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  #1  
Old 09-25-2011, 10:38 AM
JetMech07 JetMech07 is offline
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Fuel pump runs constantly

It started out w/ my wife swapping from the front fuel tank to the rear tank due to the fuel qty getting real low. Then the next day when I drove it I had bad fuel qty and ampmeter indication, I found a blown fuse on my fuel qty/ampmeter gauges. I replaced the fuse and got my indications back. Now I hear my front tank fuel pump is running constantly when selected to its position. Its now full of gas and still runs constantly. The rear tank/pump operates normal. I am wondering if I ran that pump empty and lost its prime? Any suggestions on the best way to burp it? or other suggestions?
Thanks
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  #2  
Old 09-25-2011, 08:03 PM
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Are you talking about while the truck is running? or Key off. The pump should run all of the time if the engine is running.
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Old 09-25-2011, 08:24 PM
JetMech07 JetMech07 is offline
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fuel pump running constantly

LXman
Fwd fuel pump runs constantly. eng running or not(w/ key on). I assume its not getting fuel pressure to shut it off like it will do once the key is turned on for a second before a start.
I was looking @ the fuel connections, It looks like there are the special connections which I dont have the tool to get them off w/ me. Anyplace to disconnect the line w/ a wrench? I havent dug into it too much, just l
ooking for ideas
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Old 09-26-2011, 08:16 AM
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Read below only if you have a stock fuel system and the wiring from the computer to the fuel pump relay by way of the EEC self-test plug is not shorted to ground.

There is no way the rear fuel pump can run normal if the front tank runs all the time when selected.

Fuel pressure has nothing to do with the running of the pumps. Both pumps when selected run on a one second timer inside the EEC Computer. The Computer will run the selected pump for one second when the key is first turned on and again for one second every time it sees a PIP pulse. The selected pump will not run at any other time.

You did not say if the engine runs or not so I am guessing it will not run. Unplug a fuel injector and plug the plug into a NOID light and crank the engine. If the NOID light does not flash and you have spark replace the Computer. You can rent a NOID light at most auto part stores.
If you do not have spark unplug the computer and see if you get spark then. If you have spark with the computer unplugged and not with it plugged in change out the computer.
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Old 09-27-2011, 12:01 AM
JetMech07 JetMech07 is offline
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SubFord,
the truck runs fine, I have put over a tankful(from the rear tank) and she ran fine, so yes it will run with the fwd pump running. Also the fuel pump does have something to do fuel pressure, thats their job! maybe via the pip signal but its job is fuel pressure so I am a little confused w/ what your trying to say. I am still looking for ideas on this. I plan on disconnecting fuel filter and try to purge this pump that way. I believe the computer is not an issue per the above statement so other items left are wahat, the switch and pump and regulator maybe?
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Old 09-27-2011, 07:25 AM
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You gave me the idea that you thought that the fuel pump turns off when the pressure builds up, this is not true. The computer turns the fuel pumps on and off and the computer does not monitor fuel pressure.

You may have a bad front tank boost pump drawing to much current and sticking the fuel pump relay closed when the ground goes away.

The fuel pressure regulator just bypasses fuel back to the tank when the fuel pressure goes over what the spring and vacuum says it should be. With the engine not running the regulator will by pass fuel to maintain 40-45psi and with the engine at idle with max vacuum about 30-34psi.
These are the pressures for a V-8 engine, a 4.9L is a little higher.

The HP frame fuel pump will pump to about 95-100psi into a closed head and will run all the time it has power on it. But the fuel pressure has nothing to do with the running of the fuel pump.

All three pumps are controlled by one fuel pump relay in the engine compartment and the relay is closed when the computer grounds the coil of the relay.

As said above the computer will ground the fuel pump relay for one second when the key is first turned on and again for one second every time it sees a PIP pulse. The fuel pump runs at no other time pressure or no pressure.

A PIP pulse comes faster than a second from the distributor when the engine is cranking or running so the pump will run all the time the engine is cranking or running and has nothing to do with pressure. When the engine is not cranking or running their is not a PIP train so the pumps will not run with the key just on.

All pumps run off the same fuel pump relay so there is no way one LP tank boost pump can run all the time the key is on and the other tank boost pump does not when selected. The LP boost tank pumps are selected by the switch on the dash and the HP frame pumps wiring bypasses the selector switch so it will run when ever the fuel pump relay is closed.

The power to the HP frame pump and the selector switch comes from the the Inertia fuel shut-off switch by the parking brake control on the safety wall. The Inertia fuel shut-off switch power comes from the Fuel pump relays NO contacts. The common contact of the fuel pump relay is fed from a fuse link by the starter solenoid. Power to the fuel pump relay coil is fed from the NO contact of the EEC Power Relay.

I hope this clears up your question. The above is the operation with factory parts and wiring with all good parts.

The below diagram says 302 but both the 4.9L and the 5.8L are the same. The 460 had a Carb in 1986.
Click the image to open in full size.

Note that in 86 you may have the filter inside the selector fuel valve on the frame and may or may not have a filter on the frame.
Click the image to open in full size.
/
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Old 09-27-2011, 03:05 PM
JetMech07 JetMech07 is offline
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fuel pump runs constantly

I was thinking the pressure DID shut off the pump like a bunch of folks I asked so thanks for that input!
So since the truck does run, I burned over a tank since this has started acting up! It sounds like a boost pump on my front tank per your eval. That might be why I burned a fuse around the same time I had this fuel pump problem.Although I dont see and gauge indictions in the diagram. I dont have any manuals/diagrams on this new truck(to me) yet.
I'll try and siphon my full front tank out and drop it and see what I can find. I imagine its a good idea to replace the whole unit in the tank? Thanks for not getting mad or defensive subford and thank you for your info.
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Old 09-27-2011, 03:11 PM
JetMech07 JetMech07 is offline
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oops I now see the y/w wire to the gauge! I'll ck things out tomorrow hopefully
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Old 09-28-2011, 07:38 PM
JetMech07 JetMech07 is offline
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While I was slowly syphoning gas from my full front tank I decided to try the key and see if the pump came on. I found w/ 1/2 tank of fuel, the fuel pump worked as required or came on for the 1 second and off. I tried it 4-5 times and had same effect. I'm going to run the fuel out of the fwd tank and probably pull that pump and see how it looks.
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Old 10-01-2011, 09:10 PM
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Good luck, If the gauge works fine, I would just replace the pump itself.
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Old 10-01-2011, 09:10 PM
 
 
 
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