KDO

I have a 2005 F150, with the 5.4.
Last time I gased up, the gas cap literally broke after I had filled up. The top half seperated from the bottom half. I probably drove 100 miles and then got a new STANT, OEM equivalent gas cap. I removed the broken bottom half and put the new cap on. Now my gas gauge is showing FULL, when I know it is probably only 2/3 full. I turned it on and off several times and even let it run for a few minutes before I turned it on and off, and it is still showing FJULL. Any idea on how to reset the gauge so it works?
Thanks,

kylnmeg10

It may not be the gas cap. It could be the fuel pump assembly itself. Either the slide for the float, or a wire going to the pump may have broken. Just some ideas to look into.

KDO

Because it happened immediately after the gas cap came apart, I am thinking the tank lost compression or something and so the gauge just stayed in the full position. There are no other symptoms. It runs fine, the gauge just won't move off of full.

F150Torqued

The whole thing is just a 'Murphy's Law' coincidence. No 'compression' in the tank. Evap system must have been working or you would have a DTC - now or forthcoming.

The PCM modulates the EVMV (Evaprative Vapor Management Valve - often erroneously called the Purge valve) and allows intake manifold vacuum to draw vapors from the fuel tank as it monitors Fuel Tank Pressure sensor. Seeing vacuum build in the tank is how it checks for 'Gas Cap Off', and major leaks in the system. The time it takes for the maximum vacuum to decay when the EVMV is closed, allows PCM to determines air volume that can pass through the carbon canister - via the Normally Open Carbon Canister Vent Valve.

This cycle repeats constantly (roughly once every couple of minutes) during driving. (Periodically - when a bunch of conditions are satisfied) - the PCM closes the N/O Canister Vent Valve and measures the decay rate of the vacuum built in the tank - to test for 'SMALL' leaks.

My point:

I think the irregular nature of the negative pressure in the tank, by design, suggests your tank level sensor acting up is probably unreated.

kylnmeg10

It really sounds like you have a bad sending unit. I would verify the plug is completely connected, then you could do the old school way of ohm testing on the assembly.

KDO

Thanks everyone for your input.
Murphy must have been at work.
After another gas tank fill up, the gauge started working again.

So, after 150 miles driven today, things look good.

Thanks again for taking the time to reply.

powerranger262

Yep, if you would have unhooked the battery, and did a relearn, it would have done the same thing.