The fuel selector valve works in unscreened fuel. Contaminated fuel can cause selector valve seal damage. When fueling from possibly contaminated fuel sources McFarlane recommends the use of a fuel filter, such as Mr. Funnel or another filtering device, to reduce trash in the fuel system.

A common cause for this problem is fuel starvation due to improper installation of the fuel pump. Make sure that the fuel lines are not crossed; that is, the inlet line should be connected to the inlet side of the pump (marked “IN” on the top of the port) and the outlet line should be attached to the fuel pump outlet port. Fuel should be present at the inlet side of the fuel pump.

Improper installation of the fuel pump may result in a misalignment of the operating lever with the operating plunger in the accessory case. If so, there will be no movement of the lever and no pumping motion to provide fuel flow. If misalignment is found upon removal of the pump, the lever is probably damaged or broken. If misalignment of the operating lever has occurred, it will require inspection and repair by a qualified technician.

Diaphragm fuel pumps should be replaced or overhauled:

1. At engine TBO or after ten years in service, whichever occurs first. Replace on condition if leaks develop or performance drops for example.
2. Refer to Lycoming Mandatory Service Bulletin 240W for information regarding engine TBO.

The most common reasons for low fuel flow/pressure are the following:

1. A worn or broken O-ring on the inlet side fuel fitting can cause an inlet suction leak. An inlet air leak will result in low Fuel Pump output pressure.
2. A weak operating spring in the Fuel Pump diaphragm assembly
3. A worn fuel pressure gauge
4. A clogged fuel vent or fuel cap
5. A constriction in a fuel line either externally (a kinked hose), or internally (due to a collapsed hose lining)
6. Wrong fuel pump for the installation

Fluctuating fuel pressure can be the result of the following:

1. A worn fuel pressure gauge
2. A worn or broken O-ring on the inlet side fuel fitting
3. An obstruction in the induction system (air intake)
4. A clogged or dirty fuel strainer
5. Foreign material inside the fuel body (such as fuel cell sealant, thread sealant, or a drill shaving)

Leakage of fuel or oil through the fuel pump overboard drain line indicates a worn or ruptured diaphragm inside the fuel pump. The fuel pump should be replaced before further flight.