Well here is where I am before lunch.

I was trying to run some fuel pressure and volume tests. I couldn't find any baselines so here is what I have.

In 10 seconds:
8 ounces - Rear tank directly from the pump
7 ounces - Front tank directly from the pump
7 ounces - Rear tank after the fuel filter
6 ounces - Front tank after the fuel filter

I have a question. I let the pressure out of the fuel system out of the fuel rail using my gauge. After disconnecting the fuel output line at the front pump, there was another small connector at in the fuel line. The connector has a small piece of wire mesh in it and I am assuming that it is a one-way check valve. When I pulled this connector from the fuel lines, I got a very healthy and strong spray from the line. With the system pressure let off at the fuel rail should this have happened?

fefarms - I will re-run the voltages tests with the pumps operating. The original tests wer done only at the connectors. The drop in 6.9 volts was after the initial priming of the fuel system (the time after the fuel pump runs and you actually start the truck). I really thought this should drop to zero volts though. I have the jumper ran to keep the pumps running.

I am now going to attempt to measure the fuel output at/near the fuel rail. I tried using the pressure release valve on the gauge, but the results were too low, so I am assuming that the fuel is returning to the tank and not into my container. There is a connector that looks like it requires a special tool on the fuel line to the fuel rail. Is there any other way of disconnecting this?

 

Well the running pump voltages were 10.9 volts on both pumps. I don't know if this is good or bad. I would assume that since the pump is running that this is ok. The voltage fluctuated about .1 volt when running. I should check the batteryvoltage though, since I have been running the pumps for a while without a charge.

 

Another side note is that I jumpered the pumps directly off the battery using a long patch wire. My fuel pressure readings were the same.

 

I'm not sure what the volume rating of the high pressure pumps for the I6 is supposed to be. But we can try to estimage from first principles.

6 ounces in 10 seconds is 36 ounces per minute, or 17 GPH. This is enough for 17*6 / 0.5 or 204 HP. Not enough of a safety factor to avoid lean starvation of the engine at full power. And this is with NO backpressure, as compared to the 40 to 50 PSI of backpressure needed at full throttle. A centrifugal pump follows a "pump curve", and delivers less volume at higher pressure. I'm not sure whether these (vane?) pumps are considered positive displacement or centrifugal pumps, but the volume output seems a little iffy to me.

The Dali design book recommends that you disconnect the fuel pressure regulator from the rail and plug the return line port. (Or you can plug the return line output of the regulator). Power up the pump. If you dont get at least 50PSI, the pump is bad, or the opposite tank check valve is bad, no matter how much volume you do get or don't get.

10.9 volts at the pump is OK if the battery voltage is (say) 11.5 volts from running off the battery for too long. If the battery voltage is still 12.5 volts then I'd say you are losing more voltage than you should.

The 6.9 volts you measured with the relay disngaged and no load on the fuel pump wires is about what I would expect. There is a high-impedance input on the computer pushing the wire up. With no load on the wire, nothing bleeds this voltage off, least of all your meter. I think if you try that test again with the fuel pump connected, you will see the voltage does drop to zero when the relay contacts open.

Your experience with trapped high pressure suggests to me that the Ford safety recall
93V125000 may have been performed on this truck. Rather than replace the fuel delivery module, Ford added an extra set of check valves to the pressure lines. I suspect that the pressure was trapped between these redundant check valves and the tank. If there is a screen in the check valve, I would wonder if it might be clogged due to debris in the tanks. The truck did not originally come with these check valves, and I think later models did not have them because they improved the design of the FDM. I reproduced the relevant portion of the recall below.

