Diesel fuel plumbing engine just died

[phorvati]

My main engine just died after i rebuilt racor 900 series separator.
I have a northern lights generator and ford Lehman 90hp engine. Previous owner's arrangement was to have each fuel tank go to a selector valve. Output of selector valve splits into 2, and each fuel line goes to racor 900 series separator. One for generator and one for main engine. I never ran the engine in that configuration. I had taken the tee off and only ran main engine with generator separated. Last weekend I wanted to add the generator back in, and had rebuilt both separators, and added the tee back. I primed both separators. Engine started but died after about 20 mins. I looked at the sepatators and both were half empty.
Am I supposed to have a check valve going into each fuel separator?

[phorvati]

So I shut off the line going to generator's racor water separator, and re-primed the main engine filters(both primary and secondary), which were empty. Engine started right up and ran good for about an hour. After I shut it off, i looked at the main engines Racor water separator and it was still full of fuel.

So this tells me that main engine was sucking fuel from the generator feed line instead of the tank. Whats confusing is that this is how PO had it setup and it must have been working.

Either I need to add check valves or generator is somehow faulty letting air in. Or maybe both. Generator has been sitting for 2 years without being decommissioned properly.

[Lepke]

In ships and big boats the racors are setup as dual primary filters with one on line and one ready. Each engine has it's own secondary filter. All engine fuel supply comes off the manifold after the racors w/o check valves. All my boats have been setup like this. Current boat has 2 mains and 2 generators all off a manifold after the dual racors. No check valves.

Racors usually have a check valve built in. Most have a flow rating well beyond the needs of the installation, so there should be no or little suction vacuum after the racors (where you install a vacuum gauge to show filter life remaining). With a new filter and the mains and one generator running, my vacuum gauge reads zero (or close to it). And my mains pump 70 gph.

[phorvati]

so from what I gather, I should setup my racors to be able to use either one for both engine and generator. That sound pretty smart to me since I can switch them in an emergency or at high seas when nobody wants to deal with contaminated fuel and when issues are most likely to come up.

Given that you said you have a manifold it means no check valve so for instance main engine should never suck from generator feed line instead of the tank unless there is a leak and air is coming in. Perhaps something is wrong with my generator.

[Lepke]

The main shouldn't suck fuel from the generator because the generator lift pump valves function like check valves. Pumps will always draw from the least resistance. Air pumps easier than fuel so any air leak in the fuel system allows a pump to pull air into the system. It can be a fitting, wear spot in the fuel line, or a filter gasket leaking. But it can be fuel starvation. If say the pickup tube in the tank becomes plugged, the lift pumps will create a vacuum greater than normal, sometimes drawing in air.

If you can't find the air leak, try pressurizing the fuel line and listen for the leak. Or put powder on the fittings and look for the wet spot. Kinda like fat girls.
While you're doing all this, add a vacuum gauge after the Racor. With a new filter in the Racor, see what it reads (should be zero or close to it). Later as the filter gathers debris the gauge gives an indication of how dirty the element is and if it needs changing. Each system is different, but over time you'll be able to relate the vacuum reading to filter hours. Mine starts at O and reads about 7" of Hg at 500 hours. That's my normal changing time.