1979 F150 Ranger XLT 2WD. When the fuel gauge shows 1/4 I run out of gas and can only add less than 10 gallons. Is there such a thing as a 10 gallon tank for this truck or there is something else wrong?

 

Unless your rig has a non-OEM fuel tank that is only 10 gallons, then it might be a venting issue when ya fill up or the technique ya use to fill up.

I have to invert and then insert the nozzle and hold it there. I also do not go "full on" with the filler lever.. fill it up slowly so the displaced air can extracted/escape without triggering the pump's pressure switch.

 

Might be your pickup tube in the tank is rotted off and your not emptying your tank

 

That was my first thought.....

 

Yeah, it is hard to fill to the top, I have to go very slowly, haven't tried inverting it, will try that next time, however the gauge shows full and then only goes to 1/4 before I cannot get any more gas. Could be bad sending unit?

 

That would certainly explain the symptoms. I guess I will have to drop the tank and see what it looks like.

 

could also be that your sending unit is not telling the truth when it says 1/4 tank - have you actually run it dry?

Before you pull the sending unit, if you remove the large filler hose you can see the sending unit pickup tube with a flashlight. The mesh filter , or at least the pickup tube, should reach down to the bottom of the tank.

Good thing is that if you have a standard tank the sending unit is pretty cheap.

 

You may want to ground the sending unit connector to ensure the gauge is working correctly also before pulling the tank

 

Back in the 80s I disassembled a bad sending unit from a '76 F150 to see how it worked.

The float arm wipes against a resistive winding which is supposed to transmit a proportional resistance value as the fuel tank level drops. The windings should look like a candycane.... evenly distributed.

What happens is the resistive windings" droop" and provide an unbalanced reading... typically each loop is wide at the top and the is progressively tighter towards the bottom.

That's why the indication for the upper half of the fuel level gauge seems to go real slow and then speeds up later on as if it is using more fuel.