I have a '87 F150, 4.9l, fuel-injected truck, with 2 19-gal fuel tanks. The rear tank had a severe leak when I bought the truck (used), so it never got used. Recently, I replaced the rear tank. I inspected the fuel level sending and lift pump assembly when I had the unit out of the tank. Everything checked out, that is, the filter screen was clean and no holes, and the pump operated when I hooked 12VDC across it. The only problem (as is common on these trucks) is that the carbon-track resistance pad is completely worn off on the fuel level sender unit operated by the float, so the dashboard guage registers either completely full or completely empty ... there's no "in-between" so I go by the odometer reading since the last fill-up.

Here's the problem. I filled the new rear tank with 19 gallons of gas, switched the dashboard switch to "Rear" and started to drive it. I put about 40 miles on the truck, fully expecting the engine to sputter, choke, and cough (s/c/c) at some point in time as the air in the fuel line to the rear tank passed through the fuel rail. I never felt a thing, though. Now, however, if I'm running on the rear tank it will sometimes (but not always) do the s/c/c bit. If I switch to the front tank, the problem immediately disappears and the engine fires right back up to normal operation. If I then switch over to the rear tank, I might be able to drive it for miles before the s/c/c begins, or it might just a few city blocks. If I don't switch to the front tank, the engine finally dies within a short distance (a tenth of a mile). If I cycle the key on and off several times, I can hear the fuel pump pressurizing the line, then stopping. After cycling the key, I can then start the engine on the rear tank and drive away. However, it may not begin to s/c/c for several miles or it might start again within a very short distance. The problem is clearly one of fuel delivery from the rear tank, because a switch to the front tank clears up the problem immediately.

Since each tank has its own lift pump inside so as to get the gas up to the master EFI pressure pump, there's a electrically-controlled fuel diverter unit on the inside of the frame rail under the drivers's door. This unit must switch depending on which tank is being used, and do two things: (1) connect the lift pump output from the "running tank" to the high-pressure EFI pump, and (2) return gas from the fuel rail back into the correct tank ... otherwise one tank could overflow if the returned gas got dumped into a full tank.

My son thinks that somehow air is being sucked into the diverter, due to a bad seal or "O"-ring in it when running on the rear tank. I find this illogical, though, because the diverter should always be under pressure. ALso, there's no dripping which I would expect.

I believe there's a loose electrical connection in the rear tank wiring someplace - either to the lift pump or the diverter. It could be in the dashboard switch itself. I was careful to make sure that the plug from the wiring harness into the rear tank was firmly in place when I installed the new tank, so I don't think that's the problem.

Has anybody else here on the forum encountered this problem and have any suggestions as to where I should begin to look to solve it?

Thanks in advance for your help.

 

I have a 93 and on my truck each in tank pump unit supplies fuel directly to the fuel rail ( via the changeover valve on the frame rail). Maybe the pump does not provide sufficient pressure (55psi I think) you should test it.

Also the engine computer tests to see if the fuel pump draws electric power when you KOEO the ign sw ( the fuel pressure obviously builds up but the pump does not turn off due to a pressure switch but because the ECU tells it to!)