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after 28 years, the old gas tank finally started letting some out of spots it wasnt designed to. i have read through every post i could find relating to gas tank upgrades here and can't quite come up with a decent idea so i figured i would kick the dead horse. mine is an 88 long bed with the 18 gallon tank. some people say the 18 and 23.5 tank are equal in outside dimension with a baffle internally to change the capacity. while i like this idea because it means i can upgrade without having to change anything, a look at the specs and pictures on some websites makes me think twice. rock auto says the 18 gallon is 29" long and the 23.5 is 36" long. the pictures look like the correct tank (all the holes and dents in the right spots) but the 23.5 is very obviously longer. that being said, is it still just a r&r job? i have three straps on my 18 currently, and there is the extra room there for it, i just done want to get in there (only vehicle) to find out i need a different sender or a different strap or larger flux capacitor. anyone pulled this one off?

23.5 gallon

https://www.rockauto.com/catalog/moreinfo.php?pk=17611&cc=1181584&jnid=421&jpid=4

18 gallon

https://www.rockauto.com/catalog/moreinfo.php?pk=17613&cc=1181584&jnid=421&jpid=1

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i have the long wheelbase, and the 18 gallon. both are for sure no doubt about it.

EDIT:  According to the 87-90 parts manual, MJ FUEL TANKS & PUMPS COMANCHE, the 88 LWB never came with the 18 gallon tank from the factory, only 16 and 23.5 tanks.

 

1                        TANK, Fuel

8350 2635 1                        w/16 Gal. Tank w/Carbureted Eng. - also use 53002492 - 1987- 88

8350 2632 1                        w/16 Gal. Tank w/2.5L Eng. EFI - 7 Ft. Box - also use 83504481 - 1988

8350 2632 1                        w/16 Gal. Tank w/EFI Eng. - also use 83502729 - 1987-88

8350 2632 1                        w/16 Gal. Tank w/4.0L EFI Eng. - 7 Ft. Box - also use 83504485 - 1988

8350 2635 1                        w/16 Gal. Tank w/Diesel Eng. - also use 52003644 - 1987-88

8350 2635 1                        w/16 Gal. Tank w/2.5L Eng. Carbureted - 7 Ft. Box - also use 52003652 - 1988

8350 3398 1                        w/18 Gal. Tank - EFI Eng. - 6 Ft. Box

8350 5282 1                        w/18 Gal. Tank - Carbureted Eng. - 6 Ft. Box

8350 2632 1                        w/23.5 Gal. Tank - EFI Eng. - 7 Ft. Box

8350 2635 1                        w/23.5 Gal. Tank - Carbureted or Diesel Eng. - 7 Ft. Box

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Well this adds to the confusion some, because while I know my fuel gauge reads off about 1/8th of a tank, it is fairly accurate.  Pretty sure I have the 16 gal tank, yet it is held up with 3 straps?  I know that when I am sitting at about 1/3 a tank and fill up it takes about 10-11 gallons.

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well. this is somewhat embarrassing. so yes, i have a 16 gallon tank. thing is, i knew that before, but i spent so much time online looking through prices and all that i think i just got confused? oh well, either way, i was wrong and i apologize. anyhow. i got under there and sure enough, 3 straps. i snapped a few pictures here too, with the phone and on the ground, so not too great, but you get the idea. so what is going on here? and back to the original point, can i just throw the 23.5 in with no worries even though its physically longer?

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DUNx0Rq.jpg

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well. this is somewhat embarrassing. so yes, i have a 16 gallon tank. thing is, i knew that before, but i spent so much time online looking through prices and all that i think i just got confused? oh well, either way, i was wrong and i apologize. anyhow. i got under there and sure enough, 3 straps. i snapped a few pictures here too, with the phone and on the ground, so not too great, but you get the idea. so what is going on here? and back to the original point, can i just throw the 23.5 in with no worries even though its physically longer?

 

It will fit, but the 23.5 gal. tank is deeper, so you will probably need new straps. Also the fuel sender is different; the 23.5 gal float arm is longer so it won't indicate accurately, and the 16 gal pickup may not reach the bottom of the deeper tank. The float arm may even hang up on the new tank interior baffles; I don't know. It's up to you to decide if the extra 7.5 gal capacity is worth it. It's worth a shot - go for it.

