Low fuel pressure after warm up - Diesel Forum - TheDieselStop.com
'99 & up 7.3L Power Stroke Engine and Drivetrain Discussion of the '99 & up 7.3L Power Stroke diesel engine and drivetrain in the 1999-Up Super Duty trucks and Excursions. No gas engine discussion allowed except on transmissions and drivetrain that pertain to all models. Please confine discussion of topics in this forum to those items that are specific to the 7.3L Power Stroke engine.

 
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post #1 of 12 (permalink) Old 06-22-2019, 01:34 PM Thread Starter
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Low fuel pressure after warm up

I have a 2001 E350 box truck with the 7.3L powerstroke engine.

I went on a trip yesterday and only made it about 25 miles until my fuel pressure dropped off (I have an aftermarket fuel pressure gauge hooked up to the rear port facing the firewall on the fuel bowl).

I limped it back home and in the morning changed the fuel filter, the original one was only on for a few hundred miles but I changed it anyway and the fuel pressure returned back to 60psi so I was encouraged.

I resumed my trip and this time only got about 10 miles before the same symptoms returned so I again limped it back home.

I didn’t change any filter this time but again let it set overnight to see if the fuel pressure would return. In the morning fuel pressure was good on startup so I went for a drive. It only make it about 3 miles before I had to turn back home. When I returned home I took this video.

https://youtu.be/hFy0_nz9nFo

In the video you can hear key on engine off and the fuel pump humming and failing to build pressure. Then I start it up and it only builds about 40psi at idle then as I accelerate the engine the pressure drops to 20psi on the road under hard acceleration the pressure drops to 0psi under even the smallest load.

It seems to me that my fuel pump may be to weak to maintain the pressure needed when it is warm.

Since a new replacement Bosch fuel pump is
only $80 I ordered one and will throw it in to see if that fixes the issue.

In the event that a new fuel pump doesn’t fix the pressure problems any other suggestions?

2001 7.3L E-350 box truck.
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post #2 of 12 (permalink) Old 06-23-2019, 06:25 AM
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I would say your fuel pump is most likely good as it does build pressure initially. The fact that the fuel pressure drops as the truck is running is a classic sign that you have a restriction before the pump. Faulty pump aside, properly diagnosing this requires placing a test hose at the fuel pump inlet and testing for a restriction - results will show a restriction or rule it out. Since this is a box truck I am assuming that it has a rear aft-axle fuel tank which tend to have rust and delamination issues historically. That and any debris in the fuel will cause the two screens in the thermal recirculation valve in the fuel tank pick up to clog, another common problem. The pick-up can be carefully disassembled and the screens can be cleaned or replaced. Obviously that would be the time to inspect the inside of the fuel tank and address any issues you find. And if this truck happens to have a mid-ship fuel tank I have seen a few of those delaminate.

Usually this type of failure is allowed to worsen because it starts out unnoticed. I usually see them when the truck loses pressure below 10 PSI under load or after driving a period of time and then truck stalls and restarts after a short rest and cycling the ignition a few times.

I am confident you will let us know what you eventually find to be the cause.


FYI: If this is a tank concern and it is an aft-axle tank, Titan fuel tanks makes a poly replacement tank that is less expensive than the OEM replacement and will never have issues.


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Last edited by ford_doctor; 06-23-2019 at 06:29 AM.
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post #3 of 12 (permalink) Old 06-23-2019, 07:20 AM Thread Starter
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Thank you very much for the detailed reply. I’ll be sure to let you know. The tank is also less than 1/4 full so I guess if there is debris it would be even more likely to clog with less fuel in the tank. I also appreciate the titan recommendation.

2001 7.3L E-350 box truck.
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post #4 of 12 (permalink) Old 06-23-2019, 09:02 AM
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Well, you just nailed it. The fact your below 1/4 tank suggest that as said its either plugged, or the 1/4 tank rubber valve is pooched and its sucking in air. YOu should fix it, check inside the tank for debris as stated. BUt, if you don't have time you could try filling the tank and don't let it get below 1/4 until you have time to fix it. This is real common, almost every one of these trucks has that flapper fail at some point. Safe to say, your going into the tank.

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post #5 of 12 (permalink) Old 06-23-2019, 09:52 AM Thread Starter
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I just removed the 10 gallons of fuel remaining in the tank via the built in drain. There wasn’t much debris to speak of. I’d estimate less than a tablespoon of super fine dust like rust particles and almost all of it came out in the first 2 gallons. Perhaps my issue is this flapper you are referring to. Can you explain more about that?

