Fuel Remaining Incorrect

[Jeep-A-Kneez]

The days back tooling around in the old 75 Ford F100, "E" on the gauge was shorthand for walking.

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[ShadowsPapa]

The days back tooling around in the old 75 Ford F100, "E" on the gauge was shorthand for walking.

I remember those days. My 75 3/4 ton Chevy would get 100 mpg for the first 2/3 tank and then the last third I think it was 2 gallons to the mile. Once you hit 1/4 tank you had better know where the gas station is. But that first part of the tank, I swear you could drive forever.
Compared to other vehicles I've owned and driven over the years - these JTs are amazing in their accuracy.
I put a NOS (New, Old Stock) fuel gauge in my SX4 a couple of years ago preparing for senior competition in a show. Brand new gauge. Works great - until you start moving. The buffering circuit of the gauge is bad and when you stop, you suddenly get a few more gallons of gas. Take off fast and you can watch the thing go from 3/4 to 1/2 a tank. Hit a bump and it's like roulette - where will it settle. Luckily I have another new in the box fuel gauge but taking the cluster out of that car is a real exercise in agility and patience. I may put up with the ghost in the gauge for a while. If I want to know how much is really in it, I simply stop the car and wait for the fuel to stop sloshing.

 

[Hootbro]

Well, now you know where the fuel gauge and fuel remaining situation is. It really is a buffer to protect the vehicle and protect stupid people from themselves of running out of gas.

I have been on a couple of different vehicle forums and seen some asinine arguments by some owners thinking they should be able to run their vehicle dry when the DTE indicates zero or close to it.

 

[ShadowsPapa]

Well, now you know where the fuel gauge and fuel remaining situation is. It really is a buffer to protect the vehicle and protect stupid people from themselves of running out of gas.

I have been on a couple of different vehicle forums and seen some asinine arguments by some owners thinking they should be able to run their vehicle dry when the DTE indicates zero or close to it.

It was bad enough in the days of carburetors, then along come electric pumps and guys start adding them to get rid of the mechanical pump - and those don't do well when starved. Then you add injection systems and other controls and - it gets messy if you run 'em dry and worse if you are so bloody sure there's 1 cup of gas left and you have a right to it so you crank until the battery gets low, insisting there's more in there and the computer is wrong. Such people exist. That gauge is wrong, I know it is! There HAS to be more gas in this thing and I'll be damned if I'm not going to prove it! And the boss would laugh as he towed the vehicle in and we'd work to fix the damage.
In some cases it was the guy driving didn't want to face his wife telling him 50 miles ago he should have stopped at that station. He had to prove he was right and there was more in that tank.

 

[PyrPatriot]

Anyone have any issues with the computer being off on guessing your remaining mileage? Drove to work this morning (about 20 miles) knowing I was going to get gas today bc I had about 1/8 tank. Dash told me I had about 50 miles remaining...drove my 20 miles to work...turned the truck on afterwards and it tells me 15 miles, drive about 1/2 a mile and it says "low" under miles remaining.

Me thinking I'm about to run out of gas at any minute I pull off and get some gas...clicks off after only 19 gallons in. The truck holds 22 right? Just seems like a hell of an error to be telling me literally 0 gas to having 3 gallons left.

I park in a flat parking lot, and the only thing I can think is that it's hot (Northern UT is like 95+ right now), but I have no idea why there's that much variation in guage vs actual fuel remaining.

https://www.jeepgladiatorforum.com/forum/threads/fuel-tank-capacity-low-fuel-light.24554/

Putting 19 in doesn't mean you had 3 gallons left, either - when it stops at 19 for me if I am patient I can almost always get another 1 to 1.5 gallons more in it.

Yep. I saw that in a TFL video and tried it. Give it about a minute between fills. I didn't want to re-enter my card (some pumps time out after a certain period of inactivity) so I go 30s before giving a squirt of gas, then once a minuite or so passes I set it to pump normally (on lowest click), do so a few times have gotten 1.5gal in.

Don't people want to know how far they can actually go when the "low" mileage light comes on?

When the "Low" indicator comes on there should be about 15mi, or 1gal left. In my thread above I report how even that isn't very accurate.

 

[Hootbro]

It was bad enough in the days of carburetors, then along come electric pumps and guys start adding them to get rid of the mechanical pump - and those don't do well when starved. Then you add injection systems and other controls and - it gets messy if you run 'em dry and worse if you are so bloody sure there's 1 cup of gas left and you have a right to it so you crank until the battery gets low, insisting there's more in there and the computer is wrong. Such people exist. That gauge is wrong, I know it is! There HAS to be more gas in this thing and I'll be damned if I'm not going to prove it! And the boss would laugh as he towed the vehicle in and we'd work to fix the damage.
In some cases it was the guy driving didn't want to face his wife telling him 50 miles ago he should have stopped at that station. He had to prove he was right and there was more in that tank.

