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Old 09-20-2021, 12:32 PM   #1
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Low fuel pressure - I'm stumped

Hey y'all,

This is my first post here after a fair amount of reading through stuff. I've got a 2011 Ford E350 Super Duty with the 6.8L V10 gas engine, and I'm only getting 15 psi to the fuel rail. I've had 3 mechanics say it was the fuel pump, so I replaced that, and still getting the same results. We have also replaced the throttle body, TPS, and MAF sensor. I've been stuck in Richmond, VA, for 2 weeks now trying to figure this out, and I need to get back on the road ASAP. Any ideas?

Edit to include that there is no fuel pump inertia switch on this model, and that the fuel filter is on the pump itself (no in-line filter). I'm thinking it's a wire I can't find or that the line is just clogged somehow.
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Old 09-20-2021, 12:40 PM   #2
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Are you actually looking at a fuel pressure gauge that is attached to the fuel rail port while the engine is running? Verify that the fuel pressure builds up when the key is turned on with engine off. Then start the engine and monitor the fuel pressure. Does it meet pressure specs called for?
Also try remove your gas cap to make sure there is not a vacuum in the tank due to a clogged evap system.
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Old 09-20-2021, 12:43 PM   #3
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Yes, I'm looking at the gauge while it's running. When I prime it, it only gets about 5psi at a time. Prime it 2-3 times, and it starts and runs, but with low pressure. Dies if I try to drive it. If I rev the engine, the pressure on the gauge drops. I'll try with the fuel cap off and report back.
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Old 09-20-2021, 12:52 PM   #4
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Try to check voltage at the pump, while its running. Should be 12.7+

Also use find where the ground point is near the tank.
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Old 09-20-2021, 01:02 PM   #5
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Have you dead headed the pump to check that it puts out max pressure ? Doesnít this system have a fuel pressure regulator ? If that fails you wonít build up to operating pressure.
Sounds like you are throwing parts at it now & hoping something will fix it. MAF sensor should not affect fuel pressure.
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Old 09-20-2021, 01:06 PM   #6
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Quote:
Originally Posted by Pandarevolt View Post
Hey y'all,

This is my first post here after a fair amount of reading through stuff. I've got a 2011 Ford E350 Super Duty with the 6.8L V10 gas engine, and I'm only getting 15 psi to the fuel rail. I've had 3 mechanics say it was the fuel pump, so I replaced that, and still getting the same results. We have also replaced the throttle body, TPS, and MAF sensor. I've been stuck in Richmond, VA, for 2 weeks now trying to figure this out, and I need to get back on the road ASAP. Any ideas?

Edit to include that there is no fuel pump inertia switch on this model, and that the fuel filter is on the pump itself (no in-line filter). I'm thinking it's a wire I can't find or that the line is just clogged somehow.

Replace the Fuel Rail Pressure sensor Standard part # FPS5 or Motorcraft CM5229 The parts guy might even call it the electric fuel pressure regulator. They go bad and cause this problem.
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Old 09-20-2021, 02:05 PM   #7
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Quote:
Originally Posted by Pandarevolt View Post
Yes, I'm looking at the gauge while it's running. When I prime it, it only gets about 5psi at a time. Prime it 2-3 times, and it starts and runs, but with low pressure. Dies if I try to drive it. If I rev the engine, the pressure on the gauge drops. I'll try with the fuel cap off and report back.
It's either the fuel regulator or the pump or something directly related like a relay or wire connection.
Have someone turn on the key without starting while you go back and listen to the pump. Remove gas cap and put your ear to fill tube. Does it sound normal? It should run momentarily and shut off.
As others here have said... I would bet on the regulator since the pump is new.
Dumb question but I'm going to ask anyway... There is gas in the tank correct?
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Old 09-20-2021, 04:49 PM   #8
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Quote:
Originally Posted by 77Travco View Post
It's either the fuel regulator or the pump or something directly related like a relay or wire connection.
Have someone turn on the key without starting while you go back and listen to the pump. Remove gas cap and put your ear to fill tube. Does it sound normal? It should run momentarily and shut off.
As others here have said... I would bet on the regulator since the pump is new.
Dumb question but I'm going to ask anyway... There is gas in the tank correct?

