How to Depressurize a Fuel System

Written by thomas west
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Modern automobiles and trucks use a fuel injection system to deliver the correct air and fuel mixture to be burnt in the engine cylinders. The fuel injection system operates under high pressure, therefore before any maintenance work is to be done on the fuel system, such as replacement of the fuel filter, the fuel system must be depressurised. Depressurising the fuel system will keep you from getting a dangerous spray of fuel on you when you remove the fuel filter.

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Things you need

  • Flare wrench
  • Rags

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  1. 1

    Loosen the fuel filler cap on the outside of your vehicle without removing it. Listen for the audible hiss to subside, telling you that the pressure in the fuel tank has been released.

  2. 2

    Locate the fuel lines that attach to your fuel filter. Note that in some vehicles the fuel filter is mounted underneath the vehicle on a frame rail, while in other vehicles the filter is located in the engine compartment. Check your owner's manual for the location of your fuel filter. Loosen the fittings of the fuel lines on the filter by turning them with a flare wrench in a counterclockwise direction, but do not remove the fittings completely.

  3. 3

    Wait several minutes for the pressure in the lines to release. Note that fuel will leak from the cracked-open fittings until the pressure is released. Clean up any spills with rags.

  4. 4

    Remove the fuel lines from the fuel filter if the filter is being replaced, otherwise leave them loosened until your maintenance work is complete. Tighten the fuel lines in a clockwise direction once your maintenance work has been completed and you wish to pressurise the fuel system again.

  5. 5

    Tighten the fuel filler cap.

Tips and warnings

  • If you do not wish to loosen the fuel lines to release the pressure in your fuel system, use the following procedure: Start the vehicle and let it idle. Locate the fuel pump relay fuse in your vehicle's fuse box. The fuse box in your vehicle may be located under the dashboard on the driver's side or it may be under the hood. Check the guide on the inside of the fuse box lid to find the proper fuse. Use a fuse puller and remove the fuel pump relay fuse. When the vehicle sputters and quits by itself, turn the ignition off. Once all maintenance work has been completed, push the fuel pump relay fuse back into place in the fuse box. Turn on the ignition key and wait a few seconds for the fuel system to pressurise before starting the vehicle.
  • Do not use a fuel filter designed for a carburetted vehicle in a vehicle that has fuel injection. The much higher operating pressure of the fuel-injected system could burst a fuel filter designed for a vehicle that utilises a carburettor.

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