On my 85 f250 6.9. My fuel Gauge will read full when I fill up. Then When it hits about 3/4. It will drop all the way to E. the front and back tank both do it. Any ideas what it could be?

 

bad float maybe

 

Yeah My 85 does the same damn thing when it say's E it only take's 10 gallons to fill her back up.

 

The brass floats get filled up with fuel and it causes this normally. you can get the floats for like $10

 

Who has the floats?

 

If the floats are bad and gettin fuel inside them, they will not read full, as they will not float. I would be more inclined to say the circut board, or the arm that rubs against the board is bad as the OP stated that it reads ok untill it reaches half full then drops to empty.

 

Mine does this to. Starting to get annoying

 

I got mine at the dealer but I have seen them on ebay and rock auto. just pull the sender and pop off the old and in with the new.

Mine were bad and I replaced them only to find out the the sender is wore out so it sill does not read quite right.

 

Rebuilt Fuel Sending Unit Specialist

 

Thank you!

 

My '86 IDI rear tank does this, gave me my first WHERE'S A DIESEL STATION moment as I was empty in front trying to use up what fuel was sitting in it when I bought the truck. Took 8 gallons to fill. I have since found that it shows "E" by about 100 miles, stays there to maybe 125 miles, and then comes back up to 1/4 or so and stays there until dropping back down at 200 miles.

Excellent resource, thanks so much for posting that. I can deal with my rear tank on the IDI for now but the C4 race car is darn near random so I think I will send it to them in the off-season - many thanks!

- Jeff

 

What you are describing sounds like the variable resistor part of the sender is bad. The float arm moves along the wires and it has bad spots not making good connection. Not a float problem, as it does read right when full or down a bit. The problem is the sender. A fuel gage is just a resistance gage that is calbrated E to F instead of numbers. I forget the Ford resistance values for the fuel tank sender range.

 

Mine is doing the same thing. Right after I fill up, the gauge reads about 3/4 full. My filler neck is botched up, so I only can fill it that much. After a while when the gauge reaches half, it drops to E but doesn't hit the peg. I knew I didn't burn that much fuel. I thought I was going to run out of fuel when it first happened. I always carry a spare can of fuel in the back just in case, so I wasn't too worried.