STLHD Posted August 13, 2018 Share Posted August 13, 2018 so i've been looking on this forum for any information on fuel tank swaps. what i'm looking to do or find out if it can be done is trying to find out if the MJ long bed tank can be swapped into a short bed MJ. i have a 1987 MJ and i think it has about a 14 gallon tank and for the way this truck is used it is no ware near enough. I do not want the tank to hang down lower than the unibody and really not interested in swapping in a plastic tank from a dodge. Anyway if any one out there has info on this subject please let me know, i need to find a way to get some kind of range out of this thing. thanks, Link to comment Share on other sites More sharing options...
HOrnbrod Posted August 13, 2018 Share Posted August 13, 2018 You probably have the 16 gallon tank - that was the smallest. The others were 18 and 23.5 gallons. The 18 gallon will fit in your short bed, but the 23.5 will not, at least I've never heard of anyone doing it. Link to comment Share on other sites More sharing options...
DirtyComanche Posted August 13, 2018 Share Posted August 13, 2018 The long bed tank will interfere with the rear axle if installed in a short bed. I have a YJ tank that I was planning to try putting where the spare tire (normally) goes, but I haven't pursued it as jerry cans are a lot easier than modding the truck up. Link to comment Share on other sites More sharing options...
DirtyComanche Posted August 13, 2018 Share Posted August 13, 2018 Just now, HOrnbrod said: You probably have the 16 gallon tank - that was the smallest. The others were 18 and 23.5 gallons. The 18 gallon will fit in your short bed, but the 23.5 will not, at least I've never heard of anyone doing it. Are the 16 and 18 gallon tanks any different, or is it just the height of the vent tube in them? A guy can hack the vent tube a bit to squeeze a little more in most tanks, but be aware you can cause it to leak or force gas into the charcoal canister if you fill it right full on a hot day. Link to comment Share on other sites More sharing options...
HOrnbrod Posted August 13, 2018 Share Posted August 13, 2018 13 minutes ago, DirtyComanche said: Are the 16 and 18 gallon tanks any different, or is it just the height of the vent tube in them? Don't know Dirty. Link to comment Share on other sites More sharing options...
Jeep Driver Posted August 14, 2018 Share Posted August 14, 2018 Here is the tag off my tank in my short bed. And why I photo-doc everything. Link to comment Share on other sites More sharing options...
DirtyComanche Posted August 14, 2018 Share Posted August 14, 2018 1 minute ago, Jeep Driver said: Here is the tag off my tank in my short bed. And why I photo-doc everything. How about a picture of the tank (installed) rather than the tag? The 23.5 gal tank takes three straps and is longer than the 18 gallon tank. Link to comment Share on other sites More sharing options...
Jeep Driver Posted August 14, 2018 Share Posted August 14, 2018 24 minutes ago, DirtyComanche said: How about a picture of the tank (installed) rather than the tag? The 23.5 gal tank takes three straps and is longer than the 18 gallon tank. For whatever this pic is worth, I have pics of it out but in photobucket and not worth retrieving at the moment. There was no indication that the tank had ever been out and pump appeared original when I replaced it. Tag was on top and attached to a vent hose, again, no indication or reason to question the tag. Measures 29x17x13 Yes, I see that the 23 gal is 7 inches longer, otherwise the same height and width. Link to comment Share on other sites More sharing options...
DirtyComanche Posted August 14, 2018 Share Posted August 14, 2018 Just now, Jeep Driver said: For whatever this pic is worth, I have pics of it out but in photobucket and not worth retrieving at the moment. There was no indication that the tank had ever been out and pump appeared original when I replaced it. Tag was on top and attached to a vent hose, again, no indication or reason to question the tag. Measures 29x17x13 Yes, I see that the 23 gal is 7 inches longer, otherwise the same height and width. That looks like the regular shortbox 18 gallon tank. It certainly is not the LWB 23.5 gallon tank. I'm guessing that since the tag was attached to the hose that it is not relevant, the hose assembly may have been listed for the larger tank but was determined to be fit either, or it was Monday morning or Friday afternoon. How much fuel fits in it would be the real question. Link to comment Share on other sites More sharing options...
