Low Fuel Warning Light

My 06 F-150 fuel alarm goes off when the computer estimates I have enough gas for 50 miles, not sure if the 250 diesel is the same or not.

Bob

I sure wouldn't count on 5gal. Diesel foams, it is hard to get an accurate fill unless you do it very slowly.

Fuel pumps are now inside the fuel tank and need to be covered with fuel to prevent overheating and burn out.

Tom J

Precice numbers will probably not be available. My gas powered Ford van owner's manual says that the low fuel light comes on at 1/8 of a tank. Since the gauge isn't totally accurate anyway, I'd be afraid to peg the low fuel indicator to a specific amount of fuel though.

CJB

Danny, CJB is correct on this one. ALL 04 and newer "Low Fuel" lights come on at

1/8 of a tank remaining. Just so happens when you divide 38 by 8 what do you get? By my calculations I get 4.75gal. So you are close. It will go off at that level, but what you have to remember is what Tom J said. The pumps are indeed inside the tank and use the fuel as a coolant. SO in order to allow you to keep from burning out the low fuel light and consiquently your fuel pump, you should keep it above a 1/4 tank at all times, MORE importantly is in the summer when it gets hot. As for reserve capacity, all tanks have a 2gal built in reserve, this IS NOT your low fuel light. That is from the time the guage points to E for EMPTY. You should NEVER hit your low fuel light unless you are just SOOOOO far from a gas station that it cant be helped.. I have hit my low fuel light ONCE and it was driving on a trip from Seattle to south east Kansas, and I didnt see a gas station for about 150mi, and I past the last gas station at half a tank, and didnt know my truck very well at that time. So I urge you to keep from costly fuel pump replacements, fill your tank up at or above 1/4 tank.

Ford Tech

Wives tale. Not true. No evidence to that effect. Damn, I'm tired of hearing this bullshit about fuel pumps burning up. It JUST DOESN'T HAPPEN!!

Really now? Hmm, thats why if you look at the engineering drawings for the fuel pump, you will see that the fuel does indeed contact the ENTIRE motor winding, thus creating a cooling effect and carrying away excess heat.. See fuel isnt flammable as a liquid, its the vapors that are flammable (but then you would have to get a C in chemistry to know that), so it is perfectly safe to use the fuel to cool the motor on a fuel pump. They wouldnt have to cooled if they didnt have to make 40-60psi in the fuel line.

Ford Tech

Go ahead and run your fuel to empty & then pay!!! IT DOES HAPPEN on a regular basis to people that insist on runing to near empty before adding fuel.

Tom J

Yes, and you 'll notice if you've ever sat a glass of water on the dash of a car, the liquid sloshes all over, all the time. Even with a very small amount of fuel in the tank, it will still be splashing on the outer part of the pump. Not to mention (and you should know as a ford tech) the ford pumps are enclosed in a plastic housing with a somewhat complex siphon system that keeps the pump submerged in fuel down to the last drop. As for the other makes of automobiles, the mere action of the fuel going through the pump cools it, so as long as the pump is pumping fuel, it is being cooled. When it runs out of fuel, then there is no load and no need to cool. Not to mention as soon as the engine dies, the fuel pump will quit within a few seconds of oil pressure loss.

I manage a fleet of 34 trucks and about 19 of them are Fords. Been doing this for 11 years. We routinely run the tanks out of fuel before switching. Currently we're averaging over 100,000 miles on electric in tank fuel pumps.

I am not going to say you are wrong here, but their are a couple of other things to consider.

1. In the summer, the less fuel you have the warmer the fuel gets due to more vapor in the system that heat up quicker than liquid.
2. In the winter the less fuel you have the more air in the system and subsequently moisture to cause condensation and fuel in the water, leading to a higher possibility of frozen fuel lines on cold nights.

So as you can see, its best to keep the tank above 1/4 tank.

When it runs out of fuel, then there is no load and no need to

Ford Tech

The fuel pump doesn't pump vapor.

You know, I'd expect someone who worked Fords' technical hotline to at least understand how the evaporative emissions system works.

No but it does have to pump the warmer fuel. The less fuel the easier it is to heat it up during the summer heat.

I know how it works, but it doesnt stop condensation all together, especially when the temperatures get as cold as 10-20 deg F.

So, where does vapor play a role? You must have included it for a reason, or do you get paid by the word?

It does until you remove the gas cap. Now, as far as 10-20 deg F. goes, check the current weather stats around the country, report back what the dew points are where the temperatures are 10-20 deg Fahrenheit.

The vapor comes in when you talk about heat transfer. The vapor heats up more easily than liquid (following me here?), thus allows the fuel to heat up quicker. When the fuel heats up to about 90*F on a 105*F day in the midwest, then everything on the vehicle is working harder, including the fuel pump.. Thus it allows the bearings to overheat more readily, thus causing fuel pump failure.

Actually the temp here right now according to

is 10*F and the dew point is 6*F. I never said all over the country, but to be exact about THREE-FOURTHS of the country is experiencing sub-freezing temps right now, and over half of that part is experiencing single digit and below. (Current temps as of 12:06AM CST) You can punch in 66762 as the zipcode to get the same info I just gave you.

The vapor is due to the fuel heating up and fuel sloshing, not the other way around. And why isn't the purge system working as it's supposed to? Any engine that has run long enough to create the scenario you describe has gone into closed loop long enough for the purge system to be active.

Doesn't viscosity tend to go down with heat? Why does fuel automatically become harder to pump as it gets warmer?

You do realize that a dew point of 6 is mighty dry air don't you? So, we've got cold temps and dew points in the single digits, where is all this moisture going to come from that is going to condense and cause fuel line freezing? When air is cold, it becomes more dense, squeezing any moisture out of it, lowering the moisture content. Basic seventh grade science.

Well I guess it is going to come from the same place all the condensation on the underside of my barn shed roofs is coming from (22 degrees Fahrenheit here today). When the sun heats the dark color steel roof, the frozen condensation melts and drips all over my implements... All the while the frozen water outside is freeze drying (changing from solid ice to vapor, skipping the liquid stage) and it is dry as hell out here... Go figure.

Not a very good guess...

Is the environment below your shed roof the same as the inside of a fuel tank? Does the sun directly heat the top side of a fuel tank/most fuel tanks? Does your shed have a purge system that draws fresh air in and pull vapors out? Shed floor, dirt, wood or concrete? Is the shed otherwise sealed?

FWIW, that which you call freeze drying, [ice going from solid directly to a vapor] is called 'sublimation'.