Good one on the fuel in the tank, as a fleet mechanic it had to be the first thing checked always. In this case though I'm betting from his post he has plenty of fuel.
The part numbers I gave him above are the Fuel Pressure Sensor, which on these is the fuel pressure regulator. It is not a voltage problem I'm pretty sure, a lab scope and amp probe would tell the story real quick. Pu a jumper in the fuse, amp clamp on the jumper and the wave form will tell all, voltage, amps and pump speed. I'll bet pump is running almost freeload, the FPS is allowing all the fuel to return to the tank instead of maintaining 55PSI on the injector rail.
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Old 09-21-2021, 05:53 AM   #9
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I'll bet pump is running almost freeload, the FPS is allowing all the fuel to return to the tank instead of maintaining 55PSI on the injector rail.
That's what I am betting too. Hopefully the OP replies back with results.
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Old 09-21-2021, 06:15 AM   #10
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Would have verified that the pump is running first and then gone after the pressure regulator. Easier fix.

Too late for that now.
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Old 09-21-2021, 08:40 AM   #11
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Many very often make assumptions. Well it's a new pump it must be working. Then we take different diagnostic roads which leads us down another path which leads us to nowhere. Then we are confused and don't know where to turn.

Verify, verify, verify!!!! What's really working and what's not??? Do I have 12.6+ volts where needed???? Do I have sufficient fuel pressure????? Have I checked all grounds???
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Old 09-21-2021, 09:12 AM   #12
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I'd be looking for good pump voltage. I had some wire connections corrode on the backside of my fuel pump relay that gave me some "voltage drop". I was lucky enough to spot my issue before it really hammered me. At one point I wired in a cheap solar panel type ammeter to see exactly what power my pump was being fed and what it was using. I left it there. Gives me a sort of overkill extra dash gauge but what the heck. The more sensible troubleshooting approach is to have one person at the fuel tank area with an ammeter/multimeter and another person at the ignition switch but I like to work solo.
many times mechanics will bumble around with test lights and not do voltage drop testing. You might want to look into it. An example is a 95% cut wire. I could cut your starter almost all the way through, leaving just a couple of wire strands. A test light would happily show a full 12 volts at the starter still but do you think those tiny strands would feed the starter enough power to crank the engine over? Corrosion and other bad connection problems can act the same way and a simple test light will never give an indication of an issue.
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Old 09-22-2021, 07:43 AM   #13
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GypsyR,

It's obvious you understand electricity and what it takes to diagnose. For sure skipping tests and failing to verify what your voltages are is a failure.

I very well remember my first introduction to electricity: Voltage, current & resistance. The example of water flowing through a pipe was used. Water (current) flowing is what actually does the work. Pressure/voltage is what forces the current/water to flow and resistance is what slows or stops water/current. If you're putting out a house fire you need good pressure and large hoses (volume) to carry the great amount of current/water needed.

The same holds true for voltage, current and resistance. Of all electrical devices on a vehicle the starter motor requires the greatest amount of current. It's in the range of 150 amps. A standard radio requires only 1- AMP of current to operate. It takes less than 1 amp to stop your heart but it requires more voltage to force the small current through your body. You can't electrocute yourself using a car battery. Even if they pretend do it on TV!!!!!

Those are the reasons why a weak battery still lights the head lights runs your radio and will close a starter solenoid but not provide enough current flow to make the starter crank the engine. Loose and or corroded connections are a resistance to current flow. Just like a kink in a hose to the flow of water.

It really (IMHO) does not get much easier to understand.
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Old 09-23-2021, 05:27 PM   #14
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Problem solved?

I notice that Pandarevolt the OP hasn't posted in a few days, does anybody know if he got going? And if so, what did he find to be the problem? Hopefully they will pop in and let us know what happened.
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