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Old 03-10-2008, 10:40 PM
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Low fuel pressure

Got another one that has me really puzzled. 2001 F150 4.6 running really rough and the fuel pressure is only 25 PSI, remove the vacuum from the fuel pressure regulator and the pressure goes up to about 30 PSI. Since removing the vacuum increased the pressure I assumed the regulator was working properly and so diagnosed the problem as a bad pump (new filter), replaced the pump with a new one and nothing changed I still have the exact same pressure readings. Thinking that maybe the regulator was bad after all I replaced it with one that I am sure is good but again nothing changed, exact same pressure readings! I cleared the lines with air when installing the new pump along with a new pump filter (sock) and inline fuel filter then when that did not fix the problem I installed the new regulator. The only things not changed was the lines and they are clear so what else could cause a loss of fuel pressure?

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Old 03-11-2008, 12:52 PM
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re:Low fuel pressure

Hi ok l have a 2000 f150 4x4 with 4.6 triton, so l'm sure yours is the same, but my fuel pressure is 50 psi with key on and 35 psi with the engine running? anyways, mine was running bad once, so l when threw it, fuel filter, check pressure an it turn out to be the spark plugs wires got moist, 1 stupid little drop of water got down pass the wire(3 rubber rings) to the plug, it idled alittle rough but in gear going down the road, kind of had a little miss! l'm not sure but with the vacuum pulled off and running mine would shoot up into the 70,s, l think it was 72psi, l don't think this is going to help, but maybe there are more than 1 fuel pumps for them? TROY P.S l know you'll get it!!
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Old 03-11-2008, 04:57 PM
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Fuel Pressure

Well you did the right tests, does this truck have TWO tanks with a crossover valve....I have seen bad valves between the tanks, or possibly you changed the wrong pump if there are two. You can also test the max capacity of the pump by quickly pinching the return hose shut for a second, the presssure on a good pump will basically double indicating a good pump with plenty of reserve. On this Ford you should see close to 70 PSI when you momentarily pinch the return line. Do you have the 4.6 V8 or the 4.2 ine 6 cylinder ? The V8 would run at about 36 PSI, the 6 cylinder is about 50 PSI.

Last edited by oldcpecdr; 03-11-2008 at 05:44 PM.
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Old 03-11-2008, 06:49 PM
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I have the 4.6 and the max pressure I see now is about 35 PSI, either engine off or vacuum unplugged. With the engine running I have only about 22 to 25 PSI which according to Ford the engine should not even run. When I trip the relief on the pressure test gauge I get a good 1/4" stream of gas but hardly any pressure, I have tripped that thing before on a good test and it will squirt gas for some distance. Is it possible the fuel rail could be damaged? I am thinking maybe where the regulator seats may be leaking inside however I can not imagine why this would be since until the pressure dropped the regulator had never been removed.
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Old 03-11-2008, 06:57 PM
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fuel pressure

OK, AFTER you run the engine and shut it off what is the fuel pressure reading ? The system should maintain pressure even with the pump and engine off. Are you attaching the pressure gauge to the Schrader Valve on the Injector rail ? When you turn the key on it should go to about 38-40, with the engine running about 34-36, when you shut the engine off it should maintain 30-35. If it does not then there is a leak back to the tank, most likely the pressure regulator. A sticky or open injector can also cause a leak down. Is the engine idling normally ? Never mind I just read your post and saw the answers to my questions.

So if it is maintaining 35 with key on.. either the gauge is wrong, unlikely, OR the pump is bad or the wrong one. Double check the part number...was it a NEW pump or a REBUILT pump (Airtex or other brand) ? My guess is now that you either got a bad rebuilt or a defective new pump.

If someone in town has a MotorVac CarbonClean machine, you can hook it up as a known good fuel system, makes checking fuel delivery problems easy. In about five minutes they can check static pressure, running pressure, fuel volume, leakdown, and the fuel regulator.

