How to tell if the fuel gauge is broken?

Hello all, a couple months ago I bought a 1990 Ford F350, 7.3 idi. Dont know much history on the truck other then the tranny and engine were replaced about 50,000 miles ago according to the pervious owner(he gave me the receipts). It has dual tanks and when I was test driving it I flipped the fuel tank switch to see if one it worked, and two if there was any fuel in the rear tank. According to the gauge there was about an 1/8 tank left, ok fine. The other day I went to fill up both tanks, I mainly drive this on the weekend it will be my fifth wheel hauler, and I have only used the front tank so far, anyway I went to fill up the rear tank, and it would only tank maybe a dollar of fuel, WTH? Is it already full? Is my gauge stuck on 1/8 of a tank of fuel for my rear tank? The front tank gauge seems to work just fine. So how do I check how much fuel I have in there without dropping the tank? Why would the front fuel gauge work and not the rear? Sorry for the long post, thought I would give a little background, new to diesels, not much of a mechanic but slowly learning. Thanks alot for any help!

Bad rheostat, or sunk float.

Are you sure your tank is full? Im thinking you may be just clicking off the pump.

rjjp said:

Bad rheostat, or sunk float.

Click to expand...

Thanks for a place to start I will look into it.

Cincinnati Guy said:

Are you sure your tank is full? Im thinking you may be just clicking off the pump.

Click to expand...

Maybe, but I tried for 2 minutes to put fuel in, tried going really slow, fuel just kept coming back up in the fuel tank, so not sure. Thanks for the reply though

It's probably full of fuel, you just have a bad float, very common on these trucks. Too bad it isn't your saddle tank, you can actually sneak the sending unit out of the saddle tank without removing it. I believe the rear tank is a different story, many people actually remove the bed as it's argued to be easier than dropping the tank. I might just keep an eye the mileage you run on the rear tank as you have a front tank to "get you home".

Good luck whichever way you go

And by the way: I'm in Vancouver as well, and thats a great looking truck by the way

lotzagoodstuff said:

It's probably full of fuel, you just have a bad float, very common on these trucks. Too bad it isn't your saddle tank, you can actually sneak the sending unit out of the saddle tank without removing it. I believe the rear tank is a different story, many people actually remove the bed as it's argued to be easier than dropping the tank. I might just keep an eye the mileage you run on the rear tank as you have a front tank to "get you home".

Good luck whichever way you go

Click to expand...

Thanks for the info, it is welcomed. So if you dont mind I have another question, lets say I run out of fuel running on my rear tank, cause I dont acutally know how much fuel I have. How bad is this? Could I just switch over to my main tank and be fine, or will I damage something? What would I do to restart it if I ran it out of fuel to get it re-started? I know in a carburated engine you can just pour some gas down the carb to get fuel back into the system to start up.

lotzagoodstuff said:

And by the way: I'm in Vancouver as well, and thats a great looking truck by the way

Click to expand...

Ha ha thanks that is the trucks good side, the previous owner ran into one of thoose gas pump poles on the passenger side so there is a "wrinkle" in the rear fender, not to bad though and after all it is a work truck so I think of it as charcter.

gilend said:

...lets say I run out of fuel running on my rear tank, cause I dont acutally know how much fuel I have. How bad is this? Could I just switch over to my main tank and be fine, or will I damage something? What would I do to restart it if I ran it out of fuel to get it re-started?

Click to expand...

Hows your luck? sometimes if you catch it on the fires hiccup while running a decent RPM it will light off before stalling, sometimes you will be cranking for a minute or two. (let the starter cool down after 30 seconds)

gilend said:

Thanks for the info, it is welcomed. So if you dont mind I have another question, lets say I run out of fuel running on my rear tank, cause I dont acutally know how much fuel I have. How bad is this? Could I just switch over to my main tank and be fine, or will I damage something? What would I do to restart it if I ran it out of fuel to get it re-started? I know in a carburated engine you can just pour some gas down the carb to get fuel back into the system to start up.

Click to expand...

I've had mine "burp" going down an off ramp, and I left it in gear, switched tanks and kept going. If you do run it out, do make sure you don't crank the starter for more than 15-20 seconds at a time as you'll cook the starter. Sometimes it's hard to remember that these engines have literally twice the compression of a gasser.

One reason to think about an electric lift pump as it makes it easier to purge air when you run out of fuel and/or change your fuel filter. With the mechanical pump, you are only pumping fuel when the engine is turning over, which can take some time when you get some air in the system.

One other possibility is that your FSV is bad. I'd ohm out the sender.

lotzagoodstuff said:

I've had mine "burp" going down an off ramp, and I left it in gear, switched tanks and kept going. If you do run it out, do make sure you don't crank the starter for more than 15-20 seconds at a time as you'll cook the starter. Sometimes it's hard to remember that these engines have literally twice the compression of a gasser.

One reason to think about an electric lift pump as it makes it easier to purge air when you run out of fuel and/or change your fuel filter. With the mechanical pump, you are only pumping fuel when the engine is turning over, which can take some time when you get some air in the system.

Click to expand...

So if I do run it out of fuel nothing to bad will happen? I should be able to switch tanks and try to start it, giving the starter plenty of time to cool between cranking the engine. I heard in the PSD if you run out of fuel you can destroy the fuel pump located in the fuel tank. I know I dont want to run it out of fuel, I am just thinking worse case senerio. Thanks again for the reply, I dig your race car by the way.

Agnem said:

One other possibility is that your FSV is bad. I'd ohm out the sender.

Click to expand...

Sorry man, what is a FSV? Float switch valve? I am a newbie, sorry if that is dumb question, just trying to learn and understand, I would rather save some money if I can and do something myself then pay some guy to do it for me. Thanks

Fuel Selector Valve


Only dumb question is the ones you don't ask.