Selectable Restriction?NOTE: Articles in this series were written in an order that might make "skipping around" confusing. If you didn't start at the beginning, you might want to CLICK HERE.

Owners of 94-97 "OBS" Powerstrokes struggle with performance upgrades that require larger than stock fuel pumps because it always brings up the fuel tank dilemma...sacrificing fuel tank capacity (miles per fill-up) for performance, or replacing the rear fuel tank with a "Bronco Tank" (or equivalent) in order to maintain the capacity but run on a single tank. This is all because the OEM dual tank configuration includes a Tank Selector Valve that becomes the bottleneck in the fuel system when trying to run larger than stock fuel pumps. Over the years, some truck owners have successfully run somewhat larger pumps with the selector valve in place, but the durability of the pumps in this configuration is questionable at best. In this article, I'm going to explain why the Tank Selector Valve is such a problem for higher volume electric fuel pumps.

One analogy I've made many times over the years to get a point across has been the "Drinking Straw" comparison. First, imagine you have your favorite beverage in a nice, fresh fast food cup. You pull the wrapper off the straw, poke it through the hole in the lid and go for that always satisfying first drink. Right in the middle of washing down your salty french fries your friend reaches over and pinches your straw closed...@$%#! Similarly, imagine trying to drink your beverage through one of those skinny little coffee stir sticks. That feeling in your cheeks is exactly how your fuel pump feels when something is restricting the suction side of the fuel system, and depending on how much fuel you are trying to flow in your OBS, that's exactly what the selector valve is doing to your pump.

To illustrate this more accurately, I ran an OBS Selector Valve on our fuel system flow bench and collected some real world data on just how bad it flows. I have also disassembled and examined the selector valve and I can confirm that the problem is not with the size of the hose connections, it is with the internal valve configuration.

In this chart, I am showing the FUEL PUMP FLOW (relative to a Stock Bosch Super Duty electric fuel pump) and the FUEL PUMP INLET RESTRICTION (as measured in inHg). The BASELINE for this test is not displayed, but it was the Bosch pump with 3/8" hose to the inlet, pulling through a Baldwin BF1252 filter. In subsequent tests (displayed on the chart), the selector valve was installed in the line between the filter and the tank (as it would be in a truck) using the same 3/8" hose. No other part of the configuration was changed.

In the first data set, you will see that the BLUE bar is at 0% (meaning 0% more than the baseline / exact same fuel flow rate) and the ORANGE bar is at 40% (meaning 40% more restriction than the baseline). The ONLY difference between the baseline test and this test was the insertion of the Selector Valve (pulling through the Front Tank Port). Just putting the Selector Valve into the system INCREASED the Inlet Restriction that the pump feels by 40%!

In the second data set, you will see that the pump flow is at 65% MORE THAN STOCK. This was one of the common Walbro fuel pumps that is regularly used with the Selector Valve. That 65% increase in fuel flow resulted in a 100% increase in Inlet Restriction!

In the next (4) sets of data, you will notice that (2) of them are at 103% fuel flow increase, and (2) are at 132% fuel flow increase. After I finished testing the Selector Valve at these levels, I got curious if there was any difference in flow from the Rear Tank Port on the Selector Valve. Upon moving the connection from the Front to the Rear port on the selector valve, I re-ran the last (2) fuel flow rates again and collected the data. Not only does the Inlet Restriction continue to get worse as the flow rate increases (through either port on the Selector Valve), it turns out that the Selector Valve actually flows noticeably worse when pulling through the Rear Tank port as well.

The conclusion here is pretty obvious, the Selector Valve is restrictive, especially when used with high volume fuel pumps. The level of restriction is acceptable for the OEM Bosch electric fuel pump from the 99-03 trucks, provided that the valve continues to operate/flow properly. Faulty valves that don't fully cycle can cause additional restriction, which could damage even the OEM Bosch pump. Based on this data, I do NOT recommend using the Selector Valve with ANY fuel pump larger than the OEM Bosch unit, the level of restriction at higher flow rates is enough to significantly increase the liklihood of fuel pump failure.

Hopefully this clears up any questions regarding the Selector Valve and larger fuel pumps.

Author: Dennis Schroeder - Co-Owner of Strictly Diesel

Dennis has been Designing, Building and Supporting Aftermarket Fuel Systems for 7.3L and 6.0L Powerstrokes since 2001.