ok. herei is the background. 95 F250, 460 engine. Both fuel pumps are new, front is an Ultra Power, rear is a Carter. Rear tank is brand new. Rear tank is once again bypassing fuel from the front to the back (crap check valve again). I am also having issues with the rear tank pressure/volume as the tank burns off. above 3/4 tank, runs great, no issues. at or below 1/2 tank, the truck has prostate issues and pressure/flow slow to a trickle. You can hear the pump running as strong as always, but when you have the fuel line at the filter disconnected (to drain the tank...) it starts off full flow and strong, but tapers off to barely a trickle. What could be causing this? Truck isnt on a severe incline (if anything its barely nose down from level)

Short of a prescription for Avodart into the rear tank, anyone got any ideas here?

 

If both pumps are in fact new and functioning, the last piece in the equation is your pressure regulator. You should test that and the pressure test point along the fuel rail

 

the regulator is also new, and in my example, is out of the equation. The fuel line is disconnected at the fuel filter and the pressure line is routed into a jerry can for transfer/storage. pump is hardwired ON via a jumper at the relay.

I THINK I have an idea though. hear me out on this.... see if my logic is good. The pump is in fact running out of fuel. Above 1/2 a tank, the level in the tank is above the top of the module, and being unsealed is able to keep the "cup" of the module filled. below this point, the fuel does not free flow into the module, and the sock at the bottom is the only source of entry. I looked at an old module I kept around, and the entry sock has a one way flapper valve, and a small tube that goes to a port on the pump. This tube has a very small orifice that shoots through the "outlet" into the module, creating a syphon effect to aid in getting fuel into the module. My logic here is that either the pumps are NOT pushing fuel through this orifice, or the inlet into the module (where the inner sock directly on the pump feeds from) is too small (at about 1/4 inch, its plausible), either way, the demand on the fuel "trapped" in the reservoir is not enough to maintain flow, and when flow drops, pressure will follow if the engine is using more than the pump is able to supply below 1/2 tank. I wish I had kept the original Motorcraft module that I removed from the rear tank, as this would show how Ford did it originally.

my proposed "solution" to the lack of fuel in the module is to drill a few holes into the outer casing to allow the level inside the module to be the same as the level in the tank. I may end up with a bit more "unusable" fuel inside the tank, but at this point, I am not able to use ANY of it because of low flow.

 

A plugged tank vent. If there is no way for air to get in to displace the fuel that is being pushed out a vacuum is created and eventually the pump won't be able to overpower it. Have you tried removing the filler cap when this happens? If that produces a ruch of air and full fuel flow you have found the problem.

 

tank vents are brand new, less than 2 months old. Lines as well. Doesnt make a lick of difference if the cap is on or off. I removed the cap so I could hear the pump better. Made ZERO flow change to the output going into the jerry can.

 

Bad pick up tube on the pump.
Maybe a crack, or pin hole on the pick up tube.
When fuel gets lower than that point, it starts to suck air, and you get your trickle.

 

there is no pickup tube on the pump itself. Just a screen over the inlet that is at the lowest point of the tank.

That all being said, I went into the tank (again) this morning, and did some work. Evidently when the pump was built, the bypass check valve connection was not fully seated into the cap. It had just blown off, opening the bypass full time.... easy fix. I also looked inside the "cup" and took out the inlet valve and venturi pump that helps to fill the cup. The venturi pump for those that are unfamiliar uses a small amount of pressurized fuel, and shoots it through a small orifice and through the output for the inlet valve that is in the bottom of the tank. In my case, the orifice was TINY, and flowed almost no air, so logic says that no fuel was squirting through it, improving the flow into the module "cup". I opened this up to about 1/16 inch, the original hole was smaller, and appeared to be molded, with some flash occluding the orifice. I also drilled a series of 1/4 inch holes around the bottom of the cup, to allow it to free fill to whatever level that the tank is at.

The logic behind the "cup" on the module is that the venturi pump, gravity and the return line which vents inside the module will keep the level higher than tank level during off level maneuvers or other extreme positioning. (note that other manufacturers do NOT use the module, just a pump and screen attached to the pickup tube, leading me to believe that the Ford solution is a bit overcomplicated....) What seems to be happening is that when the tank fuel level is below the top of the module (and therefore running in through the unsealed top), the venturi pump (with its undersized or occluded orifice) is not keeping up with demand, and the return is not providing enough fuel back to the tank (because of reduced flow/increased engine demand) starving the pump and therefore engine of fuel. Essentially running out of gas, regardless of how much is in the tank.

I think this is the MAIN issue with the aftermarket pumps, it isnt the pump itself that is failing, its that the molded plastic module and interior components are of "lesser" quality assurance and design than the original Motorcraft units. It shall be seen soon how the modified pump and module will work, but I am no longer having starvation issues when revving it up and holding it in the yard. Fuel pressure is holding good at 30psi. And if worse comes to worst, I have a functioning unmodified front tank. I tend to keep both tanks at least half full, so having the "good" tank empty should not be an issue.