There is a pretty massive thread on one or more of the RAM truck forums over this. Seems until a couple of years ago, RAM had a couple of different size gas tanks but the DTE software for either gas tank size bought was calibrated for the small size tank. People that bought the large size tank were finding like 7-9 gallons left when the DTE was saying zero or low. I believe RAM put out a TSB for it to reflash the software because once known, people with the larger fuel tanks were driving way past their DTE margin and not keeping tracking of actual MPG against fuel remaining and running out.

 

[ACAD_Cowboy]

Yeah baby, a raging thread about tank capacity.

So part of the problem here is tank geometry and the other part is human psychology.

With the advent of high pressure in tank pumps as opposed to lift pumps in the tank and pressuring boost pumps up by the engine or worse yet mechanical pumps on the block, these pumps do not tolerate a few things; being hot, being dry and being wet with water.

So tank designers design a well into the tank to ensure that the pump is always bathed in fuel to stay cool and lubricated. Then they design the pickup to be above the tank floor to keep the pump from sucking water; there is always some water somewhere, mostly from condensation. The water stays way down in the well, the pump relaxes in fuel and the pickup drinks clean "dry" fuel and all is good.

But then you need to package this tank and so you need to play games with shapes to make all 20ish gallons of fuel fit inside the allotted geometry. Typically this leads to some bizarre designs and these shapes cause problems with the float measuring the tank volume. If the tank is broad you can lose a lot of volume without greatly effecting the vertical measure to the same degree. So the design engineers need to monkey with programming to give an output curve that normalizes float travel to gauge needle so you the driver feel confidant.

But wait, there's more!

So humans are weird things to design around. Human dynamics can make any project that much more "extra" in terms of "Are you kidding me?!?" and why should your gas tank be any different. The biggest hurdle to get over with humans is a seemingly simple question: Would you rather the gauge be accurate or precise? The codicil being the follow on question: would you rather the gauge be more accurate when the tank is full or empty?

So obviously our attention to tank volume remaining increases as tank volume decreases so when you fill the tank you're attitude is it's full and when it's hovering near empty we are straining to calculate exactly how many more feet can we get down the road accounting for axial rotation of the earth, wind drag on the antenna and the age old drama of if I take a leak we will lose mass but the fuel used on restart out weighs the mass savings... but I gotta piss so bad I can almost taste it... and the next child that touches a window switch is gonna be beaten by the other child as a lesson!!!! turn that dome light off!!!

So the designers know this as well. They however often forget to to account properly for the amount of fuel being used to bathe the pump, caught up in the lines etc. So the needle may be welded to the peg and the light on and you'll still pump about 20 of a 22 gallon tank. It happens, there is a forbidden volume you will never be able to truly access, it's included in your window sticker even, but it's never really yours. I'd rather live with this shame than run the risk of destroying the pump trying to wring that very last drop out.

In closing I feel we need to discuss the miles remaining calculation. It's is a "simple" calculation based on running MPG average as calculated from wheel speed sensors, injector pulses and the internal float data, the highly accurate and precise numbers they hide from your mere human eyes. In short this number is very real while also being somewhat useless in that it's a running average based on the previous given period of time so changes in traffic flow, terrain, gearing... it's an optimistic post it note on your sandwich versus cold hard data. So I would't rely on it to decide if the family is gonna sleep in warm dry beds tonight.

 

[PyrPatriot]

Yeah baby, a raging thread about tank capacity.

Let's do this!

So part of the problem here is tank geometry and the other part is human psychology.

With the advent of high pressure in tank pumps as opposed to lift pumps in the tank and pressuring boost pumps up by the engine or worse yet mechanical pumps on the block, these pumps do not tolerate a few things; being hot, being dry and being wet with water.

I was not aware that pumps were so sensitive. I had read you don't want to run a pump dry, but it won't destroy the thing. And pumps get splashed with water in the engine bay. Good to keep in mind!

So tank designers design a well into the tank to ensure that the pump is always bathed in fuel to stay cool and lubricated. Then they design the pickup to be above the tank floor to keep the pump from sucking water; there is always some water somewhere, mostly from condensation. The water stays way down in the well, the pump relaxes in fuel and the pickup drinks clean "dry" fuel and all is good.

