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Old 06-12-2009, 18:01   #1 (permalink)
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1992 F150 Fuel Tank Problems

I have a 1992 F150 4x4, dual tank, 5.0L, with fuel injection. There are two high pressure pumps, one in each tank. There is no third pump on the frame rail. There is no distribution valve on the frame rail.

Here is the problem. Recently replaced the fuel tanks due to rust/leaks. After the replacement the tanks were filled with fuel. I have been running the truck with the tank selector switch set for the front tank. The fuel gage needle does not come down off of full. However, when I switch to the rear tank, I notice that the fuel gage is down quite a bit. It seems the truck is drawing fuel from the rear tank, even though the selector switch is set for the front tank.

I have gotten under the truck while a helper turns the key to the on position, engine off. The front pump runs for a short burst when the key is turned on and the selector switch is set for the front tank. I then repeated the test with the selector switch set to the rear tank and the rear fuel pump runs for a short burst.
I then held my hand against each tank to feel the vibration of the fuel pump and after verifying the front pump is in fact running started the truck. I can feel the vibration of the front fuel pump running while the engine is running.
I repeated this same test with the rear tank. I also stuck a small tube into the tank and put the other end in my ear and can hear the fuel pumps running. First with the selector switch on the front tank the then with the selector switch on the rear tank. I have also verified that only one fuel pump runs at a time (i.e. both aren't running together).
I have removed the selector switch and checked it with a digital multimeter and the switch seems to function properly. I also jumpered the connector from the selector switch to select the front fuel pump and it does operate.
I am at my wits end with this problem. Does anyone have any insight as to what is going on? How do I further diagnose the problem? I don't feel like throwing a bunch of money buying fuel pump modules if that isn't the real problem. Please help.

Thanks
eric
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Old 06-14-2009, 09:00   #2 (permalink)
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Re: 1992 F150 Fuel Tank Problems

You've correctly diagnosed & confirmed that the fuel pump corresponds to the switch setting. Now you need to confirm that the GAUGE corresponds. I can't say how it would have happened, but it seems that when the front tank pump is selected, the rear tank LEVEL SENDER is connected to the gauge, & vice versa.

There's a known issue of fuel returning to the wrong tank causing it to overflow, but that's totally different from your symptom.
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Old 06-15-2009, 05:34   #3 (permalink)
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Re: 1992 F150 Fuel Tank Problems

Steve:
Thanks for your reply. I went out and jumpered the front fuel pump, and also jumpered the front fuel sender. As expected the gauge read nearly full for the front tank. I then jumpered the rear fuel pump, and rear fuel sender. The gauge read around quarter tank. These guage readings matched the readings obtained using the switch.

I then drove the truck to a gas station, and pumped $20.00 worth of gas in the rear tank. It took it all and that pushed the gas gauge up to three quarter tank. I tried to put some gas in the front tank, but it wouldn't take it. So, I think the gauge is measuring the amount of fuel in each tank correctly.

I am aware of the fuel tank return line issue with fuel being transferred from one tank to the other. However, this truck has the one way valves for the recall installed.

Is there a way for the running pump to receive fuel from the other tank? I know there are valves and such in the fuel pump/sender modules in each tank. Is there some failure mode that would cause my symptoms?

Thanks
eric
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Old 06-17-2009, 17:35   #4 (permalink)
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Re: 1992 F150 Fuel Tank Problems

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Originally Posted by starfishkiller View Post
I went out and jumpered the front fuel pump, and also jumpered the front fuel sender. As expected the gauge read nearly full for the front tank. I then jumpered the rear fuel pump, and rear fuel sender. The gauge read around quarter tank.
Where exactly did you do this jumpering to & from? If you did it AT the switch connector, then you missed the point of the test. You need to bypass all the truck's wiring from the tanks to the switch connector.
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I have been running the truck with the tank selector switch set for the front tank. The fuel gage needle does not come down off of full.
Quote:
Originally Posted by starfishkiller View Post
I then drove the truck to a gas station, and pumped $20.00 worth of gas in the rear tank. It took it all and that pushed the gas gauge up to three quarter tank. I tried to put some gas in the front tank, but it wouldn't take it. So, I think the gauge is measuring the amount of fuel in each tank correctly.
Not from what I'm reading here. You drove on the front tank, but the fuel came from the rear tank. Something is crossed between the tanks & the switch.
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Is there a way for the running pump to receive fuel from the other tank?
No.
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Originally Posted by starfishkiller View Post
Is there some failure mode that would cause my symptoms?
I can't imagine any. I think it has to be incorrect reassembly, but I can't pinpoint where from what you're posting.

If you're absolutely certain you've tested the level sender circuits correctly, then I'd set the switch & run one tank dry to find out what's happening.
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Old 06-18-2009, 06:21   #5 (permalink)
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Re: 1992 F150 Fuel Tank Problems

Steve:
Thanks for your reply. I have since replaced the tank selector switch on the dashboard. I also have talked to people I know about the problem and a few of them thought I should run the rear tank dry to see what happens. I think you also are suggesting that idea, and I think that is what I will do. First though, I will put a 5 gal. jug of gas along with a funnel in the bed of the truck, just in case the truck stops running when the rear tank is empty.
thanks again for your help.
eric
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