It’s actually still proportional but only goes up half way all of a sudden.
Half a tank equals full. One quarter equals half a tank etc.
Does that sender have to go away or is it something else?
Probably the wire on the sender is not bent properly.
If the float was eaten by the alcohol in gas, it will read low and eventually read empty. Bending the float arm is a bad idea. You might as well bend the needle on your gauge. Bending it will possibly make it read like there is still gas in the tank when it’s empty.
If the float is good you need to open up the resistor housing and clean up the wire wound resistor and contact. These are easy to damage. I wet sand with 600 grit emery paper going only in the direction of the winding. I use 10x magnification to see what I am doing. By wet I mean using electric contact cleaner.
Normally bad contacts actually increase resistance and result in a lower reading, a full tank reading half full on the gauge for example.
I could be wrong but your voltage regulator in the dash might be on it’s way out.
Any of the other guages acting up? Because the voltage regulator controls a few If I remember correctly.
It might allow more voltage to reach your gauge resulting in a higher than normal reading.
Also if i’m misreading your topic and your fuel guage actually reads lower than what’s in the tank, then clean the contacts like stated above 
No the other gauges are dead on right.
The fuel gauge itself reads 50% of the actual amount of fuel in the tank. To the best in my knowledge, no E85 or other type of alcohol has ever been used in this vehicle.
Of course, the 10% ethanol is practically universal now.
Are these contacts outside of the fuel tank where the plug probably is or inside of the tank?
Another words, do I have to empty the tank and take out the sender to get at them or just go under the vehicle?
How common is this problem?
You have to empty the tank and pull the sender. The sender then has to be disassembled. Things to be aware of, the tabs that hold the resistor box together are really easy to break off as you bend them back. The resistor is would wire, and very fragile. You should use a good quality meter that is accurate at single digit resistance readings. Check every connection, re solder any that show measurable resistance. Basic reproduction senders are very cheap so unless you have a very valuable XR7 style low fuel sender with the three prong connector you are really better off replacing it.
So, I guess I should check the condition of the connection outside and clean it up.
If that fails to improve it, replace the sender. Jeg’s has them for under $50.
I’m also wondering if there is enough extra room on the sender to put in an attachment for a return line, which would make adding fuel injection easier in the future.
Mopar fuel senders with 3/8th outlets already have these. I’m not proposing using a Mopar one in s Mercury, just adding a fitting that can have a line attached to it in the future.
NPD has senders with return lines installed.
That’s great to know, will make upgrading to EFI much easier if I go that route.
I discovered that my fuel tank was partially collapsed by a vented fuel cap that turns out actually isn’t.
The shape of the tank is probably confusing the sender.
I’m going to try to somehow pressurize the tank to return it to is original shape or else replace it.
Yes, there are videos of people using air pressure to fix collapsed fuel tanks.
The whole fuel system can probably handle 3 or 4 psi of air pressure.
https://youtu.be/zI3nyMtVGAk
It helps to drive in the daylight at times. The mechanic I brought the car to in order to get an estimate on inspection found it first.
A dent in the gas tank would cause the opposite of this.
The fuel gauge itself reads 50% of the actual amount of fuel in the tank.
If you briefly ground the sender lead at the tank, and then turn the key to the on position. The gauge should peg on full. If it does this eliminates the other 5% chance of it being other than the sender.
The problem is with the sender most likely (about 95% probability). With the tank full the sender should measure 10 ohms resistance. The pump is designed to push gasoline. It is not a suction pump and is incapable of causing the tank to be crushed. A properly working vented gas cap lets air in but not out.
You need to look at the obvious stuff first.
The mechanic said the tank has to be fixed or replaced to pass inspection.
Besides, 17 gallons isn’t really all that big to begin with. If it’s collapsed, it might only be half as much.
The unit at Jeg’s is $99. I’ll look for better prices and quality.
I better buy 2 or 3 more gas cans!!
I’m assuming that the 20 gallon tank from the 1970 won’t fit right, correct?
I was looking at radiator pressure testing tools with hand pumps. That gas cap looks awfully like a radiator cap more then the typical gas cap.
Why not give it a try?
The tank is so flattened that the front part which is slanted just a little past vertical is more flat then slanted, which could confuse the sender.
The sender itself might’ve been damaged along with the tank.
I don’t want to hijack white Lightning’s thread, but I’d like to ask a related question.
Since disconnecting my California Cougars emissions plumbing, my fuel gauge goes well past the full mark when the gas pump shuts off. And it probably is over full, because if I park “uphill”, I have seen gas come out the back.
Not good, so my interim fix has been to shut off the pump before full. Can I fix this issue without putting all the emissions equipment back on. Also, my “vent” apparently lets air in AND out - there is no whoosh of air when I take off the cap to fill it. There is no “vented cap” for the California Cougar tank filler neck that I have found.
It doesn’t fit. The cap definitely is airtight. It was warm today. When I went to the gas station, air rushed out. Of course, I filled it to the top.
That is exactly what a properly operating vented cap will do. It lets air in but not out.
There may be a way to fix it. At night when its cold, I’ll remove the fuel cap and then put it back, I noticed when i got gas that a lot of air rushed out and the gauge went up a little bit more. I might also pressurize the tank, perhaps by disconnecting it at the fuel pump inlet and putting 2 or 3 psi into it and letting it sit. I’m sure that the fuel lines can handle pressure lower then the float on the carb.
This means that the tank expanded partially!!
I’ll have to find the right pump and fitting, fun fun fun and a trip to a plumbing store.
Why don’t you just diagnose this with a meter?
I looked under the car and the fuel tank is partially crushed.
The tank must be repaired or replaced.
BY all means replace the tank. Under $100 including shipping on Ebay. But the crushed tank is not causing the gauge to read wrong.