Usually it is the float and or sending unit in the tank. Over time the float sometimes absorbs fuel and doesn't work as well, usually indicating less than what is there. Or the sender gets corroded or sticky and reads higher or lower than it should.
Only fix that I know of is to replace the unit which means dropping the tank etc. Depending on how many miles you have, if you drop the tank, you might want to consider changing the fuel pump too since it is in the tank and requires the same work. Manytimes the float/sender/pump is all part of one unit.
Alternatively I had one car where the gauge itself was faulty and had to be replaced.
Once in a great while it is a bad wire connection.
I ownand have owned many Fords, all the fuel gauges read funny (except my 99 Ranger) Some read full until half empty,others would read empty when half full. It seems to be a pretty common.
I rented a Thunderbird with a bad fuel gauge,traded for another with similar problem.
If you have one of the new SD's with a digital odometer you can get a get a reading of the what the fuel sending unit is sending to the gauge.
Start with engine off and press the trip reset button. Turn the key to the run position (or start it), but don't let go of the trip odometer button. After a few seconds, the odometer will read "TEST". From there you can toggle through the various readings. There are 2 for fuel level. One is an instant reading, the other is for an average (smooths out fluctuations).
I am no expert, but if the reading is moving down as you drive and the gauge isn't, then a bad gauge would be the likely cause. If the reading is pegged, a sending unit or wiring issue would be where I would focus the diagnosis.
I have had the same problem. 05' SD V8 2wd. I bought this truck a few weeks ago and the gauge was stuck on empty. I lowered the tank to check the wiring connections. After I bolted the tank back in, the gauge is now stuck on full!! Ha! This leads me to belive there is a connection problem somwhere near the tank. I haven't had time to check further, but I plan to put a meter on it next week. I have checked the readout on the dash and it matches.
I agree that this is usually a float problem, but it should not be in a truck that is less than a year old.
If anyone has the schematics for the sending unit or the nominal measurement, I would appreciate them.
Well I broke down and solved my fuel sending problem.
I have an 05 2wd F250 Gas.
The truck was involved in a frontal collision, which may have caused it. The float assembly had actually come off it's bracket and was laying in the bottom of the fuel tank. It was very simple to drain the fuel out and slide it back into place, making sure the tab was bent back over to avoid another occurence.
All of this was made easier by taking the bed of the truck off. You can lower the tank as well, I just found it easier to take the bed off.
Note: The 2 lines going into the tank have connectors that require you to push in on the tabs. It looks like you pull out on them. One is green on the sending line. Not sure what the other is. To get the unit out, just use a screwdriver and hammer to knock the retainer loose.
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