OK my issue appears to be 3 fold but I have done extensive research and troubleshooting and now I am at my wits end.
First off this is a 1988 f250 7.3 with 74k miles
I got it in May and it seemed like the rear sending unit had a short so I only used the front tank. I towed cars or the boat once a week or so all summer

Sunday Last week I towed the camper home a trip to NY. I had a hard time finding diesel on my route home so I was down below 1/4 tank when I arrived home.
It had symptoms of running out of fuel when i was backing camper in driveway.
I switched to rear tank and gauge pegged and it ran for a few seconds and shut off again. I added five gallons to front tank and switched the switch back and gauge never changed back. After adding 5 gallons the truck ran fine and I drove 5 miles to station and filled front tank and another couple miles to my shop to park truck.

Monday I hopped in truck and went to run an errand and about ten minutes later the fuel filter light came on and it shut off. I got it fired up and turned around and limped back to garage with it dieing two more times under braking or decel. All 3 times I got it going with a shot of brake clean
The filter was about 400 miles since being changed for regular maintenance but I figured i sucked up crap running out of fuel

Tuesday I changed the filter and it ran fine again although I only moved it in and out of shop.

Wednesday I tested the switch and all wiring to the valve successfully
Everything is working as it should except the valve appears stuck on rear tank. The way I deduced this was that the contactors for the rear tank gauge have continuity no matter what the switch on dash says.

Friday I removed the bed and removed both sending units and rebuilt the gauges with old gas ones I had around and also both showerheads were shot so I replaced them with 3/8 fuel line and a clamp. both gauges work great now. A puzzling thing is that the front tank only had about 1/3 of a tank although I filled it Sunday afternoon. The rear tank was full although I have never put fuel in it. I had to pump 5 gallons out to work on sending unit.

Monday night I drove the truck home about 8 miles about 1/2 mile from home the filter light flashed. The filter was maybe 10 miles old.
I ate dinner and went back out. truck fired up and drove fine for about a half mile and shut off. Got it fired back up and it died twice again on decel or braking. Third time all it did was crank. even with brake clean. Called a buddy with wrecker and towed it to garage.

Tuesday night (Yesterday) It fired right up and drove in shop like normal.
removed schrader valve and attached gauge. read 4 psi after 10 seconds cranking. reconnected fss and started truck read 4-5 psi while idling for ten or 15 min in driveway. removed gauge and went for ride. After 5 min filter light flashed and died. Got her going and back to shop. Pulled into bay and died 30 seconds later with filter light. Attached gauge again and had 4 psi and it fired up over the course of 4 minutes maybe it went from 4psi to 10 deg of vacuum on gauge and died when light came on. At this point i removed the feed line to filter head and there was no fuel and inrush of air like it was under vacuum.

so now we are to the present time and my questions are, Do I need a lift pump? Is the schrader valve before or after the filter? Is it possible something is screwy with my valve causing this?
I am contemplating bypassing the valve tonight, anybody do this with rubber hose and clamps? Can plastic lines handle clamping?

Hopefully somebody has read this novel and can offer some support.
If you need any other info please dont hesitate to ask.

 

Well welcome to the forum first off.

To me, who am fairly uneducated, it sounds like a lift pump. The thing that sticks out to me is when you took the line from the lift pump to the filter head loose and there seemed to be vacuum. I have read that the IP (injection pump) will draw fuel from the tank in some cases. So that would lead me to suspect the lift pump.

Now I am not sure why the lift pump would start the truck with 4psi of fuel pressure and drop off after it was running. Someone else may/will chime in.

JM2CW

 

Classic selector valve failure. Actually, it's half-working. It's drawing from the front tank, but returning to the rear tank. That's why the front tank went down so precipitously, and the rear tank level went up.

It also explains the gauge. The gauge switching in the selector is set up not to change which tank contacts are closed unless the valve mechanism successfully changes over both the supply and the return. So it's probably working thusly - set to Rear, it's supplying from the rear, returning to the rear, and getting the rear sending unit / gauge feed. Set to Front, it's flipping the supply to the front, but the return is still on the rear, and so the gauge feed is still connected to the rear.

Long story short, selector valve needs replacement.

 

So why is he having engine issues? FSV shouldn't have much to do with the engine dying and running erratically.

Maybe he's got two different issues going on......

 

Its probably starving because it's pulling suction on the tank. If the pump is pumping fuel from the front tank and returning it to the rear tank the tank level is dropping a lot faster than normal. If the tank breather isn't working well (I'm not sure how its designed) I would guess you would pull vacuum in the front tank leading to the stumbling issue. Quick check would be to take the fuel cap off and go for a drive with it on the front tank. If this is what's happening, you might eventually overflow the rear tank or have a leak show up in an odd place as it's trying to push fuel into the full rear tank. I would say pull the valve and see if there is shower head pieces stuck in it. If you can clean it and its not burnt up, you might be able to put it back together and use it.

Edit: That would also explain why after it sat for a while it would start right back up.

 

Guess one way to check would be to remove the fuel filler caps and see if you can replicate the issue.

 

Or just switch to the rear tank and drive it a while, see if it smoothes over. Esp now that there's so much fuel in the rear tank.

 

When you replaced the 'showerhead' how long did you leave the hose, and is it cut off straight at the end? Its possible that if the end of the line is cut straight (or even at an angle, but just sliced across) the suction on the hose could be sticking it to the bottom of the tank causing the fuel supply issue. There are a few ways to get around this, one is the cut the hose so it hangs just above the bottom of the tank (might be hard to measure that correctly), a better way is to cut a V at the bottom of the hose so it can't suck it self to the bottom.

I agree that there is an issue with the tank selector valve. I'll also second that it could be a tank breathing issue. A previous owner swapped the rear tank on my '88. The breather hose was in the wrong place upon reinstall, and was squished between the tank and the truck bed effectively closing it completely. I can't remember if I had any issues while running, but whenever I loosened the gas cap it would gasp some air in.

BTW, the fuel filter light is controlled by a vacuum switch in the filter header. It comes on when there is vacuum on the injector pump side of the filter. This can be caused be a blocked filter not passing enough fuel through, or it can be caused by a restriction before the filter, like a failed lift pump, bad/blocked fuel line, blocked fuel pickup, or possibly excessive vacuum in the fuel tank from a blocked breather.

The breathers on these are simpler than the gassers. There is a rollover valve on the top of the tank that connects to a hose. I can't quite remember if there is a one way valve as part of the rollover valve, or if its just a simple ball type that will close if the vehicle flips over. I know the gassers have a more complicated vapor recovery type breather with two hoses coming off.