Recall 93V125000: Fuel Pump Check Valve Malfunction

A MALFUNCTIONING CHECK VALVE WITHIN THE FUEL PUMP ASSEMBLY CAUSES A PORTION OF THE UNUSED FUEL FROM ONE TANK TO BE RETURNED TO THE SECOND TANK. WHEN THIS OCCURS, THE CAPACITY OF THE SECOND TANK CAN BE EXCEEDED, CAUSING FUEL SPILLAGE. SPILLED FUEL CAN RESULT IN A FIRE WHEN EXPOSED TO A SOURCE OF IGNITION. REPLACEMENT OF THE FUEL PRESSURE REGULATOR AND INSTALLATION OF REDUNDANT CHECK VALVES.

 

Couldn't he test the rear check valve by squeezing off the fuel line to the rear pump and operating off of the front pump?

This is another great thread, a good read, thanks.

 

Yes, if there is a rubber line that could be squeezed down, between the tee fitting and the rear tank, he could prevent pressure loss to the rear tank by closing off this path, then running the front pump. But if the truck had the recall done there are TWO check valves in between. It is possible that BOTH of them are leaking, but at 1/4 tank per month observed fuel transfer, this should not be enough to kill all the pressure produced by the front pump.

The idea of blocking the regulator output is similar. This "deadheads" the pump no matter what the regulator wants to do. If the pump can't produce 50 PSI with NO place for the fuel to go, the pump is bad.

 

Well I ended up charging the battery overnight just to finalize the voltage tests. The batterys this morning was at 12.6 volts. I ran the fuel pumps and got 11.7-11.9 volts of power at the pump.

 

This is an acceptable voltage drop, and does not explain the low pressure. I guess you have to change out one or both pumps.

 

I ended up swapping out the front pump first. The fuel pressure went up to 60 psi with the engine off and 50 psi running.

The pumps were it!

Thanks all for the help... now to put this darn bed back on... a sheet of plywood on there is sounding like a good idea!

 

Wow, you guys are incredible. I experienced that failure you talk about. I bought a truck in 1998 (1990 ford Super duty with aux tanks) The first thing I noticed was when I filled the aux tanks, the main would runneth over! I then understood why the previous owner had disconnected the aux tank and quit using it. It was pretty severe, I mean gallons were pumping out of the mains cap as I was driving down the highway.

I received a letter from ford in 99 or 00, (can't remember) and was shocked that there was a recall on a truck that was 10 years old. They did exactly as you said. Ever since then, I have had trouble with the tanks. Do you think I'm experiencing a problem from the extra check valves they installed?

 

It isn't clear whether Skyforum's truck has the standard dual tanks or some factory or aftermarket option with three tanks.

With the factory fuel delivery module, you can experience fuel transfer due to a leaking check valve or a sticking shuttle valve in the unused tank or tanks. If the recall has been performed then both the in-tank check valve and the added external check valve have to leak to have fuel transfer.

The check valves added by the recall will increase the back pressure on the pump, and may slightly reduce its cooling. But a healthy pump should be avle to overcome this.

The added check valves could also get plugged up with gunk over time, especially if they have a filter screen on the inlet. That would tend to muck up the functioning of the aux tanks.

If the pump in the aux tanks had not been used in a long time (because the previous owner disconnected it), and then you started trying to use it, the pump output may be impaired by an accumulation of varnish and gunk from the stale residual gas in the aux tanks. On a similar note, the aux tank pumps may have been damaged during the massive fuel transfer episodes, since they were being forced to pump a lot of fuel that was not coming back to the tank, thus impairing the cooiling of the aux pumps.

To figure out why the aux tanks don't work right, do the same sort of pressure/volume tests that Mr. plapczynski did, focusing on the aux tanks.

 

Thanks for posting the solution to the problem, we don't hear the end of most of these stories.

Where did you get your pumps? How much? What came with them?

 

I got the pumps at NAPA.

They were about 155 each after my AAA discount.

The came with just the pump, new pump filter and gaskets. You have to unbolt the old pump and sending unit and bolt them back on and splice the positive and negative wires back in.

I think the hardest part (other than removing the bed/dropping the tank) was to get the old fuel pump retaining ring loose.