Last edited by oldcpecdr; 03-11-2008 at 07:18 PM.
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Old 03-11-2008, 07:32 PM
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With the key on, engine off, the pressure goes to 33 PSI the same as it does with the engine running and the vacuum unplugged. With the vacuum attached and the engine running it only goes up to about 25 PSI. The gauge is attached to the schrader valve on the fuel rail and it is an accurate gauge. The pressure will stay at the 25 PSI reading for about 20 minutes or so after the engine is shut off and while that is probably not normal a leak-down that slow should not cause a problem such as this, or could it? I guess that also answers the question about the regulator seating in the fuel rail, if it did not seat the pressure should fall as soon as the engine is shut down. I am beginning to suspect the new (Auto Zone) fuel pump but since the pressure readings are exactly the same as before changing the pump I think the pump is probably not the problem.
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Old 03-11-2008, 07:46 PM
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fuel presssure

I think it has to be the pump. Even a damaged fuel line or rail would restrict Volume but not change the Pressure. The fuel pressure dropping about 5 pounds indicates that the fuel pressure regulator is working as intended. That doesn't leave much except the pump..the only way to check that easily that I am aware of is with the MotorVac system or actually changing the pump again. I talk to quite a few shops that are unhappy the last few years with the quality of rebuilt fuel pump units, you might have to bite the bullet and get a Ford OEM pump if you plan on keeping the truck for a while. There is another method of checking fuel pumps by amperage draw. You could probably google that method and check it out... my old brain can't fathom anything but seeing or changing the part.

OK another thought, try restricting the return line to raise the pressure and see if that cures the rough running...find a flexible part of the return line and watch the pressure rise as you restrict the flow. When it hits 36-38 the engine rpm should straighten out, the pump fully restricted should aprox double its unrestricted PSI..but partially restricting it will give you the pressure you need to see if fuel pressure is the problem. If restricting the return line does NOT raise the pressure then the pump is definitely bad.

Last edited by oldcpecdr; 03-11-2008 at 09:15 PM.
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Old 03-11-2008, 10:02 PM
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re: fuel pressure

Had the same problem,low pressure all tests said bad pump,changed pump same problem ,pulled pump (easy this time took bed off first time) close inspection found a split in the hose that adapts pump to hard line. Replaced same ,put it back in powered up pump had 43 psi.
Also I ran across this on a company truck, the change over valve got hung up and the pump in one tank was pumping gas into the other tank!! had about 22psi
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Old 03-12-2008, 06:28 AM
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I replaced the adapter hose with the new one that came with the pump so that probably is not leaking and it only has one tank. Thinking about that hose however has me thinking that maybe the line could be cracked somewhere on the unit, maybe where it comes through the part that fits to the tank? It has a solder joint there and I suppose maybe vibration could have cracked it, never thought to check for any of that when I changed the pump. Next step will be to drop the tank again, easy to get off but a real bear to put back on!
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Old 03-12-2008, 12:28 PM
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fuel pressure

Before you drop the tank, restict the return hose and see if the pressure increases.
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Old 03-12-2008, 12:48 PM
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Quote:
Originally Posted by oldcpecdr
Before you drop the tank, restict the return hose and see if the pressure increases.

Will do! ***************
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Old 04-01-2008, 11:14 AM
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Well I got the darn thing fixed but I am even more puzzled than before. Due to a back problem I simply was unable to drop that tank again so I had the Ford dealer repair it and what they did makes no sense to me. They said the miss was due to a bad coil pack on no.5 cylinder (checking for the cause of the miss was how I discovered the low pressure) and after replacing that it runs fine but now it has nearly 50 PSI with the key on-engine off and about 38 PSI running! He said the coil pack was all they did and that is all that is on the bill but is this guy on the level here? What could a coil pack possibly have to do with fuel pressure? The truck runs fine and as I said the fuel pressure is normal now but I sure as heck would like to know what happened?

BTW when I first changed the pump I checked the pressure with another gauge and got exactly the same reading and the gauge now showing the right pressure is the same one I started with.
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Old 04-01-2008, 11:41 AM
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Low fuel pressure

Hi Well l see that you have it fixed, and as l had posted before, your running now about the same as mine, to my understanding the computer controls the spark, so if the wires or coil is not working proper, the computer will shut down the pressure (supply and demand), as this is part of my problem that l had with the wet plugs an wires! For me to find this out after a few hours on my own messing with it, l took it to a friends that had a code reader for free, whereas Ford wanted $98 for a read, when you can buy the scanner at Walmart for about $130, cheap to buy if your going to keep the truck around. TROY
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