fuel situation
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Trailering
Joined: Apr 2025
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fuel situation
I have a 1986 f350 6.9 idi dual tank. When i bought it i was told the rear tank didn’t work. I’ve switched it to the rear to see if it did work but how do i know the switch is actually changing the fuel over. not positive on how these fuel systems work each tank has a pump and the switch switches which pump is drawing gas right? so i’m wondering if the switch doesn’t work or if the pump itself doesn’t work and which is more likely since i know these have those issues. I’m fine with buying a new tank and pump, even lines just don’t want to do all of that if it’s just something electrical. appreciate all help.
Lead Driver
Joined: Nov 2013
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1986 wiring diagrams here:
https://www.garysgaragemahal.com/1986-evtm.html
See page 98 for the diesel. I'm not familiar with the diesel models, but that diagram does not show an in-tank pump.
Note the tank selector valve is motor-driven. That means it stays in its last commanded position when power is removed. This is not to be confused with a spring-loaded valve (like on my '84 351W), which.automatically returns to the default position without power.
https://www.garysgaragemahal.com/1986-evtm.html
See page 98 for the diesel. I'm not familiar with the diesel models, but that diagram does not show an in-tank pump.
Note the tank selector valve is motor-driven. That means it stays in its last commanded position when power is removed. This is not to be confused with a spring-loaded valve (like on my '84 351W), which.automatically returns to the default position without power.
Lead Driver
Joined: Apr 2002
Posts: 5,352
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The diesel engines came stock with an engine mounted mechanical fuel pump. Some older IDIs may have been modified with an electric fuel pump. But it will not be in tank. Usually located on a frame rail or even in the engine compartment adjacent to where the mechanical pump was. A block off plate will sometimes be installed where the mechanical pump is normally installed.
If you flip the switch to the rear you should also see the fuel guage move as the switch for the sender is mounted on the valve. These valves are known to stick and sometimes are stuck partially where you are drawing from both tanks. Try topping off both tanks and switch to the rear tank and drive 20 or 40 miles and see which tank is drawn down. If your in tank senders are working that can be helpful. But even if they don't work perfectly you should see the fuel guage needle move when switching.
If you flip the switch to the rear you should also see the fuel guage move as the switch for the sender is mounted on the valve. These valves are known to stick and sometimes are stuck partially where you are drawing from both tanks. Try topping off both tanks and switch to the rear tank and drive 20 or 40 miles and see which tank is drawn down. If your in tank senders are working that can be helpful. But even if they don't work perfectly you should see the fuel guage needle move when switching.
Last edited by BigBlue2; Oct 21, 2025 at 07:45 AM.
Lead Driver
Joined: Apr 2002
Posts: 5,352
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I have had some fuel tank valve issues lately. This post may have some solutions:
https://www.ford-trucks.com/forums/1...d-86-6-9l.html
On these valves if they are not switched often, like leaving it on the front tank all the time, they will become stuck. I suspect some debris may have entered my valve. Switching back and forth while driving might flush out the valve. I recently did that and it seems to have worked. See the last few posts in the thread above. On these valves use them or lose them.
https://www.ford-trucks.com/forums/1...d-86-6-9l.html
On these valves if they are not switched often, like leaving it on the front tank all the time, they will become stuck. I suspect some debris may have entered my valve. Switching back and forth while driving might flush out the valve. I recently did that and it seems to have worked. See the last few posts in the thread above. On these valves use them or lose them.
Moderator
Joined: Jan 2001
Posts: 56,999
Likes: 2,746
From: Virginia
Turn the key to run, then flip the dash tank selector switch back and forth. With the engine off, key on, you should hear a thunk under the truck from the valve switching. What was said is correct, you do not have lift pumps in the tanks. Your original lift pump is on the pass side front of the engine. It's difficult to see, but you can trace the fuel lines and see if it's still being used.
FTE Legend
Joined: Nov 2015
Posts: 30,940
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From: Angier, NC
1986 wiring diagrams here:
https://www.garysgaragemahal.com/1986-evtm.html
See page 98 for the diesel. I'm not familiar with the diesel models, but that diagram does not show an in-tank pump.
Note the tank selector valve is motor-driven. That means it stays in its last commanded position when power is removed. This is not to be confused with a spring-loaded valve (like on my '84 351W), which.automatically returns to the default position without power.
https://www.garysgaragemahal.com/1986-evtm.html
See page 98 for the diesel. I'm not familiar with the diesel models, but that diagram does not show an in-tank pump.
Note the tank selector valve is motor-driven. That means it stays in its last commanded position when power is removed. This is not to be confused with a spring-loaded valve (like on my '84 351W), which.automatically returns to the default position without power.
Turn the key to run, then flip the dash tank selector switch back and forth. With the engine off, key on, you should hear a thunk under the truck from the valve switching. What was said is correct, you do not have lift pumps in the tanks. Your original lift pump is on the pass side front of the engine. It's difficult to see, but you can trace the fuel lines and see if it's still being used.
Now this 3 port valve dose thunk when you switch but it also defaults to only 1 port, rear tank in my case, but the senders still work as they should as the gauge switching is in the dash switch not the valve..
I normally use the smaller front tank first then switch to the rear. There was 1 time I did not see the front tank dropping like it should and when I switched to the rear found it was lower.
A blown fuse was the cause of no power to the dash switch and then to the valve so it defaulted to the rear tank.
Dave ----
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From: Virginia
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Lead Driver
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I've never heard mine clunk. It would be rare for me to switch tanks with the key on motor off though. Might be easier to hear with the door open. With the key off flip the tank switch. Then open the door and turn the key on. Maybe stand outside as low as you can and reach up for the key? Or get a helper to do that while you are close to the valve. The senders will also switch and you can usually see the fuel guage move. Beware that you can be stuck part way in the valve but the sender will probably indicate the tank with the widest opening. I saw that happen with mine.
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Brett Foote
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From: Angier, NC
Dave ----
Lead Driver
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I have never heard mine thunk. Might try that next time I'm going somewhere. I can see why you would think it wouldnt. I was wondering the configuration. Maybe Garys site has more valve information? Also depending on your WB and cab. My supercab might have the valve further aft?
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