Fuel gauge erratic reading
 

Hi all,
I've found that the reading of my fuel gauge is false ! When i totally fill the rear tank the reading is not at "full" but maybe between 3/4 and F. Next day after 20 or 30 miles the reading is below 1/2. And some days later, without driving, the reading is at 1/4 ! Seems that the reading changes at every start ! This happens on the rear tank, the front tank seems to have a correct and constant reading.
Problem of tank gauge (i have electric fuel pumps) ?
Thanks

I would say the sender for that rear tank is going or is bad.
Could be just the float but being you have to pull the sender to do the float I would do the whole sender.
Now I don't know if you can get just the sender or if you have to buy the sender & pump as a unit?

I just did a quick check (86 F250 w/460 2wd), Rock Auto & Auto Zone, for the sender and all I find in the 19 gal. side mount tank $68 & $100 no rear tank listing.
It did not come with the electric pump and the line fittings was quick disconnect.
Don't know if that unit can be used in the rear tank but thinking no.
Dave ----

Ther're some differences between the shop manual and Rockauto !
https://cimg8.ibsrv.net/gimg/www.for...4fa61e0015.jpg
From service manual
https://cimg9.ibsrv.net/gimg/www.for...adf5cb5c34.jpg
from Rockauto

You said rear tank and the Rock Auto one is for the midship or side tank.
When I was looking for senders for my truck it use different senders for side & Rear tanks.
As for pictures you can't always go by then when shopping sometimes they use a picture that is close.
Just check that the sender you buy is for the tank you need it for.
Dave - - - -

If your tank is steel you will have the sending unit fuel pump assembly like the one pictured from Rock Auto which installs with a locking ring. The one pictured in your manual is a plastic tank with a threaded connection. Some after market plastic tanks have a locking ring and sending unit similar to the metal tank.

If your tank is a rear behind the axle type you can crawl up over the axle with a flash light and a mechanics mirror and see the type of sending unit you have. You can also see the type of tank by peeking around the skid plate, if you have one, or look over the frame rail near the tank filler tube. It's kind of hard to see since it's tucked up between the frames and behind the axle.

Ok, i'll try to see what unit i've, i'm 99% sure that it's a steel tank. The FSM says also that the rear (auxiliary) tank is a steel tank.
Problem, as stated by FuzzFace2, is that the sending unit from Rockauto is for front (midship) tank, not sure if it's the same in the rear tank ?

The sending units for the front & rear tanks are different, to accommodate the different shapes of each tank.

True, The top of the sending unit will look similar but the length of the pick up line and throw of the float could be different. You probably have a 19 gallon rear tank. What ever the tank is you need to get the sending unit for that specific tank.

When I replaced my rear tank I used the original sending unit even. It doesn't read accurately either but since I have a trip meter on the odometer I don't need an accurate gauge so I didn't replace it. If my tank didn't have a leak in it I wouldn't have replaced it either. If the only problem I had was the fuel gauge wasn't accurate I wouldn't have a problem at all. Removing the rear tank is not that much fun so I'd wait for a "real" reason to do that ...like a fuel leak!:D

Depending on the condition of the rear tank, you might want to replace everything, tank and sending unit. Since you have FI, if you do drop the tank to replace the sending unit then it would be a good idea to replace the fuel pump too. I don't have that worry as my fuel pump is mechanical outside the tank.

I haven't FI on the 460 ! But two electric pumps ...
If the replacement of the unit needs a removing of the tank i'll also wait ... The tank is well a 19 gallons but front is also a 19 gallons. I'll take a look at other years on rockauto, problem would be to find a rear unit with a return line (and the right connector).

Edit: after some research i found a (1987) 460 SPECTRA PREMIUM FG33C {#E8TZ9H307D, E8TZ9H307DA} Includes Strainer and Float Rear Steel Tank (same as the '86 302 FI rear unit FG33B).
https://cimg2.ibsrv.net/gimg/www.for...5bd52ceb21.jpg
https://cimg9.ibsrv.net/gimg/www.for...e5a3270cb6.jpg
https://cimg4.ibsrv.net/gimg/www.for...545b416f2b.jpg

A 1987 sending unit won't work in a 1986 vehicle, Ford changed how their dash gauges operate in 1987+. Sure, a 1987 sending unit will physically fit and plug in, but the dash gauge will read opposite of how it should (i.e. the gauge will read Empty when the tank is actually Full and vice versa).

that mean he could cut and swap the wires?

Why things are not simple ? So i forget the '87 unit (about swaping wires i see only one blue for the sending unit).
Curiously the '86 unit for 302 ci FI is pictured the same on Rockauto: Spectra Premium FG33B but the informations tell all ! See below the resistance full and empty.
As the '86 unit is for FI i don't know now if the non FI electric pump could be installed ....
https://cimg0.ibsrv.net/gimg/www.for...ceacb3dd17.jpg
https://cimg9.ibsrv.net/gimg/www.for...151cb77b11.jpg

I look at a '84 19 gal. rear unit (460 ci) https://www.rockauto.com/en/moreinfo...123816&jsn=737 and even if it'll not work because no provisions for electric pump, it has the same resistances as the '86 FG33B

Be sure to check the size of hole in the tank, all I remember is that there are 2 sizes - the "small" hole, around 2.5", used on tanks without electric pumps, and the "large" hole, something around 5.25" used with in-tank fuel pump configurations.

Sometime, around 1985(?), the sending unit hole in the tank became larger for all models, due to the fact that the 5.0L engine started to have FI while the 4.9L and perhaps the larger engines [5.8L] were still carbureted and had mechanical fuel pumps.