Howard Beal wrote:

Car is a 2003 mercury sable GS, V6 engine.

Fuel is leaking from the top of the gas tank only when the
engine is running. With the key in the run position i can
hear the fuel pump turn on and pressurize the system
and there are no leaks. It only leaks when the engine is
running. The fuel pressure is within specs. Only 50k
miles. New fuel filter installed. Using shell regular unleaded gas.
No OBD codes when scanned. Check engine light is off.

Any idea why it only leaks when the engine is running?

I did a google search for this problem, no usefull results.

Best Regards
Tom.

The return line to the tank from the fuel pressure regulator probably
has a leak where it connects to the top of the tank. When the engine is
running there is return flow, when not running no return flow. If you
jumper the fuel pump relay to run the pump with the engine not running
you should see the same leak. When you turn the key on, it only pumps
for a few seconds, not enough to push much if any fuel back through the
return line.