1985 dual tank question
11-04-2009, 09:17 PM
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1985 dual tank question
I recently aquired a 1985 f150 from my brother-in-law which has a dual tank fuel system (5.0 EFI, Automatic). He told me that only the rear tank worked and he thought that the in tank pump in the front tank was bad. He is not real good with mechanical stuff so the other day I switched to the front tank to see what would happen.
The rear tank was at 3/4 tank and the front (bad) tank was full (a friend filled it up for me not knowing the front tank didn't work). To my surprise, the engine didn't die but kept running normally. After about 15 min of idling in the driveway, I switched it back to the rear tank only to discover that it was now at 1/2 tank instead of 3/4. My guess is the fuel went into the other (much larger) tank. I'm not familier with these dual tank systems so I crawled underneath to see what I could see.
From what I can tell, there is a fuel pump in each tank which feeds some sort of filter/resevoir which is then fed into another (probably high pressure) fuel pump. I didn't notice any kind of switching valve.
My question is how do these fuel systems work? Do both pumps run continously or does the switch in the cab control power to each tank pump? If so, I would think there would be some sort of check valve somewhere. If so, where would it be? I did a brief forum search but was unable to find a clear answer.
The rear tank was at 3/4 tank and the front (bad) tank was full (a friend filled it up for me not knowing the front tank didn't work). To my surprise, the engine didn't die but kept running normally. After about 15 min of idling in the driveway, I switched it back to the rear tank only to discover that it was now at 1/2 tank instead of 3/4. My guess is the fuel went into the other (much larger) tank. I'm not familier with these dual tank systems so I crawled underneath to see what I could see.
From what I can tell, there is a fuel pump in each tank which feeds some sort of filter/resevoir which is then fed into another (probably high pressure) fuel pump. I didn't notice any kind of switching valve.
My question is how do these fuel systems work? Do both pumps run continously or does the switch in the cab control power to each tank pump? If so, I would think there would be some sort of check valve somewhere. If so, where would it be? I did a brief forum search but was unable to find a clear answer.
11-05-2009, 09:01 AM
#2
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The dash tank switch will turn power on to only one tank pump at a time.
The filter reservoir thing is the tank selector valve (Non electric). The valve works by fuel pressure provided by the running tank pump. The primary function of this valve is to switch the fuel return lines from the engine EFI system back to the selected tank.
If one tank pump does not run when it should AND/OR if the fuel selector valve is not functioning then fuel can be returned from the engine to the wrong tank. If the valve is really messed up, it is possible for fuel to be directly pumped from one tank to the other.
Also if the selected tank pump is not working then the engine may still run, but you will hear the high pressure pump straining.
Look for my replies in these threads for more details:
https://www.f150forum.com/showthread...985#post148985
https://www.f150forum.com/f13/86-f-1...working-26443/
https://www.f150forum.com/f13/1985-f...302-efi-27233/
https://www.f150forum.com/f13/1986-1...em-woes-33018/
https://www.f150forum.com/f13/1986-f...roblems-33656/
The filter reservoir thing is the tank selector valve (Non electric). The valve works by fuel pressure provided by the running tank pump. The primary function of this valve is to switch the fuel return lines from the engine EFI system back to the selected tank.
If one tank pump does not run when it should AND/OR if the fuel selector valve is not functioning then fuel can be returned from the engine to the wrong tank. If the valve is really messed up, it is possible for fuel to be directly pumped from one tank to the other.
Also if the selected tank pump is not working then the engine may still run, but you will hear the high pressure pump straining.
Look for my replies in these threads for more details:
https://www.f150forum.com/showthread...985#post148985
https://www.f150forum.com/f13/86-f-1...working-26443/
https://www.f150forum.com/f13/1985-f...302-efi-27233/
https://www.f150forum.com/f13/1986-1...em-woes-33018/
https://www.f150forum.com/f13/1986-f...roblems-33656/
11-05-2009, 08:14 PM
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Thanks
Thanks for the info. I fear that due to the age of this vehicle, I may have multiple issues. I think I will start by ensuring the tank pumps work then pulling the switch/resevoir and cleaning it up (probably replace filter too). I want to avoid dropping the tank and having to see how much gunk is in there (ignorance is bliss) but I may end up doing that if there is a bunch of gunk in the tank switch/resevoir.