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I was out driving and noticed I am overflowing gas from my front tank. Everything is stock on the truck. The truck runs just fine off of both fuel tanks, I was running on the rear tank when I started to overfill the front. Neither of the fuel level gauges work reliably, I know that they floats tend to rot out on them over time. I am guessing there is some sort of return fuel switch I need to troubleshoot? Where would it be located? Any ideas on how to test it?
Since you didn't say what year-model truck your talking about we can only guess, but you've got either the "dreaded fuel reservoir" failure or a bad FDM (fuel delivery module). Basically the return fuel from the motor is going to the wrong tank. If you have a pre-'91 (?) the fuel reservoir on the frame will only deliver return fuel to the front tank, so running on the front will work fine. If you have later than that, the front tank FDM will need to be replaced.
Others will chime in with more exact diagnosis.
The cutoff was '89-90, '89 and older has a frame mounted switching valve and high pressure pump with low pressure in-tank pumps, '90+ just has in-tank high pressure pumps and all fuel flow control happens with valves inside the FDM in each tank.
Sounds like a malfunctioning DFR! (Dual Function Reservoir) if your truck is of that vintage. BTDT. Run off one tank, the front in your case, and look for a replacement DFR, probably from a wrecking yard. My new-to-me DFR
If it’s the 93 in your profile, you have a stuck check valve in your front fuel tank.
The sending units have a check valve in them that close when the tank is not selected. The check valve opens for the return on the tank selected.
Ie. if your running on the rear tank, and the front tank check valve is stuck in the open position, the return will also return to the front tank. There is only one return line, it splits off close to the fuel tanks.
There is no fix that I’m aware of. Replace the faulty sending unit.
It is the 93. I thought that popped up in my signature automatically. 1993 f150 dual tank. So then it is the check valve in the front sending unit? So its just two sending units that are switched on and off.
So the return line goes from the front take to the rear, then the check valves determine which get filled? or is there a return line switching unit anywhere?
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