Actual gas tank capacity

I have a '98 XJ 4.0 with 75k miles. As is the case as these later models age, the gas gauges many times seem to loose accuracy due mostly to crappy quality. The other day i decided to drive lower on the gauge than i would normally do, down to a needle thickness below the empty line. The thing is i was able to only fill just over 16 gals. Don't these tanks hold 21 gals??

My thoughts are, and help me with this- Is this correct for a 'norman' gauge?? Am not convinced it is actually malfunctioning because of this: I am aware it is possible to only fill these tanks to 85% capacity to allow for expansion and proper emissions processes. If this were true, 85% of 21 gals is 17.85 gallons, and with that in mind, my tank really did only have around a gallon of gas remaining?? Or, is that 21 usable gallons on what would be a 21 gal tank with an additional 15% expansion space?

I am preparing to move 2/3 way across the country for school to Utah and to know this could be some help.

Thanks guys, Judd

For practical purposes they are 20 gal. I ran out of gas once, and I was only able to put just over 20 gal. in. I believe it actually physically can hold more, but it won't let you put it in unless you go really really slowly. Something like that anyway...

I ran out of gas once and was only able to put 18.5 gallons in, weird.

The factory gauges are calibrated to show empty before they are in order to protect people from themselves. Case in point:

My brother is a BMW service writer. When he was service manager at another dealership, he got a call from a very irate female customer whose new BMW had run out of gas. She claimed the on-board computer was defective because it showed she still had 2 miles remaining before she ran out of gas when the engine died.

When I had the '99 WJ it had a trip computer as well as the standard electronic odometer. What used to irk me no end was pulling into a gas station to fill up and finding that the two odometers didn't agree with each other. Usually the discrepancy was only a few 10ths, but it might have been a mile or so on some occasions. Not earth-shattering, but also not right.

The moral is -- don't trust the gas gauge. Find out what your mileage is, use that to figure out a safe range per tank, and start looking for a station when you get close. I'm good for 350 to 400 miles on a tank but I start looking when I hit 250.

Eagle is right on with his comments here. Consumer protection issues he cited aside, making a gas guage that is truly precise would be an expensive proposition, given all the variables that exist in the sensing system used in most automotive gas guage systems.

What you are asking for from your gas guage is laboratory-quality measuring precision. (If I interpret you correctly, you want the guage to tell you the tank's real status to within a few tenths of a gallon, which is a level of precision of less than 2%)

An automotive gas guage (and all the other guages, for that matter) are more properly called "trend indicators", meaning that their precision of measurement (usually within 5-10%) are NOT good enough for real accuracy, but good enough to warn you BEFORE a situation develops.

As Eagle said, learn your gas guage's "trends", and then do you fill ups accordingly.

I remember a 1970 Ford I owned that had 5 gallons left when it hit "E". It also went from "Full" to "1/2" in about 75 miles. Like I said, they are only "trend indicators"...........

More Gas

Extra fuel

Check out this article. 5 more gallons per tank?

Judd W. VA said:

I am preparing to move 2/3 way across the country for school to Utah and to know this could be some help.

Thanks guys, Judd

Click to expand...

So where are you moving to Judd? We'll have to hook up once you get here......remember, Moab is only 235 miles from SLC.........

I read about the mod on the gojeep site a while back and finally got around to doing it a few weeks ago. I simply pulled the tank sending unit and reached inside (those of you with beefy forearms won't like this part) to the upper drivers side of the tank. You can feel the fuel fill tube and above that, the smaller tube is the vent tube.
I decided to try and bend the vent tube up, and would up using a long, clean prybar to do this because the angle was all wrong to do it with brute strength and when you flex your forearm.........the hole in the tank tends to cut into the skin. The vent tube was approx. 4 fingers width (just over 3" or 8 cm) from the top of the vent tube to the top of the tank before I started, when I was done bending the tube has a 2 finger width gap or 1.25"

I then filled the tank and got 21.5 gals in and I figured that I had about 3 gals left before fill up. I usually only get in about 17.5. I headed down to Moab a few days later, but didn't keep track of mileage on the way down. On the way home I filled up in Moab and didn't have to gas up until I hit 365 miles.......and that was after pushing a head wind for 4 hours/270 miles home from Moab running 70-75 most of the way, climbed over a 7500ft pass in a fully loaded Jeep with family and gear (a few pounds of moab rocks that my son just had to have ) and a motor with 180,000 miles on it. Did I mention the ultra low drag co-efficient of a 6" lift?
Anyway, making a short story long, the mod works, you can get an extra 3.5 to 4 gallons in and still allow for expansion.

Cheap, easy and as far as I can tell, causes no harm to whales, dolphins, desert tortise, spotted owls, striped sand fleas, black footed ferrets, fanatical environmentalist, or albino penguins.

XJEEPER

Okay.....

