how old is the fuel fillter, the fillter housing is prone to tiny leak at the fuel heater wire port, on top,,small o ring up there...will leak tiny amount of fuel when engine is running and drip down the back side so u have trouble seeing the leak,,and lets air in over nite,,serious air leaks,,the engine will start,,then die,, and need two or more min of cranking,,to start, one way to find leaks is to temprarily install electric pump, mount it near tank to pressureize the hole system,,temp block off return line's and look for leak's