fuel pressure low
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fuel pressure low
hi!! I have a 1986 ford f250 truck xl with a 7.5 liter (460 ci) engine, duel tanks, electric fuel pumps in the tanks. Heres the problem: i had to replace the carb cuz of a fire. I can get the truck to start for about 30 seconds then dies. I have fuel flow but no pressure at all that i can tell. I have replaced all fuel filters, all rubber hosing. It seems the fuel tank switch and valve are working ok. Everything worked before the fire. Why i dont have fuel pressure?? I had a carb mechanic and mechanic both say that i need pressure to the carb. I was also told that i can put an electric fuel pump just before the in-line fuel filter to boost the pressure to the carb, it might fix the problem. Is the true? I was also told about the oil pressure switch which shuts the fuel pump off if the switch is bad or no oil pressure. I have oil pressure. I would like to know how to jumper the oil pressure switch to see if thats the problem. any other help would be appreciated.
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From: Virginia
The oil pressure switch has two wires. When you have pressure these two wires are electrically tied together. So just unplug the switch and jumper the terminals in the plug to simulate oil pressure.
Here's a diagram of your system. Just make sure your pumps in the tanks are actually running. I would not add a fuel pump, the ones in the tank can do the job if they are working correctly.
See that wire hooked to the starter relay(I believe it will be dark blue)? That bypasses the oil pressure switch/fuel pump relay while cranking, so the carb is supplied fuel before the engine starts, since it doesn't have fuel pressure till it's running. You might want to try and hot wire this wire to the battery and see if you can get it running. That might clue you in to where the problem is if it will or won't run with this wire jumped to the battery.
Here's a diagram of your system. Just make sure your pumps in the tanks are actually running. I would not add a fuel pump, the ones in the tank can do the job if they are working correctly.
See that wire hooked to the starter relay(I believe it will be dark blue)? That bypasses the oil pressure switch/fuel pump relay while cranking, so the carb is supplied fuel before the engine starts, since it doesn't have fuel pressure till it's running. You might want to try and hot wire this wire to the battery and see if you can get it running. That might clue you in to where the problem is if it will or won't run with this wire jumped to the battery.
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ok here it goes now i'm lost. I tried the oil pressure switch, did not solve the problem. Did what you asked about the d blue wire on the starter relay. Fuel buzzed, pumped fuel to the in-line filter and filled it, filled the carb. Started it and purred like a kitten. Took hot wire off, it ran until fuel ran out. So now i know fuel pumps work, fuel tank switch over on the dash works. What should i do now??? I'm stumped. thanks for the advice so far.
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by the way i'm to square 1 after i hooked the blue wire to the battery and the truck ran fine. The fuel pump just kept going after i shut the truck off. I hooked all the wires back and it started all over again. No fuel pressure. Where do i go from here any ideas?
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From: Exmore, VA
Franklin, wrong diagram. 86 doesn't use a tank selector relay. My website has the 86 7.5L wiring diagram. Fuel pressure is real low, on the order of 3-5 psi. The pumps are centrifugal electric in-tank pumps.
The blue wire from the starter relay bypasses the oil pressure switch and relay, it directly powers the pump circuit.
Go to my website, downloads are on left side, center download column is Ford, 86 7.5L is sixth one down.
The blue wire from the starter relay bypasses the oil pressure switch and relay, it directly powers the pump circuit.
Go to my website, downloads are on left side, center download column is Ford, 86 7.5L is sixth one down.
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From: Virginia
Might be wrong diagram, but from the experiment he ran by powering the blue wire directly, we know it's in the control circuit in the engine compartment.
Look around for the fuel pump relay. I believe it might be mounted over near the brake booster area under the hood. If you can find another, swap it in and see what happens. That's what the oil pressure switch controls, is the pump relay.
The fuel pumps kept running when you powered the blue wire, because that was just an experiment to see what was working and what wasn't. Of course you just can't leave this wire hot wired to the battery and call it good. If you are desperate and need this truck running right away, you can hook the blue wire to a switch, and hook the other side of the switch to the battery +. You will need to turn this switch off each time you turn the truck off, but if you really need this truck you can run it this way till you figure out the real problem.
Look around for the fuel pump relay. I believe it might be mounted over near the brake booster area under the hood. If you can find another, swap it in and see what happens. That's what the oil pressure switch controls, is the pump relay.
The fuel pumps kept running when you powered the blue wire, because that was just an experiment to see what was working and what wasn't. Of course you just can't leave this wire hot wired to the battery and call it good. If you are desperate and need this truck running right away, you can hook the blue wire to a switch, and hook the other side of the switch to the battery +. You will need to turn this switch off each time you turn the truck off, but if you really need this truck you can run it this way till you figure out the real problem.
Old School Hot Rodder
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From: Exmore, VA
I would be willing to bet some of the wiring was damaged when he had the fire. The wiring runs down the inside of the left valve cover and passes real close to the fuel inlet fitting. He didn't say what caused it.
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what caused the fire was the float on the carb was not set right after rebuilding the carb. I checked all the wiring, only one wire burnt off. Only the top part of the engine caught. Fire was put out pretty quickly.I will look for the relay and see about getting another one and let you know what happen. I was thinking about running wires and a off-on switch in the cab to control the fuel pumps for now cuz i have no other wheels. thanks for the advice. i will let you the outcome.
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If you do the wire from the battery, install an inline fuse near the battery.
I'd think it'd be easier and faster to just find a different relay from a JY or sumthin, or do some tests if you have a volt/ohm meter.
I'd think it'd be easier and faster to just find a different relay from a JY or sumthin, or do some tests if you have a volt/ohm meter.
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i"m back to square 1 again No fuel pressure. I had a minor set the last couple of days. My flywheel ring teeth were ground down and some missing. Fixed that, went to start the truck and back to no fuel pressure. I tried the hot wire trick and still no fuel pressure. In the hot wire mode i can hear the fuel pump work. I tried switching tanks and can hear the fuel pump work(running) with still no pressure. Any ideas as to what it could be? Could it be thetank selector valve stuck? any information would be appreciated. thanks
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From: Virginia
Pull some of the lines off underneath at the switching valve. Do something like this; Switch to the front tank, verify the front pump is making noise, then turn it off. Pull the line off the valve that comes from the front tank, extend it into a bucket, and see if the bucket fills with fuel when the pump is turned on.
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both fuel pumps make noise, i used the switch in the cab to change the tanks, that works. ok will try again to pull lines off and try that. Its snowing and cold here in callif. So i will try it. will let you know the outcome
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alright got the truck running. The fuel pumps both work, pump fuel to the switching valve. I took switching off and hooked the fuel lines up to the valve 1 tank at a time and hot wired the fuel pumps and valve to see if fuel would go through the valve and it does. I then hooked the blue wire back to the starter relay and the pumps dont pump. Could the problem be the starter relay? Everything else checked out fine. In the mean time I have a wire going from the blue wire to the fuse box so when i turn the key on the fuel pumps will work and I can drive the truck. Any other feed back will help. And by the way THANK-YOU