What causes "dieseling"

It started life as a step side but either the factory or someone put the tool boxes on, which is the main reason I bought it because I wanted a tool truck because I am too lazy to lug heavy tools everywhere I go.

I'm not sure what a fleet side is but it is a shorty.

At this time I am not selling, because I want to play with it.

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That's kinda cool! Better have good locks on that tool body!