Procedure 4. Checking for Air LeaksIf stalling or rough operation only occurs after cold startup, check for fuel leaking backwards or air leaking into the
fuel supply lines. Fuel leaking backwards may be caused by faulty check valves in supply pump. To check for this
condition, remove return line connector fitting (ball check) from the top of pump and install a plain fitting in its
place. Connect a section of clear hose approximately 3 feet long to the fitting and route to a container. Start engine
and allow to idle. Stop engine. Suspend clear hose from hood of vehicle and mark fuel level in hose.
— NOTE —
Allow sufficient room in hose for level of fuel to rise due to thermal expansion. If level
drops over a period of several hours (or sooner), a leak back condition is indicated.
Occasionally a very small air leak will only let enough air into the system to cause a stalling problem or rough
running condition after the vehicle is shut down for many hours (such as overnight). Double check all fittings,
clamps and fuel lines and do not overlook components after the supply pump such as the fuel filter element or
base. There have been cases where tiny holes in the filter base casting or in the sealant used in the manufacture of
the filter element have allowed air to enter but no external fuel leakage to occur.
Air Leak DiagnosisAIR LEAK ON THE SUCTION SIDEThe housing vent wire may allow minute quantities of air to pass harmlessly out of the pump. However, at some
point there w ill be more air than can go out the vent and the air will go into the charging circuit. This air will
compress and upset fuel flow in the lines which will create a rough running engine. A plastic line placed on the
fuel return fitting will show up this condition. To isolate the air leak, it may be necessary to feed the system out of
a container and hook it up at various places to bypass certain sections of the fuel system.
AIR LEAK ON SUCTION OR RETURN SIDEAn air leak in either the suction or return side can cause starting problems. Air entering the system will allow fuel
to drain back towards the tank. If an air leak occurring at some point like the filter, the fuel will drain back then the
fuel on the return side will be heavier and it will pull fuel out of the filter through the pump and back towards the
tank. Because there is no fuel in the filter or pump, the engine will not start. When it finally does, surging will be
very evident.