Why is my rear tank draining my front tank? - Diesel Truck Forum - TheDieselGarage.com
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post #1 of 9 (permalink) Old 10-21-2010, 09:40 PM Thread Starter
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Why is my rear tank draining my front tank?

When we are driving our 96 F250 gasser on the rear tank is sputters and tries to die out. Even with a full tank it runs like it is almost empty. The strange thing is it is sucking fuel from the front tank and over filling. The truck runs fine on the front tank. At first I thought it was the selector valve. I thought it was somehow redirecting fuel into the rear tank due to a malfunction. We even went to the parts store and bought a new one. Well guess what… this truck does not have one. It has a block that all the fuel lines converge into. So I now believe the sending unit in the rear tank is bad. That would explain the sputtering. But how do I explain the removal of fuel from the front tank to the rear?
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post #2 of 9 (permalink) Old 10-21-2010, 10:01 PM
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whens the last time you changed the filter ?
could the pickup/foot/screen in the rear tank be clogged w/ dirt ?

I had a 96 (f150 w/ dual tanks) - the sending unit on the the front tank went,
gauge would lay down on E when the tank was full (i thought someone was siphoning
my gas....back when it hit 1.25 a gallon HAH!)

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post #3 of 9 (permalink) Old 10-21-2010, 10:45 PM Thread Starter
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I just replaced it a couple of weeks ago thinking that might be the issue. i know the problem you speak of. The little circut board on the sending unit goes bad and the gauge will not work. That is not the problem here. Both tanks reads correclty as far as the fuel level. I did not think of the screen being clogged though. It looks as though I can not longer avoid dropping the tank. So I will be doing so I hope tomorrow. I will know then if that is the issue.
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post #4 of 9 (permalink) Old 10-21-2010, 11:25 PM
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bummer - always happens when the freekin' tank is full

check all your connections....i forget but isnt the pump on that pickup too ???
maybe when you swapped out the sending unit you forgot to tighten something and
now its intermittently coming loose.

make sure youre not putting a jack in direct contact w/ the TANK...i built/used an
"I" beam outta some 2x4's so that the tank was lowered and raises w/out contact
to the center of the tank (where the tank could flex & contact the pickup/foot).

I'm bewildered by the siphoning from one tank to the other that youre experiencing.

Don R.

'04 350 4WD Lariat Crew Longbed (built 05.04)
-------------------

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Gear Vendors Dbl Overdrive Install w/ Pics.
135 Amp Alternator Upgrade/Replacement
50 Gal. Aux Tank; Pump, Filter, Wiring
Truck Audio,
CUSTOM Valentine One Install - LINK
$10 Grill Mod. (protects the coolers) !

Block Heater Cord (Rear Mounted w/ Timer) - LINK

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post #5 of 9 (permalink) Old 10-22-2010, 01:04 AM Thread Starter
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My wife had it done while I was in Iraq. So I am not sure how the install went. The going from one tank to the other is what has me hung up as well.
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post #6 of 9 (permalink) Old 10-23-2010, 08:10 AM
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How the dual tank system works is when you toggle the switch is transfers the power from one fuel pump module to the other. The fuel supply line is shared between the 2 fuel pumps so it basically relies on a internal check valve built into each pump.

If a check valve fails the other pump is not only going to supply fuel to the engine is also going to be pushing fuel into the other tank through the failed check valve. Probably the reason for the stuttering is the pump does not have enough volume to supply the engine and also cross flow to the other tank.

There was a recall that addressed this concern on older F-series trucks in the early 90's (90-93ish) for cross flow conditions. It involved putting inline check valves on the fuel supply lines at the pumps. I dont know if the parts are still available or not, or if they are compatible with a 96 but it could possibly be a cheaper fix than replacing the pump.

The only thing needed to put the check valves on was disconnecting the fuel supply line at the pump and snapping the inline check valve on and snapping the supply line back onto the check valve. Granted it sounds easy but getting to the lines at the pump and then releasing the retainers was sometimes a PITA.

The latest recall number was 01M05, the earlier recall number was 93S68. The latest part number I have for the check valve kit is 1L3Z-9J274-AA.

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post #7 of 9 (permalink) Old 10-23-2010, 09:27 AM Thread Starter
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I think I am fallowing what you are saying. Where I am confused is which fuel pump is bad? The front one seems to feed the engine fine. But the rear one is what makes the trucks sputter. But the fuel from the front tank is going to the rear. So does that mean the front tank has a bad valve and is sending gas to the rear? Or, is the rear fuel pump going bad and pulling it.
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post #8 of 9 (permalink) Old 10-23-2010, 02:55 PM
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I kind mis-read the cross flow part, kind of strange... If it is transfering fuel from the front to the rear it would be the check valve in the rear pump that has failed. Maybe since the check valve has failed it has also caused a restriction which could explain the running issue. It could also be that the rear fuel pump itself is failing too. If you had a fuel pressure guage to hook up to the fuel rail you might be able to verify this. Regardless of if you have a check valve or fuel pump problem in the rear tank, both concerns would be fixed with a fuel pump replacement.

My motto: Be nice, be polite, but have have a plan to kill everyone that you meet.

94 F250 IDI turbo, auto 2wd.. ATS housing, 3" down, 4" exh, guages. 104k miles got totalled by a damn mustang (parts now).

98 12v, 4x4 quad cab. auto with BD T/C & V/B, Banks Big head, Exh brake, AFC modded, 16.5ish*, KDP gone, 60lb springs, 4K-GSK, BHAF, pyro, trans, and boost guages. 4" straight pipe,

72 AM General M35A2, LDT465-1C "Whistler" Turbo, PTO winch.
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post #9 of 9 (permalink) Old 10-24-2010, 06:53 AM Thread Starter
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Thanks! That is the clarification I was looking for.
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