Gas gauge works sometimes in 1999 expedition - 1999 Ford Expedition

Check the link. It is a known issue

https://www.google.com/search?q=2000+ford+expedition+5.4+gas+gauge+not+working

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a guess
thats a lot of sensors to fail all at once
may need an auto electrician
.any fuses blown
if happened all at once
voltage overload/spike from alternator regulator
or huge short / wire loom melt ....

This sounds more like a faulty alternator. Sometimes an alternator can still produce adequate voltage and amperage to keep everything running, but they are governed by a voltage regulator. If the regulator goes bad, the alternator may have little of no output, or it will not limit itself properly causing gauge problems, and various light on the dash to not work or sporadically come on.
I'd take the vehicle to have the alternator tested at least.
Don't settle for one of the part store tests, they won't give you all the information you will need. A properly working alternator should charge anywhere between 13-14.6 volts and depending on the vehicle 90-150 amps.
I hope this helps in some way

SOUNDS LIKE YOUR FUEL PUMP PICK UP TUBE HAS A PIN HOLE IN ITWHICH IS CAUSING IT TO SUCK AIR. MAY HAVE TO REPLACE FUEL PUMP.

First the problem is caused by a defective fuel level sending unit, that is located inside the gas tank, the gas tank must be removed to access the part for replacement. As to the how much fuel remains there is not way to know this.

GO TO AUTO PARTS STORE BUY A FUEL PRESSURE GAUGE THAT CAN CHECK FUEL PRESSURE UP TO 100 PSI. LOCATE YOUR FUEL PRESSURE TEST PORT.LOOK FOR SCHRADER VALVE ON FUEL LINE NEXT TO EGR VALVE.PLACE A RAG NEAR THE SCHRADER VALVE AS YOU INSTALL FUEL PRESSURE GAUGE TO CATCH GAS SPILL AS YOU SCREW IN FUEL PRESSURE GAUGE HOSE FUEL PRESSURE 30 - 45 PSI WITH ENGINE RUNNING. THE ENGINE MUST BE OFF HOOK UP THE FUEL PRESSURE GAUGE.IF YOU HAVE TROUBLE THE FUEL PRESSURE GAUGE WILL HAVE INSTRUCTION HOW TO HOOK UP FUEL PRESURE GAUGE.BE CAREFUL CHECKING FUEL PRESSURE IS DANGEROUS JOB.DONT ALLOW NO MASSIVE FUEL SPRAYING WHEN HOOKING UP FUEL PRESSURE GAUGE YOU WILL GET CAR FIRE.

Hi. Check to be sure that the dimmer switch for these bulbs is turned all the way on. If still dark, the bulb may well be blown. If that is the case, call your nearest autoelectric repair shop and they will get you up and running in no time. J.

Mine does the same thing(same year) and my best guess is....
It only seems to do it when its really low on gas. On the expedition the pump is located near the back of the gas tank(when backed into a driveway the gas gauge reads significantly less gas than what is really in there) so when accelerating the gas sloshes around and the pump doesnt get enough gas to keep it running. Hope this helps.

The 99 expedition comes with a 26 gal tank as standard equip. and there is an optional 30 gal available. I suspect the gas pump you used is not accurate, and needs to be recalibrated. I think it's just a coincidence that the gauge is acting up now. Hopefully I'm wrong, and by some miracle you actually are now getting 300 miles on a half tank of gas! You may have a bad sending unit, this would result in inaccurate fuel tank readings.

this is caused by a defective gas gauge sending unit, u must remove the gas tank to replace it.