1. I have a 1985 FLC and I believe it has the original fuel tanks on it. It does not have any numbers stamped on it, so how is a good way to figure out what size it is?

2. The fuel gauge does not work on this truck except for when I fill it up. Then the needle will go from E To F real fast several times, and then go back to E and stay there until I fill it up again? Any suggestions on how to figure out what it could be.

 

As to the capacity, say your tank is 4 feet long and 2.5' in diameter. Find the area of the circle. Divide the 2.5' by two to get the radius of the tank (1.25'), now square the radius and you get 1.5625. Multiply that by pi (3.14) and you get the area of the tank end to be 4.9 square feet. Now multiply that by the tank length, I said 4 feet, and you get 19.6 cubic feet roughly. Now multiply that by 7.48 to get the gallons, 147 gallons in this case but you can only fill (IIRC) 80 or 90 percent of that. Does that make sense I hope? The formula I just described is (D/2)^2*pi*L where D=diameter and L=length of tank.

The other question sounds like the fuel tank sender is "plumb broke". Replace it.

Birken

 

If you are like me and are math impaired you can use this site to determine tank capacity. Take note that it asks for feet of tank size so you will need to use fractions.

http://www.watertanks.com/calc.asp

Might be worth taking the sending unit out and cleaning it up real good before you throw it in the trash. Also check for a short in the sending wire from the gauge to the sending unit.

 

Sounds more like a bad ground than a short....make sure the ground is not bad and check the other wiring but you probably need a new sending unit based on the age. Thin film tchnology has STILL not been perfected!!!

 

the whole fuel sender circuit, including the variable resistor pot is the ground for the gauge head. A short in the wiring lays the needle on the "full" mark.


test it - with key on;

note that the gauge currently reads "empty".

take the wire off the sending unit and connect it to ground. the needle should go full deflection (full mark).

if it does, that eliminates the wiring and gauge --- the sending unit is bad.

I would just put a new one for all they're worth, the job of changing it might be anything short of amusing.

keep in mind; the sending unit is indexed (it will only fit in one way).

 

I think on those the tank capacity is kinda etched into the tank itself in a circular fashion around the filler opening. It's not real bold and it can get grimy around there, so spiff it up real good and you should see it.

It's actually a federal law that the tank capacity be specified on the tank.

 

I thought the same thing, but I'm not sure if it was the FMCSA rule back in 1985.

 

I did find the fuel tank size stamped on the tank, just not where I have seen them before. It is in very fine print, and almost impossible to read.
Thank You for the advice.

 

Was it around the filler neck?

 

No, this is where I have always seen them, but this one was in small letters, about 10 font size on the top side.