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  • 1 month later...
  • 9 months later...

oh man, totally lost track of this thread, but I will resurrect it because I know it's a hot topic. So,mine worked out strangely butto my advantage. So after some debating and inability to really figure out what I could and could not do (only vehicle so it couldn't be down for more than a day or two) I decided to just order the 16g from Rockauto. got straps and pump and all that too. Unsurprisingly, had to cut the old straps and heat the J bolts to get the nuts off then clean the threads, but thats not the interesting thing... Part number I ordered was KIT1282 and I didn't look at much besides the fact that it was dimensionally accurate and said for long wheelbase while the other said short. had i looked more at the details it says long wheelbase 23.5G and short wheelbase 16.5G. I laid it on the ground next to the old one, identical. same holes everywhere and indents for mounting straps and all, so I put it in. now, i know this will sound like I must be some kind of idiot or something, and i assure you, i am most surely an idiot, but i double and triple checked all this. that aside though, my old tank (drove it for 3 years like this) was a 16.5. my new one (identical when laid on the ground next to it) is the 23.5. i am told and assured by everyone that all the LB had the bigger one, but i most definitely had the 16.5. how this happened, i don't know, but what i then know about it, is that somehow there are externally dimensionally identical fuel tanks with different capacities. when researching this i saw a few people say that the small ones had baffles inside to lower the capacity for some emissions reason or production reason or something and most people seemed skeptical of this. So i have been a mechanic for almost a decade, so i know that they were identical externally without a doubt. rockauto lists it as an approximately 7 inch length difference. anywho, just thought i would finish up this story and add to the debate of whether its external size or internal baffles that make the difference in fuel capacity.

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VERY interesting!  The thing I see missing here is just exactly how long  was/is the replacement tank?

 

I used to think all 3 strap tanks were 23.5 gallons before this thread.

 

What is the biggest refill/refuel you have managed to put in the new tank?  With my long bed, I have once or twice manage to run it all the way to the bottom and still make it to the gas pump.  Then fill it up with 23.8-24 gallons.  Way too close I know.  But it has helped me calibrate my fuel gauge down to a gallon or so.  My low fuel light goes on with about 7 gallons remaining, so I can watch the odometer from that point for an estimate of miles remaining in the tank.

 

For some interesting reading about getting more fuel in the gas tank for a Cherokee, check out GoJeep's web site.  He added about 5 gallons of capacity to the Cherokee's 20 gallon capacity.  One of these days, I will try the same tricks in my Comanche fuel tank

 

http://gojeep.willyshotrod.com/

 

Fuel tank info here:

 

http://gojeep.willyshotrod.com/HowtoFuelTank.htm

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I'm confused. I have a long wheel base but a 18 gallon tank. Do most have 23.5s with long wheel base

 

The parts manual doesn't even list an 18 gallon tank for the 87-90 longbeds, only 16 and 23.5 tanks. Some of the 87-90 shortbeds did come with an 18 gallon tank however. BUT, anything's possible.........

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I'm confused. I have a long wheel base but a 18 gallon tank. Do most have 23.5s with long wheel base

The parts manual doesn't even list an 18 gallon tank for the 87-90 longbeds, only 16 and 23.5 tanks. Some of the 87-90 shortbeds did come with an 18 gallon tank however. BUT, anything's possible.........

Ya I have a 16 for sure I just thought maybe I was being dumb and it actually was 18. But would a stock 23.5 bolt right in on my long bed?

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I'd say yes for sure. The filler hose assembly is the same, but you would need another J-hook and strap, the 23.5 sender, plus the longer plastic shield under the tank if you use one. The metal tank skid is the same for all the long beds too.

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Curious, I have an 88 longbed parts truck that I just pulled the sending unit on... the tank looked just like the one pictured. 3 straps and a skid plate. The wires to the sending unit had the paper tag still on it saying 18.5 gallon on it...

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  • 5 months later...

What I would love to see or find is a direct comparison between the 16 and 23 gal sending units.  I hate having a 16gal capacity.  It just feels like I am filling up every other day.

 

As far as I can tell you, they are identical as mine came out of a 16 and went into a 23. I replaced the pump with the unit the tank came with (mostly direct fit, a little persuading but nothing big) so go for it. I can no longer say for sure what went in what, but someone else owned mine for the first 250,000 miles of its life, so there is a good chance it was swapped before. But to clarify, I had a 16g unit (I'm one of those guys with a notebook in my glovebox who writes down mileage and gallons and does the math for MPG per tank for every fill-up, so it's well documented) that used all three straps and looked identical when placed beside the new one. Wish I had taken some more pictures but I didn't expect this.

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Jeep did the same thing with the early Cherokees -- the base models came with a smaller capacity tank, to lower the curb weight and help boost the fuel economy a fraction of a MPG. The difference was an internal baffle - the tanks looked the same externally.

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