Fuel System / Fluid Dynamics Question

[da4wheeler]

Hi Dr. Lee,

I'm sure you will love this question as it involves all sorts of down-home engineering estimates.

I recently experienced a fuel tank selector valve failure in my 83 K10 6.2. The electronics switched the fuel sending unit, but not the valve, so while I happily cruised along, I was actually running on an almost empty tank instead of the full one that showed on the dash! Pure distilled happiness when it died and left me in 105 degree heat amost 30 miles from home. Anyway... after pulling the valve hoses off and jumping the valve with a section of axle vent tube, I was still unable to start the now airlocked system. I broke down and used my new corporate "F#$D Powerstroke" to tow home my beloved Chevy.

This got me thinking... there must be a better way. Here is my solution under construction.

I am replacing the single mechanical lift pump and fuel selector valve with dual electrical lift pumps. Each tank will have a pump that is activated via relay by the switching of a tank selection rocker switch. This will also switch on an indicator light over the appropriate one of the dual fuel gauges that I installed with the installation of my custom dash. Each supply line will have a check valve after the pump to prevent simply pumping fuel from one tank to the other. The return line is where my question lies.

Can I simply Tee into the return line and have the pressure differential caused by suction on the active tank cause the fuel to return mostly to the active tank? If not, do I still need a selector valve in the return line? Do I need a return line at all? I suspect that the pressure difference will do the trick, but I really don't know. Do you know of anyone else that has tried this?

Before everyone tells me how stupid this idea is, consider the following supoporting points:

1) I will now have the ability to replace one pump with the identical one from the other side in the case of an on-highway failure.
2) Selector valves are notorious for failure, and replacement of this one does not mean that I will never have to do it again.
3) Electric lift pump on both tanks allows power bleeding (many of you have already done this).
4) I'm doing it anyway so you can't talk me out of it.

Any opinion that you can put forth would be valued. If there are any other potential problems you see, please feel free to mention them.

Regards,

Nate

[CleviteKid]

Hey Nate,

I think you are on the right path. With your dual fuel gages you will know soon enuf if the return is going to the correct tank. And yes, you do need the return. Fuel flow thru the pump helps keep it cool, and the amount used by the engine isn't enuf. Also, your injectors are bleeding off leakage around the plunger, and that fuel needs to go back to the tank too, unless you want to just dump it down the side of your engine and make your rear tires into REAL SLICK slicks.

You might try a check valve in each tank's return line -- that might keep the working tank from sucking air out of the idle tank.

You might be interested in my cross-connected fuel fillers on my '84. I made up Tee's that go on top of each tank, so you can fill both from either side. When the near tank gets full, the fuel just runs across the transverse line to the other tank. I take both fill caps off, and have built a special vent for the non-fill side by putting a piece of tube in an extra filler cap, extended up above the top of the bed. Keeps me from having to reposition the truck at the island, or drag a hose across my paint. Think about it.

Good luck with your project -- you need something to keep you busy on those long winter nights on the UP.

Dr. Lee :cool:

[da4wheeler]

Long winter nights... wow I miss those. I've re-located to Central California after my recent graduation.

Fuel fill tee's sound like a great idea. I had a plan at one time to add a second filler neck and fuel door to my blazer, but I was having a rough time finding someone to weld another fitting on the gas tank. ( I don't know why either )I didn't even think of the problem with sucking air out of the other tank, I'm glad you mentioned it. Is there any way to get an estimate of return line pressure and volume? My electric pumps claim to push 35 Gallons per hour at 4-7 PSI. I am assuming that if this is true, most of the fuel will be returning. I am also assuming that the pump will simply build up to max pressure and then stall until it can supply more volume. These are PeP boys off-the-shelf pumps, an upgrade may be necessary...

Thanks for the input...

Nate

[rjwest]

I added a tank( and gage)_ to my 96, pump from aux to main keep main tank 1/2 to full ( fill vents "t" together in case I forgot transfer, just returns to aux)
eliminates all the other problems...

[da4wheeler]

Well,

I finally dragged myself out after work enough nights to get this project up and running. With just over 250 miles on the new pumps, lines, wiring, etc... it seems to be working just fine. All of the fuel is returning to the proper tank, and there are no starvation issues at high RPM levels. In the end, was it cheaper than just replacing the selector valve? No... it probably broke even. But... it was a lot more fun. I still haven't tested it in a long-term drive where one or both tanks end up mostly empty, and checked to make sure that the fuel returns properly in that case, but I suspect it will work out just fine. The only complaint I have is that the pumps are fairly loud, oh well... you only hear them until you turn the key!

Thanks for the help everyone.

Nate