C Series Fuel System - Overview Section F - Familiarization Page F-5 Air from uncorrected leaks in the supply circuit will make the engine: Difficult to start Run rough Misfire Produce low power Emit excessive smoke Produce fuel knock. A source, often overlooked for air to enter the fuel system, is between the inlet of the prefilter and the suction tube in the tank. Fuel tanks that have the outlet fitting at the top will have a suction tube that extends to the bottom of the tank. Cracks or pin holes in the weld that joins the tube to the fitting can allow air to enter the fuel system.
Since the fuel transfer pump provides positive pressure through the fuel filter and supply line to the fuel injection pump, loose connections or defective seals will show as a fuel leak.