I have a 1988 -250 with dual tanks...
There is a low pressure fuel pump in each of the tanks, and a high pressure pump mounted on the frame....

The problematic In-tank pump pumps fuel out, but produces very low pressure, it doesn't even register on my pressure gauge.

Does anyone know if I were to simply disconnect the supply line to the high pressure pump and stick it in a clean container of gasoline, should the rail develop proper pressure (30-45 PSI) ????

In other words, does the high pressure pump require it's supply to be provided under the normal pressure (8-10 PSI), or should it be able to run ok if it is just supplied with free flowing fuel from a container?

The reason I ask is that I'm trying to make sure before I get parts that the in-tank pump is the only part of the system which has an issue.
I don't want to go buy parts,replace the in-tank pump and find that the high pressure pump needs to be replaced too.

Many thanks!

 

I have the same set up as you do and I have just got done doing a lot of work to mine. Your in-tank low pressure pumps only make about 5-10 pis. I have a i6 in my truck and it makes 42psi on the fuel rail when it is running. You can do a search and find out how much pressure your motor is suppose to be making. Test the pressure on both tanks and see what you get. If one is different most likely it is the low pressure. When my low pressure pump went out in my front tank my truck would not run at all. And when my high pressure pump went out it was not making enough pressure to keep the truck going down the road. What kind of problem are you having?

 

Yes it should if the fuel pump has fuel in it and also the fuel line is full of fuel to the container when you turn it on.

No it does not require it's supply to be provided under the normal pressure (8-10 PSI). But it must be primed to pump (have fuel in it and its fuel lines).

Note:
It may require the in tank pump if going up a hill, high acceleration and to prevent surging.

 

OK, I pulled the line off of the supply side of the high pressure pump and replaced it with a piece of hose directed into a container of gasoline....

(I disconnected the in tank pump by unplugging it's harness behind the tank so as to avoid pumping fuel everywhere from the now disconnected hose)

As soon as I turned the key, the pump ran and sucked gas up the hose..... It seems that it will indeed prime itself, although the hose is only about a foot long, so that short length may be just enough to get by.

The van fires right up and runs great with about 30-32PSI at the rail.... I think that replacing the in tank pump so that it produces pressure 8-10 PSI will help to boost the rail pressure a few more PSI once everything is straightened out.
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The issue has now been diagnosed and the information to know here is that just because the in tank pump has a clean filter and is getting good electrical supply to it, and just because it runs and pumps fuel at the proper volume (about 1 liter in 20-30 seconds) does NOT mean that the pump is properly operational, unless it also produces the required pressure (8-10 PSI)

And this condition is easily isolated by running a piece of hose from the supply to the high pressure pump into a can of fuel, then running the vehicle/checking for normal pressure at the rail.



Thanks Guys!!!

 

The tank pumps have nothing to do with the pressure at the fuel rail.

30-32 PSI is normal at idle. The pressure will jump to about 40 PSI at WOT (low vacuum).

The pressure from the tank pumps is only to switch the selector valve and fill the selector valve with fuel for the HP pump to use.