It can be a messy adventure any way you go about it in my experience. I've done it when I thought the tank was "empty" after draining it out via the supply hose, but it still had a few gallons in it due to a clogged tank strainer not allowing all the fuel out. I've come out from under the car soaked in diesel, and I've come out with out dripping a drop. The shop vac trick works well, if you don't want to drain the tank, but there is definitely some room for ol' Murphy to drop in.

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