'86er

Hey all,

I'm having some issues with my dual tank system on my 1986 F150 5.0. I normally get 2 weeks out of front and rear tanks combined, but so far, I've gotten 2 weeks out of my front tank alone. The gas gauge seems to have stalled out at 1/2 tank. When I turn the car off, the needle goes down to the peg, but when I switched tanks, the rear supposedly has a half tank left too. I know thats not correct because I filled it up a few weeks ago and haven't used the rear tank yet. I need some help diagnosing the issue. I'm thinking it could be the switch, the gauge, or the tanks some how. Any help is much appreciated.

Thanks,
Max

Good old Bill

Am a little confused. To be on safe side, go fill up one tank only and see if gauge reads full. Switch tanks, it should be much lower, if not, drop tank with the least fuel and manually move sender up and down. Does gauge read accordingly?

klricks

Do you have the (non electric) fuel pressure operated tank switching valve?
Looks like this:
http://i608.photobucket.com/albums/t...ordfuelres.jpg
Located on the driver side frame rail behind the High Pressure fuel pump.

If the valve is not working then you can have strange things happening like fuel from the EFI fuel return being sent back to the wrong tank. Or fuel being drawn from both tanks at the same time etc.
Note that both in-tank fuel pumps must be working correctly in order for the selector to work properly.

Do a forum search dual tanks etc. , or search on my username. I have posted extensive info on fixing the dual tank system.

'86er

yeah, so i filled up my truck the other day and the gauge is working but i never switched tanks in the 2 weeks it took to get to the point where i needed to fill up. This leads me to believe that im drawing from both tanks at the same time. Is there any problem with that? Gas is being taken from both tanks, but i dont seem to be getting any worse mileage. Can i just leave it, or will something go wrong later on?

FordFiddler

I have had a similar problem with my '86 F150. I found, (after replacing regulator, and filter ) that the tank selector valve was contaminated with a substance that looked and felt like yellow clay. The selector valves were stuck partially open, and the 20psi diverter valve open. The o-ring on the diverter valve was in the bottom of the resovoir, The resovoir filter was completely clogged. I disassembled the unit, cleaned it and replaced the filter.
When I ran on the rear tank from full to empty, I would also find I had used about one-half of the front tank, with the selector switch on the rear tank.

'86er

Yeah, i have that valve. pressure is good, both pumps are working. I had the electronics checked out and they said it was the front tank "sender unit". whatever that is. The front tank still reads full after 2 weeks of the switch never leaving the front tank and the rear tank is down to just 1/2 tank. the needle on the front tank has gone from way above the full line to just over the full line, so it seems like gas is mostly being taken from the rear tank and some is being taken from the front tank. that or the valve decides to work every once in a while. still tryin to figure this out. the quote for new parts was $380, but i refused to have the work done.

klricks

The sending unit is the part which fits into the tank and tells the fuel gauge how much fuel is in the tank. It has coil of resistance wire. A float on a pivot arm moves a contact point across the coil to change the resistance. Just a variable resistor much like a volume control. A voltage is applied to the circuit so as the fuel level changes, a corresponding voltage is sent back to the gauge circuits.

Trucks with fuel injected engines will also have an electric low pressure fuel pump attached to the sender unit.

The low pressure pump(s) feed the high pressure pump. Measuring the pressure of the HP fuel pump does not tell you if the tank pumps are operating. You have to remove the tank fuel lines from the switching valve and run the fuel into a container to measure the flow rate.

The switching valve can be dissembled and cleaned up but if parts are worn or broken you won't find any parts to fix it.