RubiJK

Ok, well as promised in the "skid plate must have" thread, I intentionally ran out of gas. I have to say, it sucked driving around for a couple of days wondering when it would happen.

Here is the background... My gas tank skid is pretty well bashed up, as are many others. If you read that thread, you will see that there is a question of whether "my tank wont hold as much gas" is due to the sending unit, or the tank physically.

Typically, I get around 13mpg. My low fuel light comes on around 180-190 miles on the tank. I fill up, and it only takes about 16gal give or take. When it is full, my dash fuel gauge does not quite point to the F line, so I assumed the sensor was a part of the mystery.

My 4 door tank is 22.5 gallons per the manual. At 13 mpg I should be able to drive 292 miles before running out. The quantity left when the low fuel light comes on is "approximately 2 gallons" per the manual. So, I should run out about 26 miles after the light comes on.

Ok, here are the facts with my "experiment"...

I went camping over the weekend and drove in the mountains winding up the RPMs on some big hway hill climbs, so I knew I would get less than 13mpg on this tank.
*173 miles, my low fuel light came on
*243 miles, I ran out of gas (70 miles later)
*[Added a few gallons from my jerry can]
*19.9 gals of gas were used to fill up the jerry can and the tank
*9.7 miles were driven between filling up the can/tank (approx 3/4 gal of gas)

*22.5 gal capacity - 19.9 gal to fill - .75 gal to gas station = 1.85 gallons missing

So what did this "prove" (in theory anyways)? That BOTH my sensor and physical tank are jacked up. It seems that the trigger for the low fuel light is based on the orientation of the fuel gauge in the dash. I drove 70 miles after my low fuel light came on... This is obviously much more than 2 gals of gas. Based on 13 mpg, there was 5.4 gals left at that point. My gauge does not read "full" when it is filled up, and it dipped way below "empty" when it was really empty.



However, aside from the discrepancy in the gauge/sensor, the fact remains that I was physically unable to put 22.5 gallons of gas into my empty tank. I have to admit, I expected that I might be 1/2 gallon off, maybe as much as a full gallon at the worst case. I am very surprised that I am approx 1.85 gals short.

While this isnt the most accurately controlled experiment, it does prove these points. Since there was not a control group, it would be interesting to see if somebody is able to put in the full 22.5 gals into a 4 door. [What is the most you have ever put in when you have really pushed your luck driving after your low fuel light came on?] I guess this would begin to help us understand if the reason behind the lack of fuel capacity is due to a dented and smashed skid/tank or if the stated capacity might be inaccurate...

RubiJK

FYI... My 2007 manual states the fuel capacity for a 2 door is 18.5 gallons and the 4 door is 22.5 gallons...

wayoflife

wow, that is so cool that you were willing and able to do this. when you say you ran out of gas, is it safe to assume that you tried starting up the engine again a few times without it kicking in even for just a bit? also, what position was your jeep sitting in at the time you were supposedly empty. just so that it's clear, the sending unit is located at the back of the very long tank. if in fact your jeep were sitting even slightly with the nose down, your sending unit could have been sitting high and dry while a substantial amount of gas could have been held in the foward part of your tank. of course, if in fact your tank were dimpled up a bit in the center, this might even be enough to keep gas from reaching the sending unit. last but not least what would be helpful in making your case is if we could get someone with a brand new JK to do the same test, drive their jeep till it's empty - COMPLETELY EMPTY and then fill it back up. having owned older jeeps and having worke on their gas tanks before, i'm not entirely convinced that the JK's can be filled to an actual 22.5 gal even though that's what their said capacity states.

again, thank you for doing this but as you stated, we have no control to guage your findings but i sure would be interested to have this looked into more.

JayhawkJeep

One thing you have to consider is did you truly run out of gas or lose suction? Don't know about the JK but other systems I have dealt with would "run out of gas" even though there is some left in the tank.

wayoflife

good question and one that has real merit. unfortunately, it would seem the only way you could know for sure if you were empty is to drop the tank, physically empty it out and then refill it. of course, that would be a chore to do.

RubiJK

Yep, there were some minor holes in the "experiment" but I think its pretty telling. I agree, it would be very interesting for a brand new rig to do the same thing. Most of the other vehicles I have owned actually were capable of holding MORE fuel than what is stated by the manufacturer. Either way, just an observation.

I was actually on the freeway when I ran out of gas... taking my kid to school this morning! It was perfectly flat, and I was running about 65mph. After I coasted to a good spot on the shoulder, I tried to start it back up. It tried and tried, but never turned over. Its true that there could be a bend or dent that is acting like a dam preventing the fuel from reaching the sending unit... but 1.85 gallons seems to tell something.

Again, this wasnt perfectly controlled, but it was controlled enough to confirm that I could not put 22.5 gals into my tank. Now... the REASON for this cannot be deduced 100%... But I think it is pretty safe to say that dents in the skid/tank at least contribute to it... And, equally important, is that my sensor/gauge/sending unit is jacked up.

highoctane

I can attest to the jk 4 dr tank taking all 22.5 gal. I dropped my tank to take a look at my frame(different story there). Ran my jeep till the low fuel light was on, drove another day to work and did it then. Pulled the hose off the pump output and cycled the key on/off multiple times to empty the tank. Once the tank was out it was literally empty, small amount sloshing around, half gallon at most. Once it was all done we flat towed to the gas station on base, took 22.3 gallon to fill it up.

wayoflife

being that the gas tank is a little over 5' long and that the sending unit is located in the back of it, i still have my doubts. even if you were perfectly level, a perfectly level JK sits nose down and by quite a bit from the factory. this is refered to as a rake. even people who level their JK's out by installing coil spacers up front will still find that the foward most point of their frame rails (behind the front tires) still sits a bit lower than the rear (in front of the rear tires). over the span of 5 feet, even a degree or two down would create enough of an area to hold a substantial amount of fluid.

however inconclusive, i think your experiment is awesome and i'm glad that you did it.

wayoflife

good to know. assuming your data is accurate, it's one less thing to take into consideration.

RubiJK

My first reaction to this was that Chrysler wouldnt be so stupid as to design a tank that wouldnt allow nearly 2 gallons of gas to be utilized due to the rake of the vehicle... but then I remembered all of the idiotic design flaws I have come across while working on my JK and would say that it is plausible.

Anyway, the data and circumstances are here for everybody to draw their own conclusions from...