AZJeff Posted August 14, 2018 Share Posted August 14, 2018 I put a "plastic gas tank from a Dodge" in my SWB MJ, and the tank sticks down below the "frame" rails less than 1 inch at it's lowest point. Since I have a 4" lift already, that protrusion is not very much. FWIW...... Link to comment Share on other sites More sharing options...
DirtyComanche Posted August 14, 2018 Share Posted August 14, 2018 8 minutes ago, AZJeff said: I put a "plastic gas tank from a Dodge" in my SWB MJ, and the tank sticks down below the "frame" rails less than 1 inch at it's lowest point. Since I have a 4" lift already, that protrusion is not very much. FWIW...... The sucky thing about the Doooodge tank for him is that it won't play nice with his fuel gauge. My sender/pickup is screwed now and I'm considering the Dakota tank along with swapping to a HO cluster and speedo setup... I just don't have any of the parts on hand. Of course that still doesn't really fix the fuel gauge, since the Dakota is the insane new Chrysler resistance range... Not interested in doing the new style dash and interior. Link to comment Share on other sites More sharing options...
Eagle Posted August 14, 2018 Share Posted August 14, 2018 11 hours ago, STLHD said: so i've been looking on this forum for any information on fuel tank swaps. what i'm looking to do or find out if it can be done is trying to find out if the MJ long bed tank can be swapped into a short bed MJ. i have a 1987 MJ and i think it has about a 14 gallon tank and for the way this truck is used it is no ware near enough. I do not want the tank to hang down lower than the unibody and really not interested in swapping in a plastic tank from a dodge. Anyway if any one out there has info on this subject please let me know, i need to find a way to get some kind of range out of this thing. You have a shortbed? The tank is 18 gallons. There was only one size tank for the shortbeds. Long bed could be 16 or 23.5 gallons. As others have commented, the tank from the longbeds is too long to fit into the shortbed chassis. What kind of range are you looking for? My '88 was good for 19 MPG in general, mixed driving. Figuring 16 gallons usable, that gives a range of 304 miles. I can't drive 300 miles in an MJ without stopping. Link to comment Share on other sites More sharing options...
HOrnbrod Posted August 14, 2018 Share Posted August 14, 2018 6 hours ago, Eagle said: You have a shortbed? The tank is 18 gallons. There was only one size tank for the shortbeds. Long bed could be 16 or 23.5 gallons. As others have commented, the tank from the longbeds is too long to fit into the shortbed chassis. For the HO years, the 16 gallon tank was not offered, only the 18 and 23.5 gallon tanks. 1 TANK, Fuel 8350 3398 1 18 Gal. - Gas 8350 2632 1 23.5 Gal. - Gas 8350 2635 1 Diesel Link to comment Share on other sites More sharing options...
STLHD Posted August 14, 2018 Author Share Posted August 14, 2018 I am the third owner of this truck and really have little history on what was was done to it before me. when i purchased the truck it was in okay shape with some 97+ stuff done it had a lift pieced together and drove really bad but had a new fuel tank and aluminum skid plate for the tank. I am thinking at some point the tank was replaced with the 16 gallon from a long bed or at least that's what the parts house had ( When filling the fuel from empty with the low fuel light on i am only able to get 14 gallons in it ), judging from some of the work i found on this truck that would be my guess. Does anyone reproduce the 18 gallon tank? Has anyone done a dual tank where the spare tire is mounted? Best i can figure is i'm getting around 14mpg, and being in Nevada nothing is close including gas stations. Thanks all for the input on this matter. Link to comment Share on other sites More sharing options...
DirtyComanche Posted August 15, 2018 Share Posted August 15, 2018 There's a few companies re-popping the 18 gallon tanks. Or maybe there's only one place and they're sold under multiple brands. I bought this one from Spectra (it's like $65 US or something). https://www.rockauto.com/en/moreinfo.php?pk=17613&cc=1181720&jsn=1979 It's fine, but a little thin, like most reproduction tanks. I cleaned it thoroughly and painted it because rust is an issue here. Link to comment Share on other sites More sharing options...
