How to Bleed a Diesel Fuel Injection System

Written by kyle mcbride
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Diesel injector fuel lines carry fuel from the lift pump and deliver it to the injector supply lines or the fuel gallery. As the fuel travels through the injectors, small amounts of fuel are injected into the combustion chamber. The excess fuel, called spill fuel or return fuel, cools the injectors and is returned to the fuel tank to diffuse the heat. Air bubbles introduced into the supply lines by a fuel filter change or by running out of fuel will prevent the engine from starting and running normally. The air must be bled out until the system is purged.

Skill level:

Things you need

  • Mechanic's tools
  • Shop rags or Oilsorb rags

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  1. 1

    Ensure that the fuel filters are installed properly and not clogged. Check for fuel leaks that may indicate an air leak at all fuel line connections from the tank to the lift pump and from the lift pump to the fuel filters.

  2. 2

    Gain access to the injector tops and fuel lines. Some engines have their injectors exposed, while others mount the injectors under the valve cover. Remove the valve cover if applicable.

  3. 3

    Ensure that the fuel cut-off valve is open. Break the torque with a wrench on the fuel bolt of the last injector in the series to get fuel. On most models, this will be the forward-most injector on the engine. Check with the manufacturer's specifications for your engine for this detail.

  4. 4

    Place the engine control in the "No-Fuel" position. Actuate the starter and turn the engine over. Observe the fuel leaking from the loosened fuel bolt. Allow all the air to be forced out and tighten the fuel bolt fully with a wrench once the fuel is seen to be pure, with no air bubbles evident.

  5. 5

    Clean up all spilt fuel with shop or Oilsorb rags. Replace the valve cover if applicable. Test start and run the engine.

Tips and warnings

  • Purging the fuel system should be done only once every filter change. If air bubbles persist, or if the engine acts as if it is out of fuel intermittently, then there is likely an air leak somewhere on the vacuum side of the system, or between the lift pump and the fuel tank. This must be rectified before any amount of bleeding will actually remove the air from the system.

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