But then you need to package this tank and so you need to play games with shapes to make all 20ish gallons of fuel fit inside the allotted geometry. Typically this leads to some bizarre designs and these shapes cause problems with the float measuring the tank volume. If the tank is broad you can lose a lot of volume without greatly effecting the vertical measure to the same degree. So the design engineers need to monkey with programming to give an output curve that normalizes float travel to gauge needle so you the driver feel confidant.

Engineers are interesting folks.

But wait, there's more!

I'm shamwow'd!

So humans are weird things to design around. Human dynamics can make any project that much more "extra" in terms of "Are you kidding me?!?" and why should your gas tank be any different. The biggest hurdle to get over with humans is a seemingly simple question: Would you rather the gauge be accurate or precise? The codicil being the follow on question: would you rather the gauge be more accurate when the tank is full or empty?

Accurate when the tank is full. I can then keep an estimated calculation on miles driven based on approximate MPGs.

So obviously our attention to tank volume remaining increases as tank volume decreases so when you fill the tank you're attitude is it's full and when it's hovering near empty we are straining to calculate exactly how many more feet can we get down the road accounting for axial rotation of the earth, wind drag on the antenna and the age old drama of if I take a leak we will lose mass but the fuel used on restart out weighs the mass savings... but I gotta piss so bad I can almost taste it... and the next child that touches a window switch is gonna be beaten by the other child as a lesson!!!! turn that dome light off!!!

? ? ? ? ? ?

So the designers know this as well. They however often forget to to account properly for the amount of fuel being used to bathe the pump, caught up in the lines etc. So the needle may be welded to the peg and the light on and you'll still pump about 20 of a 22 gallon tank. It happens, there is a forbidden volume you will never be able to truly access, it's included in your window sticker even, but it's never really yours. I'd rather live with this shame than run the risk of destroying the pump trying to wring that very last drop out.

So that's what the 4gal of extra fuel is!

In closing I feel we need to discuss the miles remaining calculation. It's is a "simple" calculation based on running MPG average as calculated from wheel speed sensors, injector pulses and the internal float data, the highly accurate and precise numbers they hide from your mere human eyes. In short this number is very real while also being somewhat useless in that it's a running average based on the previous given period of time so changes in traffic flow, terrain, gearing... it's an optimistic post it note on your sandwich versus cold hard data. So I would't rely on it to decide if the family is gonna sleep in warm dry beds tonight.

Now I want to run my tank dry just to see. I don't recall seeing any dire warning in the user manual on the subject of running to empty. And heck, shouldn't it be EXPECTED and anticipated by engineers that at some point, not often, but it will happen to a number of people in any liquid fuel fed vehicle, that it will run out of gas?'


Good write up, learned a bit.

 

[ACAD_Cowboy]

These high volume high pressure in tank pumps respond negatively when water is pumped through them. Gasoline has a lubricity that water does not on their components. You'll hear them squeal if they suck water through. Bad bad.

They will tolerate some abuse but only some and only for limited periods.

As for sucking dry, you can suck it only as low as the pickup which is above the low limit of the pump. Dry for the pickup is jot dry for the tank.

And speaking of tank geometry, we haven't even touched on eulage. So let's touch it, all over.

Eulage is the expansion of the liquid in the tank due to environmental heating. If you have a 22gal tank and you put all 22gal in, you have no space for vapors which will lead to tank overpressure which will lead to burping liquid fuel or spitting & spraying if it's hot enough. So as the liquid heats it expands and generates vapors which will also expand.

Heat comes from the air and the road but also from the fuel system. The injectors have a specific volume they can discharge per injection cycle but you need to provide extra to damped the pulses. This reject has been heated by pumping and hanging out around the engine and goes back to the tank further heating it.

As the tank heats the density of the fuel decreases and the float isn't as accurate as it might otherwise be.

So... the gauge isn't always telling you truth but it's not lying.

And so help me god turn that dome light off or you're walking...

 

[PyrPatriot]

And speaking of tank geometry, we haven't even touched on eulage. So let's touch it, all over.

Do you mean ullage?

Eulage is the expansion of the liquid in the tank due to environmental heating. If you have a 22gal tank and you put all 22gal in, you have no space for vapors which will lead to tank overpressure which will lead to burping liquid fuel or spitting & spraying if it's hot enough. So as the liquid heats it expands and generates vapors which will also expand.

I see. But would you need 2gal worth of expansion ability? I suppose you might, I'm basing my thought on the 5gal gerry cans: they don't seem to need too much expansion. But mulitiply that by 4 and whatever additional factor you need that comes with volumetric increase in fluid, could result in 2gal.