2001 7.3L E-350 box truck.
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post #6 of 12 (permalink) Old 06-23-2019, 12:12 PM
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In the tank, on the fuel pick up assembly. At the 1/4 tank level on the pick up tube. You will see an opening with a rubber stopper type system that can open and close. They rot out and break and allow air to enter once the fuel level goes below that valve. What it is, its for the fact diesel fuel carries a lot of water in it. In freezing temperatures the water trapped in the tank will freeze and block off the pick up tube. Its common in cold places. When that happens it creates vaccum in the pick up tube, sucking that 1/4 tank valve to open and let fuel get past the blockage until return warm fuel melts the ice and lets fuel resume its normal route. Its a safety valve, and its a POS. Research Hutch Mod and you will find the simple elimination process. IF your in a cold place, use anti gel stuff in your fuel after you remove it. Clean, or complete the mod in order to get the crap out of the screens. If a table spoon came out, that means to me that the screens are so jammed full that the table spoon didn't get trapped. Any sign of crap means new filters, or no filters and an after filter system like a fass system. Your screens will be dirty and a good reason it may be relying on the 1/4 valve for fuel entry, and the valve could be working correctly, and just the pick up is plugged as if frozen. So quit being lazy and drop the tank and get that thing cleaned up. Or it that's too much work, cut a hole in the floor and get in that way. Make a service hatch so to speak. lol

2002 F350 Super Duty, Crew Cab,Long box,4x4, 7.3 PSD, Complete rebuild .010 over by myself except balancing, 2000hp billet connecting rods, ARP main and head studs, ceramic coated pistons, Smith Bros Chrome Molly push rods, Comp Cams springs, shims and seals. Competion complete valve set and head grinding and vacuum tested, Adrenaline HPOP, DP Tuner Infinity DX programmer and full load of tunes modified for my new Stage 3 250 shot injectors, ceramic coated exhaust manifolds and up pipes and collector, BD Diesel triple clutch furnace brazed billet torque converter and BD pro engine brake and compressor set up, MBRP 4" stainless turbo back, Driven Diesel regulated fuel return system, AFE pro stage 2 cool air intake, riffraff billet Whicked Wheel 2 compressor wheel in a GTP38R turbo, riffraff blue cooler boot kit and coil clamps, International Double Ceramic Coated Bellowed Up Pipes
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post #7 of 12 (permalink) Old 06-23-2019, 01:41 PM Thread Starter
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I dropped the tank and the screens were completely clogged and the pickup foot was broken too. The other rubber parts seem fine but I may investigate the hutch mod now that the tank is out. I’ll keep the fuel pump on the shelf if I need it in the future 🙂

2001 7.3L E-350 box truck.
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post #8 of 12 (permalink) Old 06-23-2019, 07:38 PM Thread Starter
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I think the foot was broken off because it was in direct contact with the bottom of the tank instead of slightly above it. I can see the outline of the base of the foot etched into the metal of the tank bottom. I plan to place it 1/4” off the bottom so maybe it won’t break next time. Thoughts?

2001 7.3L E-350 box truck.

Last edited by tpendergraft; 06-23-2019 at 07:41 PM.
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post #9 of 12 (permalink) Old 06-24-2019, 04:38 AM
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It is meant to be on the bottom AND the "flapper" is actually a bypass in the event the screen on the bottom gets clogged. Yes, that too is a pattern failure.


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post #10 of 12 (permalink) Old 06-25-2019, 12:23 PM
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When I did my Hutch Mod (Strictly Diesel) the directions recommended placing the empty tank on 2X4’s where the tank straps go so that the bottom of the tank can sway down naturally. Then adjust the pick-up umbrella (bottom), if needed, so that the flange at the bung hole has to be pushed down 1/8 to 1/4 inch, allowing the threaded ring to be tightened down, and putting pressure on bottom of pick-up against the bottom of the tank. I measured my pick-up tube before cutting and adding extension but still had to make a small adjustment to get that correct fit.

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post #11 of 12 (permalink) Old 06-26-2019, 07:52 AM
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Well, after reading my post ^^^^^ above I probably didn’t do a good job describing this so here attached is the directions straight out of the PDF instructions.
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2002 F350 7.3L / 4R100 - 4WD CC DRW 8 Box, AIH Delete, Gauges- Pyro, Trans Temp, Boost, Fuel Pressure, TS-6 tuner, Zoodad Mod, Mishimoto 31 row Trans cooler, Mobil 1 ATF, Trans return line ATF filter, Hutch/Harpoon Mod, Air Lift Load Lifter 5000 rear air springs, Rancho 9000 shocks, Timbrens on the front.
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post #12 of 12 (permalink) Old 07-06-2019, 06:44 AM Thread Starter
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Hutch mod complete with external filter. No more fuel pressure problems. I did not mount my pickup directly on the bottom of the tank as advised though. I believe that is why mine broke originally so I want to avoid going back in the tank ever again. I can get by without that last 1/4" of fuel

Thank you all for your helpful advice as always.

2001 7.3L E-350 box truck.
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