Trying to get my head aroud this. Is there really any differance between having negative pressure on one side (suction) or positive pressure on the other side as far as the filter/seperator is concerned? The filter see the same pressure differential regarless. If flow thru the filter is the same for both scenarios, which I assume it would be because of idenitical regulator settings and injector consumption, I don't see how pushing fuel thru or sucking fuel thru the filter is any different.
You would need a fuel tank sump to have a true free flow system otherwise your primary or lift pump will have to provide suction/negative pressure to the system. You've made that task even harder for the primary pump by sticking a pre-pump filter in the way. That right there gives a lot of reason for having a sump and/or lift pump.

 

It depends on the type of separator you use and the type of filtering media within that filter. I am using a prepump (suction) filter that was designed for that purpose and used for large diesel engine applications of ~ 1.5- 2 times the size of a 7.3 PSD. It's flow rate is ~ 4 times the free flow output rate of the stock 7.3 fuel pump. It can used in pressure applications when the comparable filter with synthetic, or coated cellulose, media is used. Without the proper media when used for pressure applications, reduced water filtration can result as it can be forced though the standard media. This fact is most important when a fuel bowl delete is done and the stock water separating feature is removed.The following pdf explains how a fuel water separator works. Be sure and read both pages. http://www.donaldson.com/en/engine/shoptalk/070451.pdf
It boils down to what kind of filter you are using. I'm not saying pressurizing a water separator filter is a bad thing. It just depends on which one you are using and purpose for which it's designed.

 

It was just that your comments were leading people to believe that you shouldn't pressurize the prepump filter/seperator or that it is undesirable, or that they don't work as well when in fact any of the common fuel filter/water seperators available work just fine down stream of the low pressure Carter lift pump.

 

I hope my last post cleared up a few things. I never meant to confuse anyone.
The Baldwin BF1212, or it's equivalent, was meant for suction side applications and is commonly used in the prepump/intank kits. You can pressurize it when still using the post pump stock bowl and filter. I am heavily concerned with water separation capablilites due to the removal of my stock bowl. That is why I reverted it back to its original application and using a remote mounted post pump spin-on filter. For those interested, the Baldwin BF1259 is ~2" longer, is better suited for pressure side applications, and provides for better water removal.

 

Update to an old thread, learned something useful today.

I started having my fuel pressure drop under heavy load back in November, problem has progressively got worse. To make a long story short I narrowed the problem down to my nearly 7 year old Carter 4070 pre-pump last weekend. While ordering a new pump I discovered the availability of pump rotor rebulid kits for it, but didn't order a kit because I though my issue was the pump motor.

Removed the old pump today and started to dismantle it for an autopsy. Upon opening the the pump I found a very fine mesh fuel screen surrounding and protecting the rotor was nearly completely blocked with the normal black diesel fuel "crud". Removed the screen and cleaned, reassembled, and reinstalled the old pump and I'm back up to full pressure at WOT runs.

Now I have a spare 4070 on the shelf. Maybe this will save somebody else $75 on a new pump.

 

Nice find and having a back up pump is always a good thing. I had forgot all about this tread and since I ended up leaving the #4070 on my motorhome a few years back. I thought by now that my stock fuel pump wouldn't last without a lift pump. Well it has been working fine without it. I might just steal it off the motorhome and put it back on my truck this spring.