 

[ACAD_Cowboy]

Likely...

 

[ShadowsPapa]

These high volume high pressure in tank pumps respond negatively when water is pumped through them. Gasoline has a lubricity that water does not on their components. You'll hear them squeal if they suck water through. Bad bad.

They will tolerate some abuse but only some and only for limited periods.

As for sucking dry, you can suck it only as low as the pickup which is above the low limit of the pump. Dry for the pickup is jot dry for the tank.

And speaking of tank geometry, we haven't even touched on eulage. So let's touch it, all over.

Eulage is the expansion of the liquid in the tank due to environmental heating. If you have a 22gal tank and you put all 22gal in, you have no space for vapors which will lead to tank overpressure which will lead to burping liquid fuel or spitting & spraying if it's hot enough. So as the liquid heats it expands and generates vapors which will also expand.

Heat comes from the air and the road but also from the fuel system. The injectors have a specific volume they can discharge per injection cycle but you need to provide extra to damped the pulses. This reject has been heated by pumping and hanging out around the engine and goes back to the tank further heating it.

As the tank heats the density of the fuel decreases and the float isn't as accurate as it might otherwise be.

So... the gauge isn't always telling you truth but it's not lying.

And so help me god turn that dome light off or you're walking...

YES. Those of us who have moved from mechanical engine-based pumps to other pumps - electric types - frame mounted, in-tank types, etc. know well the differences. Run even the after-market pumps dry you have troubles. I have a Jeep in-tank pump in my SX4 fuel tank (oh, that was fun) and I make really sure to fill it when I get to about 1/4 or a bit less because my tank is a 1982 tank made with no baffles, made for mechanical type pumps. (and my engine hiccups if I let it get too low and take a corner fast - sucking air for a split second)
Electric pumps come in various types.
And the return line - in the old days this was to keep fuel moving to keep it cool - prevent vapor lock (a real issue with mechanical pumps PULLING fuel from 13 feet away and then pushing it up past a 1,000 degree exhaust system) so there was a return system devised - fuel filter with a 1/4" fitting and some of the fuel went right back to the tank to keep fuel moving. Otherwise under conditions of low fuel use and high heat (like driving in LA or NYC) the fuel would sit in the lines until the float dropped a bit and let more into the bowl - and it would get hot and boil and bam, vapor lock. Keep it moving, keep it cooler.
Today it's a mixed bag- move it past the engine in that hot engine bay warms it up.

It's an engineer's crazy balancing act.

I'm just happy the Jeep in-tank pump I used to feed the 4.0 in my SX4 has held up - I don't like the idea of dropping the tank - worse, trying to find another pump like that. I forgot to write down what model Jeep it was for and it's the one part I didn't keep track of the part number for when I did the conversion!

 

[ACAD_Cowboy]

I see. But would you need 2gal worth of expansion ability? I suppose you might, I'm basing my thought on the 5gal gerry cans: they don't seem to need too much expansion. But mulitiply that by 4 and whatever additional factor you need that comes with volumetric increase in fluid, could result in 2gal.

Why 2 gallons worth? No idea. I suspect it is more a function of the floor area of the tank, 1 gallon is 231 cubic inches so 2 gallons is 462 square inches 1 inch deep which is 3.2 square feet which tracks with JK/JL/JT tank shapes.

 

[Gossamer]

Don't people want to know how far they can actually go when the "low" mileage light comes on? I like to know how far I can actually go just incase I'm somewhere in between gas stations on a long stretch. I usually push it a little more every so often around town of course where a gas station is close by or in an area where I know my road side cover with my insurance is available. Never know when the zombie apocalypse will hit and you'll have to get 100 miles out of that low gas light. i think the most I've put into the tank on fill up was around 20 gals. I also use 89 or every so often 93 just to keep the system cleaner because I do run the tank so low. My best mpg was like 20.4 after a 70mph 4 hour road trip and the worst I've gotten was like 13.8 over the winter.

If you're at that point (light on), does it really matter how many miles you have? You've already on the way to a possible issue. So, you'll either make it to the next gas station or you won't. Knowing the mileage becomes sort of pointless.

 

[PyrPatriot]

I recently filled up when the "Low" light should have come on (15mi remaining) but it did not. Put in 19gal in my tank. So I either had 3gal of fuel left when I filled up, or like above was stated, the sensor for the cutoff is situated such that it leaves 3gal space in the tank for gas expansion.

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