Is it necessary to depressurize fuel filter on 2004 Trailblazer before changing fuel filter?

Open the gas cap to depresurize. Have a container ready as you may get half a cup of fuel that comes out.

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I have to respectfully disagree. Removing the fuel cap will only depressurize the system if there are no in-line check valves in the pump, filter, or pressure regulator. Since the Trailblazer is a port injected vehicle, it maintains at least 40 psi in the lines while parked to allow for a fast startup. Pull the fuel pump relay and start the truck. After a few seconds the residual fuel pressure will be used up and the engine will stall. There will still be some pressure left, but far less than the 40-80 psi most vehicles run at.