LOW fuel level equals LOW fuel pressure

[cranetruck]

One feature of my dual tank control panel is the "Fuel Flow" indicator.
It is controlled by a couple of relays and a low fuel pressure switch in such a way that it turns on when the in-tank fuel pump and the return fuel don't match OR if the fuel pressure is low.

Today while doing some crane work the "Fuel Flow" warning light came on. The pump in the main tank was used and the return flow was routed to it (they match), so that ment that the fuel pressure was low (less than 5 psi).

I checked the fuel level and it was low, about 2 inches on the dip stick. I added biodiesel and the "Fuel Flow" indicator turned off when the fuel level was about 3 inches on the stick.

This means that a low fuel pressure switch installed before the primary filter may be used as a "Low Fuel" dummy warning light.

Images below show the control panel with the "Fuel Flow" light and the fuel pressure switch used.
The fuel pressure switch is another "gift" from Kenny in Florida and has NSN # 5930-01-161-9580. It is normally used on the M923 series 5-ton trucks.

 

[Desert Rat]

Holy guacamollie!!!!! Is there ever any part you can't graft onto a Deuce Bjorn?!?!?!? I am in awe! BTW, does VA require you to have a pilots licence to understand all the information on your dash as well?

 

[Westech]

hmm... I do alot of work on high psi fuel injected trucks and cars and if the psi drops more then 2-4psi most injectors will not fire. now you can and i have seen many cars come in to my shop out of gas. if it were to some how lower the psi when the tank got low it would shut it down with gas still in the tank. I dont think the psi was droping because of low fuel in the tank, well it could have been sucking air in the pick up (low fuel) but not just because there was less fuel. I think you have a good idea on the early warning of low fuel, but i think if you only looked at that to see how much fuel you had left, get a good pair of walking shoes.

 

[cranetruck]

Well John, it behaved the way I described it, I'll let it run low again to see what's going on.
Need to learn exactly how the pump works.
Thanks for the input!

 

[mangus580]

Bjorn, I noticed the same thing the other day.... I had NO power, and was limited on RPM's.... When I went off to get fuel, I was lucky I made it there It was acting like it was running out by that time, as it was so low...

 

[cranetruck]

Since I was only running the crane any loss of power was not noticed. May have been a different story on the road.

 

[rdixiemiller]

My truck gets sluggish when it gets down to 3" of fuel. Goes dead at stoplights. I try to always keep 1/2 tank in there just in case.

 

[gringeltaube]

This means that a low fuel pressure switch installed before the primary filter may be used as a "Low Fuel" dummy warning light.

I’ve always wondered what happens when the primary intank fuel pump starts sucking air at low fuel level (2” or less), specially while driving on bumpy roads. In the worst condition the fuel/air mixture would not separate in the filters but be stable enough to reach the IP! Not to talk about more viscous alternative fuels like SVO! And somewhere below 20 – 25 psi the relief valve for the fuel return line would remain closed which means that accumulated air also could find its way to the IP…!
Good idea to have a pressure switch (or gauge?) to indicate a low fuel pressure condition. I would install it behind fuel filters to ensure that there always is fuel in excess (exceptionally with some air) returning into the tank. Whatever causes low fuel pressure, it should be corrected before it gets worse.

Gerhard

 

[cranetruck]

Gerhard, the primary purpose for my low pressure switch is to monitor the operation of the valves. If a valve fails, one pump can simply run the fuel from one tank to the other, a condition which will reduce the fuel pressure.
For monitoring fuel pressure in general, I agree that there are better places.
Today's observation may have been a bit hasty, but seems to indicate that pumping some air will also reduce the fuel pressure.
I might install a transparent in-line fuel filter just to see if any bubbles are pushed through. It is possible that the impeller design is such that air or gases are somehow separated from the fuel.

 

[Recovry4x4]

On somewhat of a related issue, my wrecker has been a yard truck since I got it running. When it has adequate fuel it starts right up. When the fuel is low, you need to crank some time to get it to light. I think with the design of the screens on the side of the in tank pump it starts moving air with the fuel when low and causes the diminished fuel flow. Since I've figured that out, I've never checked the fuel in the wrecker. When it gets hard to light, I just add fuel and the issue goes away.

 

[mangus580]

I should add to this.... that I dont think my in tank pump is working (and hasnt been for quite some time) That might make my observation a little farther to the extreme than one with a running pump.

 

[cranetruck]

I googled the problem, but didn't get any conclusive answers.

In general, it appears that air bubbles caused by pumping from a low fuel level or sloshing are simply returned to the tank (via the fuel return line) and the engine will exhibit fuel starvation symptoms.

I observed the low fuel pressure, so now I just need to confirm that air is returned to the tank when this happens.
It still looks like the "low fuel pressure" warning is also a "low fuel level" warning.

 

[Recovry4x4]

So where did you plumb in that switch again? I might have to come up with a low air/fuel warining system like that. Wonder if I have any of them switches left?

 

[Dieselsmoke]

I read an artice in one of the PS magazines about not letting the deuce get below 1/4 tank of fuel. Don't remember which PS issue it was, but was for the multifuel.

 

[cranetruck]

Kenny, you may use the fitting right on the fuel pump plate on top of the fuel tank. It's 1/8 NPT so no need for inverted flare fittings.
You should have one more down there, you offered me one additional last time I saw you. If you can't find it, let me know and I'll stick one of the ones I have here in the mail for you.

The switch is normally open, which means that when the pressure exceeds 5-6 psi, it closes. That may work against you for a simple connection to a light and you may need a relay. Let the pressure switch operate the relay and use the relay contacts for the light of whatever.

 

[rmgill]

If below 1/4 tank of fuel, big hills will slosh your fuel to the back of the fuel tank and you will run out on the hill.

 

[Recovry4x4]

Maybe I'll simplify things and have an adequate fuel pressure light. When the light goes out I'll know somethings up. Wonder what roll dirty filters will play with this?

 

[cranetruck]

If the primary filter gets clogged, the pump will have to work harder and you'll see a pressure increase.
Driving off-road requires more fuel than will be indicated by this kind of warning light, so it won't do much good there. When the light goes out/on there is only a couple of gallons left to drive on, good for crane operators, though.

 

[Boatcarpenter]

In a emergency, could you close the vent and pressurize the fuel tank through the plug in the top of the tank with system air and a regulator set at 4 or 5 PSI? Especially when low on fuel and the return line is making lots of bubbles right near the pickup.
Just thinking online!!

 

[DrFoster]

I guess that explains why in the heck the reserve is so vast on these tanks... wasn't it like 8 gallons or more? it must have a weak or low volume discount type pump and requires a bit of gravity feed to assist.