Fuel System Diagnosis

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When a problem exists within the fuel system, many different symptoms might be exhibited. Some common symptoms are:

  • Engine crank, no start
  • Hard starting/long cranking
  • Lack of power
  • Service Engine Soon lamp with mixture faults stored
  • Excessive emissions

Begin diagnosis by verifying that there is fuel in the fuel tank. Verify that the fuel is being transferred from the right side of the tank to the left side (pump side). The fuel levels and total fuel in liters can be accessed through the Instrument cluster electronics test functions.


Next, check fuel pressure by connecting a fuel pressure gauge to the vehicle. Fuel pressure can range from the typical 3.5bar up to 5-6bar. Engine's with direct injection can range from 6bar to 120bar of pressure. Be sure that the proper tool is used for the vehicle when checking fuel pressure. Some vehicles offer a Schrader valve on the fuel rail to check fuel pressure. Others require the use of a T-fitting to access the fuel system before the fuel rail. Newer vehicles require the use of a special tool. Note: Always observe safety precautions when working with fuel systems. Always have a fire extinguisher near by in case of emergency.


Depending on the results of the fuel pressure check, if no pressure is present, start diagnosis at the fuel pump. Earlier fuel pump circuits were simple, consisting of a power supply and ground. Newer vehicles may have a control unit such as an EKP controlling the fuel pump operation; this should be taken into consideration when performing diagnosis. If fuel pump operation is found to be within specification, the next step would be to check fuel volume. With a suitable container used to catch the fuel, measure the fuel volume coming from the system. A general rule of thumb for the proper fuel volume is approximately 1 liter in 30 seconds.


Loss of residual pressure can also be caused by other faults within the fuel system. If the residual pressure bleeds off, the engine will have an extended cranking time and possibly black smoke upon startup. The fuel pump check valve, fuel pressure regulator and fuel injectors hold residual pressure within the system. To monitor the residual fuel pressure, connect a fuel pressure gauge to the vehicle. Then start and run the vehicle for a short period of time. Note the fuel pressure while the engine is running. Shut the engine off and monitor the fuel pressure over an extended period of time. Pressure drops of more than .5bar within 30 minutes should be investigated for leaks. Diagnosing older vehicles is simple, clamping off certain components can identify the cause of the leak down. New non-return systems contain most of the fuel system components within the fuel tank. The process of elimination is required on these systems due to their complexity. A “bubble test” can be performed if one or more of the injectors are suspect of leaking down. This requires the use of a special tool. With the injectors connected to the special tool, the injectors are pressurized with compressed air and submerged in water to check for bubbles that would indicate a leak. Another technique for checking whether one or more injectors are leaking down requires monitoring the injectors visually with the rail and injectors removed and pressurized by the vehicle's fuel system.

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