Maybe this part is obvious, but in case it isn't. If there is air in the fuel system, the leak has to be on the vacuum side. If there is fuel coming out there is a leak on the pressure side. I just went through this on my boat which turned out to be multiple leaks
that finally got bad enough to stop the engine. I got a short length of clear tygon tube and hose barb unions. I inserted the tygon in the system in different places to watch the bubbles go by. I disconnected the fuel line somewhere on the pressure side of the sytem and put the end in a container, then turned on the electric
fuel pump. There were no bubbles upstream of the Racor
filter. I found air bubbles in the line downstream of the Racor
filter, so I got a new filter, problem solved
there. I then moved the tygon to the inlet on the electric pump
, no air there, so it had to be the fitting on the electric pump inlet, or the pump itself. Regooping and tightening the fitting didn't work, taking out the pump did.
For trouble shooting between the electric fuel pump and the mechanical pump in a situation where the electric pump isn't always on, I would start by turning on the electric pump and look for fuel leaks
between the two.
On my boat the fuel tank
is low enough that the vertical lift
is out of spec for the mechanical pump, so an electric pump was added which is on all of the time, but the mech pump seems to work fine without the assist. I will replace the electric pump.
And before someone says it, I did not leave the tygon in my fuel system, it was only temporarily installed for troubleshooting.