Newps wrote:
G Farris wrote:



I have heard that the left tank becomes slightly pressurized, due to the
forward-facing vent on that tank, which is why it drains faster than the
other. I do not claim this to be a fact - just something I've heard.

The left tank becomes pressurized because the vent tube is slightly out
of rig. It's supposed to be directly behind the strut. If it gets a
clear view of the incoming slipstream it forces a lot more air into the
tank. That forces fuel from the left to the right tank via the vent
line above the headliner in the cockpit. You will never solve the
problem until the vent tube under the left wing is positioned correctly
and then you only mitigate the problem to a certain degree.

Seems to be a popular misconception as to how that fuel gets
across to the other side. It can flow through the FUEL lines, at any
fuel level, while the selector is on both. If, for instance, the
airplane is parked left wing low, the fuel will run down through the
fuel line from the right tank, through the selector valve, and up the
other fuel line into the left tank. No mystery at all. If the tanks
have enough fuel, it will run out the vent onto the ramp. Stopping it
requires placing the selector on L or R or OFF. The old Cessna 150s,
with their on/off system, had the lines teed together before the valve
and would do this anytime, valve on or off.
The fuel will run across the VENT line only if the tanks are
full.
The fuel venting system applies the same pressure to both tanks.
The vent, whether behind the strut or below it, leads to the left tank,
and the vent line from that tank across the right tank makes sure that
the pressures are equal. Unequal pressures are a result of leaking fuel
caps, either at the gasket or through a defective vent check valve, and
air will flow through the cap into the tank and cause airflow one way
or the other through the vent line, upsetting the pressure balance and
causing one tank to drain faster than the other. Pressures are usually
negative if the check valve leaks, and possibly could go positive if
the gasket had a forward-facing leak. I've seen split gaskets on fuel
caps.
The vent is located aft of the strut to minimize icing blockage,
and one or both caps has a silicone check valve to let air into the
tanks in case of vent icing. That valve deteriorates over time and lets
air out of the tank. The valve disc isn't available as a separate part;
the cap must be replaced. Thankfully, they aren't expensive like
Piper's.

Dan