تكاثف الماء في خزان الوقود

Water condensing inside the tank from humid air drawn in as fuel is used. Every tank breathes, and warm damp air entering a cool tank leaves water behind — which then sits under the fuel because it is denser.

It is worse in vehicles used little and kept part-full, since a large air space cycles more air through more temperature swings. The symptoms are erratic and follow fuel level: misfires and pressure faults as the pickup draws slugs of water, worse as the tank empties. Keeping the tank fuller reduces it, which is a genuine piece of advice rather than folklore. On a diesel it reaches the water trap, which is what the trap is for; on petrol it can freeze in the lines in winter, which is the version that strands people.