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Discussion Starter #1
The other day I noticed that even with a full tank of fuel in the front tank, the gauge read empty. I flipped the toggle switch between the front and back several times, and it went up and down. Down for the front, and full for the rear, even though both tanks were equally full. I thought "encouraging it" might do something magical... It didn't.

I drove around on the supposedly empty front tank and went for many, many miles, so it is holding and sending fuel. Eventually, the gauge decided to sort of work and registered a 1/4 tank. Then another 1/4 more, and then another. It went back to being normally operational until I refilled the front tank with fuel and it repeated the whole process. The gauge is completely normal when I'm using the rear tank, so it's not the gauge. What could it be? Is there a little trick to this, kind of like when you used to be able to tap the top of the tv years ago and fix reception, or do I need to get something replaced?

Thanks

Andy
 

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Discussion Starter #3
Most likely the sending unit in the front tank is going (gone) bad.
But wouldn't the truck not run as well, at least intermittently, if that was failing? There's no change whatsoever in performance, just a non accurate fuel gauge.

Is there a sensor of sorts in there that's different from the sending unit that measures the amount of fuel so it's displayed properly on the fuel gauge?
 

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The sending unit is just a small coil of copper wire that creates resistance as the float electrode moves up and down against the coil of wire - telling the gauge in the cab where the float is at the top of the fuel level in the tank.
The fuel gauge system has nothing to do with how any other part the truck works (motor / computer / etc.)

Like your situation -
My front tank sending unit died years ago (always says EMPTY - even when its full).
My rear tank sending unit works the cab gauge perfectly.

It doesn't hurt anything - you just have to be careful to not run the front tank out of fuel - or your other option is to drop the front tank and replace the sending unit.
 

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Discussion Starter #5
Gotcha. I assumed the sending unit was responsible for sending fuel from the tank to the injectors. I'll check it out. Thanks for the help!
 

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As you have learned the fuel gauge sending unit has nothing to do with either performance or fuel delivery.

Also just so that you know your truck has a two stage fuel pump located in the valley behind the fuel filter housing. It draws fuel from the tank and pushes it through the fuel filter and then takes it and pressurizes it to send fuel to the injectors. Then the unused fuel returns to the FPR or fuel pressure regulator on the filter housing before being sent back to the tank.

Here is a flow diagram of the fuel from the tank and back.
 

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^^^^^ This applies to all vehicles. The term "sending unit" has nothing to do with fuel DELIVERY. Vehicles had sending units decades before the first in-tank fuel pump appeard. The pickup, strainer, fuel pump (if applicable), etc. are all independent of the sending unit.

That said, your sending unit may be fine, and it may be something as simple as a stuck float. The float rises and falls on a pivoting arm. The float can develop a leak and become sodden with fuel, thus sinking even when the tank is full. But if this were true in your case, the gauge would not work intermittently; it would always read low/empty. In your case, if the sending unit itself is not failed as suggested above, the float pivot just may be sticky. That's happening with one of our trucks. For months, the front tank would read nearly empty. We stopped using that tank for a while, anticipating having to drop the tank to replace the float and/or sending unit. We filled the tank again recently, figuring we wouldn't be dropping the tank this time of year. This time, the float responded and the gauge reads fine on that tank now. What I didn't try doing, when the gauge was acting up, was to give the tank a thump with a rubber mallet, to see if a brief shock might un-stick the float. Easier said than done with a 4x4 and a skid plate.
 

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Discussion Starter #8
Thanks for that post. I wonder too if something is stuck. I hoped bouncing around (my truck is anything but subtle over the bumps) would jar something loose and it seemed to work. I guess it could go either way so I'll wait a bit and see if it works itself out. My gauge used to stick on way overfull several years ago, and that resolved as hoped.
 

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Way past full is a different story. That typically means an electrical break in the circuit. Open circuit --> infinite resistance --> gauge pegs to the far right. If you get that again, your problem/solution will most likely be electrical.
 

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What kind of electrical problem could cause the gauge to be pegged all the way to the right? My rear tank in my 97.5 didn't do it at first but slowly it has gotten worse to where it seems that its almost 90 degrees to the right.
 
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