of injectors and fuel nozzles
  • Leak testing of lines and tank systems
  • Fuel identification
  • Checking the fuel flow for air bubbles
  • Measuring fuel consumption

  By combining several test procedures, the cause of a fuel system malfunction can be narrowed down more precisely.

• What is the difference between low pressure and high pressure in the fuel system?

  In the low-pressure section, fuel is delivered from the tank to the high-pressure pump or the injection system. The pressures required for this are relatively low and vary depending on the vehicle and fuel system.
  
  In a common-rail diesel system, the high-pressure pump then generates a significantly higher fuel pressure. The pressurized fuel is stored in the rail and distributed from there to the individual injectors.
  
  Different measuring devices and connections are required for both systems. Therefore, a low-pressure tester must not be used to measure common-rail high pressure.

• How is a common-rail system tested?

  When diagnosing a common-rail system, several tests can be combined.

  The common-rail high-pressure test determines whether the high-pressure pump and the system are building up the required rail pressure.

  A return flow test allows for a comparison of the fuel return flow rates of individual injectors.

  For certain injector systems, the return pressure can also be tested.

  In addition, the fuel supply in the low-pressure range should be checked. This is because insufficient feed pressure, air in the fuel system, or a restricted fuel supply can also prevent the required rail pressure from being reached.

• What could be the cause if a diesel engine starts poorly or not at all?

  Starting problems can have numerous causes. In the fuel system, the following potential sources of failure are among others:

  • insufficient feed pressure
  • air in the fuel system
  • clogged fuel filter
  • leaking fuel line
  • faulty fuel pump
  • insufficient rail pressure
  • worn or faulty injectors
  • excessive return flow from one or more injectors
  • faulty pressure regulation
  • Fuel of unsuitable quality

  Therefore, one should not rely solely on the analysis of the fault memory. Mechanical pressure, flow, and return flow tests can provide additional information about the actual condition of the fuel system.

• Why is mechanical fuel pressure measurement still useful despite vehicle diagnostics?

  The engine control unit measures many pressure values using electronic sensors. These values are very helpful for diagnostics, but they depend on the proper functioning of the sensor, wiring, and control unit.
  
  A separate mechanical or independent pressure measurement may therefore be useful if a displayed pressure reading appears implausible or if it is necessary to distinguish between the actual system pressure and the sensor signal.
  
  The measurement method that can be used depends on the specific fuel system and the manufacturer’s specifications.

• What does a return flow test reveal for common-rail injectors?

  During operation, a portion of the fuel flows back from the injector through the return line.

  During the common-rail return flow test, the return flow rates of the individual injectors are measured and compared with each other and with the manufacturer’s specifications.

  Significant deviations in return flow rates may indicate a faulty or worn injector.

  However, a return flow rate test does not replace every other type of injector test. Additional test procedures or an injector test bench may be required for a comprehensive evaluation.

• What is the difference between return flow rate and return pressure?

  The return flow test measures the volume of fuel that flows back through an injector’s return line.
  
  In contrast, the return flow pressure measurement checks the pressure in the injector’s return line.
  
  These two parameters address different diagnostic issues and should not be confused with one another.
  
  In particular, for certain piezo common-rail injectors, a defined return flow pressure can be relevant for operation and diagnostics.

• How can you detect air in the fuel system?

  Air can enter, for example, through leaks on the suction side, faulty connections, a filter change, or a completely drained fuel system.
  
  The fuel flow can be observed using appropriate testing equipment with a transparent test block or viewing tube. This makes it possible to detect air bubbles in the fuel.
  
  In addition, pressure and vacuum measurements can help narrow down the cause within the fuel supply system.

• When does a diesel fuel system need to be bled?

  Bleeding the system may be necessary, especially after work during which air has entered the fuel system.

  These include, for example:

  • Fuel filter replacement
  • Work on fuel lines
  • Replacement of pumps or components
  • Running the engine until the tank is completely empty or draining the system
  • Repairs to the fuel system

  For modern common-rail systems, the bleeding procedure specified by the vehicle manufacturer should be followed.

• What safety rules apply when working on common-rail systems?

  Common-rail systems operate at extremely high fuel pressures. Fuel escaping under high pressure can cause serious injury.
  
  High-pressure lines or other pressurized components must therefore never be opened without proper control. Before connecting, removing, or testing the system, it must be depressurized in accordance with the manufacturer’s specifications, and the prescribed safety measures must be followed.
  
  Only measuring devices, hoses, and adapters designed for the respective pressure range may be used.

• What kind of fuel testing equipment is right for my workshop?

  The selection should be based on the vehicle fleet and the diagnostic tasks that arise on a regular basis.

  For multi-brand repair shops, the following test areas are of particular interest:

  • Low-pressure fuel and pre-feed pressure
  • Common-rail high pressure
  • Common-rail return flow
  • Injector testing
  • Fuel identification
  • Diesel venting
  • Fuel tank system leak test

  Workshops specializing in diesel and common-rail vehicles often benefit from a combination of high-pressure, low-pressure, and injector testers.