Well, that clears up the matter of increased pressure when I jumpered it directly. Got up around 8.5 or more.

 

Come to think of it, maybe running it for extended periods with that jumper and no added resistance would be outside the design parameters. I don't want to cook a fuel pump.

 

I don't like what I'm learning, but at least I'm learning:

I pulled the plug off the close-on-rise oil pressure switch (convenient how they locate it way at the back of a big engine, with things in the way), and inserted a jumper wire in the plug. Oh goody! Now I had fuel pressure when the key was on.

Still NO fuel from the front tank. So I suspected the selector switch, and I started digging. It's a chore to remove the ashtray, the center dash section and the aftermarket stereo, but I was finally able to get my hands on the back of the switch. A but more struggling, squirming and cussing, and I got it unplugged. Everything looked good. I grabbed a couple of jumper leads and my multimeter, then set out to "impersonate" the switch.

When I jumpered the plug to simulate the selector in "rear," I got fuel pressure. When I jumpered to simulate "front" selected, I got no pressure. My meter confirmed the jumper connections were good. Worse yet, it confirmed a voltage drop when the connections to the front tank were made up. Current is flowing, but I get no "front" fuel pressure.

My only remaining hope is the diagrams Piffery1 just sent me. Maybe that switch is not picking up the load of a bad pump, but only the load of the Tank Selector Relay. I could still be open circuit past the relay, or have a bad contact in the relay.

Does anyone know where the Tank Selector Relay and the Fuel Pump Cutoff Relay might be located on my '86 F250?

By the way, unplugging the return line on the "hot fuel" system caused a drop of a pound or so. Maybe 4 psi with the key on, and 5 psi with engine running and full charging voltage on system.

Right now, it appears I have a truck that runs, but only on the rear tank.

I'm wondering if a bad relay could cause a loss of fuel gauge reading.

This definitely appears to be an electrical problem (or problems). Fuel is either on or off. I intend to upgrade filtering, but I don't think the problem is anything crudding up.


One good thing about all that poking around: I realized there's a lot of dead space above the glove compartment in my non-A/C truck. Maybe I can invent a camouflaged "second glove compartment" for that space.