Generator Fuel Tank Return Line

"tank is filled above the return point."

There will be problem, sure enough. The return line normally works on gravity flow.

Isn't a fuel loop under pressure by virtue of the fuel pump?

The return line normally works on gravity flow.

not true

Not sure about your exact question. But I installed an extended tank on a diesel generator once that originally just had a tank in the base. The owner's manual warned not to allow the fuel level above a certain point on the engine to avoid leakage into the engine. Check with your manufacturer. I had to have a tank made with a smaller diameter than standard for the capacity we wanted.

The return may be above the engine but should return to top of tank to avoid excessive restriction on the return fuel. You do not mention specifics so its hard to say for sure how much pressure your application will have.

Unnecessary pressure will promote leaks and then compound into fuel dilution in your engines lube oil, not to mention excessive wear on all other components in your engines fuel system.

Remember return fuel in a diesel engine in most applications also acts as a coolant for the system, low flow will lead to a long list of problems.

The return line should empty into top of tank as a general rule, the tank may be higher than the engine but there are limits there as well, better you be more specific as to your tank size, height, head above fuel return, engine type if possible.

"the tank may be higher than the engine" - not in the case I mentioned. I think there was a special component (valve?) available, but I chose the simple route.

i wouldnt connect the return to the standalone tank you have too many risks with backpressure and overfueling , injector bypass , engine oil dilution etc

let the injector return drain back to the day tank in the base and install a ballcock valve in the day tank which is then fed from the backup standalone tank when the fuel level drops down to 1/3 full for example.

the issue with this is if your ball valve leaks then the day tank will overflow and the injector return has nowhere to go

allow for the fact that diesel expands when its hot too

out of curiosity is there a breather on the day tank ?

you might be able to by-pass the day tank and and push the fuel with a 12v fuel punp if nessessary. install a pressure regulator inline with the pump or gravity pressure on the exit side. if you need to use the day tank along with the storage tank, i'ld install a three way pitcock. they use them on motocycles for the reserve tanks. [on/off/reseserve] i don't know if they make an pitcock that is automatically actuated by a float?