Thanks for the responses. So what am i to get from this...that it IS possible to fill the tank 21 gals, and that it is a 21 gallon capacity with required 15% additional space needed for expansion? I do understand gauge innacuracies and how it can vary from vehicle brand to vehicle, but if my gauge is this innacurate then i would consider replacement even if it is so expensive being combined with the fuel pump...or is it even screwing up?? Is this a normal characteristic? The thing is that i would like to be able to diagnose this potential problem/situation in this maner rather than to have to drop hitch, skid, tank, and look in there to see actual gasolene contained. Maybe cut a hole in rear floor, another in the tank, install covers so that periodically i would be agble to actually 'stick' the tank?! Dang!!

Also, am understanding if these tanks are 'overfilled' there is some kind of indicator that senses this and that small purge motor under the hood is switched to 'on' till the level drops to required level. They surely are anal about this kind of thing anymore!!

XJeeper, am moving to Ogden. That would add approx another hour to your distance, but yes, would be great to hook up at some point. One of the things i plan on doing is to make the most of my time out there and to hit Moab and surrounding areas, and to be shown some of the high points by a local would be totally cool. Will keep in touch-

How did I get jipped out of the 20 gal tank I only got the 13.5:rattle:

I have same problem. I know I have the 21 gal tank but can only fill to 14 gal. The other 7 gals I can add in about 20 more minutes if I go rrrrreeeeeaaaaaalllllll slow. WTF I think it is the vent tube but not sure.

Re: Okay.....

Judd W. VA said:

Also, am understanding if these tanks are 'overfilled' there is some kind of indicator that senses this and that small purge motor under the hood is switched to 'on' till the level drops to required level. They surely are anal about this kind of thing anymore!!

XJeeper, am moving to Ogden. That would add approx another hour to your distance, but yes, would be great to hook up at some point. One of the things i plan on doing is to make the most of my time out there and to hit Moab and surrounding areas, and to be shown some of the high points by a local would be totally cool. Will keep in touch-

Click to expand...

Don't know anything about a "purge motor", not on an 89 at least. I guess the only true way to know what your total tank capacity is would be to carry a spare 5 gals of fuel with you, run the tank empty, put in enough fuel to restart and get to a gas station, then keep accurate notes on how much fuel you add to fill it. Another good thing to know is what your range is from the time the Low Fuel light comes on until you run dry. Mine used to be about 3.5 gals.

Moving to Ogden? Attending WSU? Still time to change your mind?
Sorry..........actually I've lived in the SLC and Ogden area for many years. I currentlyy live in the So Ogden/Uintah area so we'll be in the same neck of the woods. There are places to explore around here......and there's always Moab!
Hit me backchannel and we can swap contact info, also let me know when you'll be hitting town.

XJEEPER

i've let my fuel gauge go down *way* below "E" (cuz i was curious and i have extra gas up top) several times...

each time i've only been able to put just a bit more than 17 gallons in.

so who knows.
?!

I may as well chime in on this one.

I have a 2000 XJ. I normally fill up right after the "low fuel" light comes on which is right at the 1/8 mark on the guage. (Normally I get 250 miles when that light comes on) Anyways I'm rather paranoid about running out of gas, and I've heard its not good for the fuel pump because its cooled by the fuel in the tank, so I never run it too low. Tonight I drove it 12 miles on the highway after the low fuel light came on. It may be due to paranoia but it seemed like the gauge dropped really fast in those 12 miles to half way between 1/8 and E. So I get to the gas station to fill up... only 15.887 gallons to the first click. WTF!!! 4 gallons in the tank seems like a lot for the gauge to be teetering on empty.

You other guys with 2000 XJs, are your gauges reading like mine? How low do you let it go before you fill (or run out)?

I never heard anything about a purge motor, and I don't know what it would do with the "extra" fuel. Not saying it isn't there, but it doesn't make a lot of sense to me.

As to the "small" tanks, remember that in the early years of the XJ the standard gas tank on base models and up at least through the Pioneer trim level was a 13.5-gallon tank. This was probably to reduce curb weight. When I bought my '88 Pioneer new, I ordered the optional 20.2 (NOT21) gallon tank, and had to take a mandatory tire upgrade -- all the way from a 195/75R15 to a humongous 205/75R15!

I have been told that the 13.5 gallon tank and the 20.2 gallon tank are the same external dimensions but the "small" tank used either an internal baffle (one story) or a different fuel vent (the other story) to artificially reduce the capacity. The idea of a tank shutting off at 14 gallons but then being able to coax another 5 or 6 gallons into it supports the vent theory.

EricsXJ......that is exactly the same thing my '98 seems to do. Sometimes after the low fuel waring light comes on it hangs there a while, then others it seems to drop like a rock!! I might not be quite as paranoid as you (but not far behind) about the gas level in the tank, but would prefer a little more consistency in the indication!! Is generally thought to run a tank dry does invite problems. Does make sense the fuelpump would rely on the gasolene for cooling, however i may try the run out of gas thing and carying gas can test just to see. Not sure- Wonder if there are any TSB's on this matter??

The purge motor is on the newer XJ's.

XJEEPER...will get in touch with you.