Fuel gauge and tank weirdness

[Mastiff]

A few days I pulled into the gas station with the gauge on "E". I put 20 gallons in and when I fired up the truck, the gauge was WAY above "F", like the needle is horizontal to the right. I think the sender is 0 empty and 90 ohms full, so I figured the wiring somehow got disconnected so the circuit was open.

Tonight I started poking around and got to where I was ohming it out right at the sender (I can barely reach from the front of the tank). The thing is reading open circuit right at the sender, or at least above the 200 ohm setting on my meter... somehow it seems that the sending unit has failed catastrophically, just like that.

Then I noticed one other thing. While banging the tank with a mallet in a desperate attempt to break something loose, I notice that the tank has a pretty major caved in spot underneath. This is a poly MTS tank that I put in maybe a year or two ago. There's a place in the middle driver's side, hard to see with the skid plate on, where it just goes up and in like six inches. It's about where the strap is.

So now I'm really confused. Did the tank collapse due to pressure and somehow destroy the sender? But if that's the case, why was it working before I took the cap off to put fuel in? Should a working gas cap allow air in to the tank and not out, or what? I had this thing vented to the world until a year ago when I had to get smogged. Then I put a charcoal canister in a plumbed into that, along with a new cap.

Any ideas? I'll almost certainly have to drop the tank, but I'd still like to know what the heck is going on.

 

[K85 Octane]

dumb question
are you sure the ground coming from the sender is good? It's on the top of the frame drivers side.

If nothing strange happens when you take the cap of (suck/puff...pressure sound) then I'd say the system is good. As for the dent, we here at CK5 love pictures.

 

[Mastiff]

I measured the ohms AT the sender, like from the output pin to the mounting plate. I did check the ground and wiring too, and it was good. I'll pull the skid plate off tomorrow and post a pic.

 

[Fordum]

Those sensors, as far as I know, are still wire wound on a plastic board that a wiper rubs across.
Its not uncommon for them to break as the wiper moves across. Probably happened when you filled it.

On my old Jeep, with the tank under the driver's seat, I had multiple failures due to a voltage regulator problem.

I would get in in the morning, turn the key on, and all the gauges would immediately peg, hang there for a second and then slowly move back after they burned up.

Meanwhile, if I listened closely, I could hear a faint hissing sound as white hot pieces of the sending unit wire melted and fell into the gas under my butt.

I could reach out and take off the cap and see some white vapor come out the tank.

Didn't make me nervous at all......................much.

I don't think yours uses a voltage regulator, so thats not it.

 

[K85 Octane]

Did it make ur *cough cough* hairs stand on end

any goose bumps on the lower extremities?