Eagle Posted August 15, 2018 Share Posted August 15, 2018 20 hours ago, STLHD said: I am thinking at some point the tank was replaced with the 16 gallon from a long bed It doesn't fit. Link to comment Share on other sites More sharing options...
HOrnbrod Posted August 15, 2018 Share Posted August 15, 2018 2 hours ago, Eagle said: It doesn't fit. My my, aren't we cranky today. Link to comment Share on other sites More sharing options...
DirtyComanche Posted August 16, 2018 Share Posted August 16, 2018 9 hours ago, HOrnbrod said: My my, aren't we cranky today. He has a point though, the tank should be the 18 gallon, and nobody makes the 16 gallon anyways. Now that we're talking about it I seem to recall that the 16 gallon long bed tank is just the 23.5 gallon with a longer vent tube so you can't fill it any more, it occupies the same physical area and is the same shell. I could be wrong though... I'm guessing the fuel sender is a bit messed up and OP can run it lower or get more fuel in it than he realizes. But still, I feel his pain, I don't have enough range to do a lot of what I want to do... But jerry cans are cheap and easy. Link to comment Share on other sites More sharing options...
Eagle Posted August 16, 2018 Share Posted August 16, 2018 7 hours ago, DirtyComanche said: He has a point though, the tank should be the 18 gallon, and nobody makes the 16 gallon anyways. Now that we're talking about it I seem to recall that the 16 gallon long bed tank is just the 23.5 gallon with a longer vent tube so you can't fill it any more, it occupies the same physical area and is the same shell. That's correct. AMC did the same thing with the XJ -- the early ones had either a 20-gallon tank or a 12-gallon tank. They were the same tank externally, the smaller one was just set up to accept less gas. They did it because the most base of the base models had such small tires that a full 20 gallons of gas put them over their rated load! Link to comment Share on other sites More sharing options...
Eagle Posted August 16, 2018 Share Posted August 16, 2018 On 8/14/2018 at 4:47 PM, STLHD said: Does anyone reproduce the 18 gallon tank? https://www.autozone.com/fuel-delivery/fuel-tank/spectra-premium-fuel-tank/163611_127816_0_7348_40163 Quote Best i can figure is i'm getting around 14mpg, and being in Nevada nothing is close including gas stations. 14 x 18 is 252 miles. That should get you to a gas station. Even if you only use 14 gallons, that's still 196 miles. But 14 MPG is pretty awful for an MJ. Maybe it needs a new oxygen sensor? It should get closer to 20. Link to comment Share on other sites More sharing options...
STLHD Posted August 16, 2018 Author Share Posted August 16, 2018 buddy we'll do five hundred miles of dirt with no towns, in town i wouldnt care if i could only go 40 miles on a tank!! Link to comment Share on other sites More sharing options...
Eagle Posted August 16, 2018 Share Posted August 16, 2018 2 hours ago, STLHD said: buddy we'll do five hundred miles of dirt with no towns, in town i wouldnt care if i could only go 40 miles on a tank!! At 14 MPG, that's 36 gallons of fuel. You don't need a 23.5 gallon tank, you need a Jerry can rack. And find out why your gas mileage is so bad. Link to comment Share on other sites More sharing options...
johnj92131 Posted August 16, 2018 Share Posted August 16, 2018 I have been looking at a second gas tank for the Comanche for years. Best options I have found: 19 gallon tank from 90's F-150 pickup to replace spare tire Gets you to 35 gallons. At 14mpg, that gets you to 490 miles Carry one 5 gallon Jerry can for security. Gets you to 580 miles. Don't like that? 22 gallon Dakota tank and move Stock 16/18 gallon tank to passenger side Relocate exhaust system as required. Chapest solution? Carry 6 Jerry cans I have 2 Jerry cans for more than 20 years. Never used them to fill an empty tank Your miliage may vary Link to comment Share on other sites More sharing options...
STLHD Posted August 17, 2018 Author Share Posted August 17, 2018 all good ideas not sure why the mileage is so off if you all are saying it should be around 19mpg. we are at 5000 feet and everything from there is up the truck has 4.88 gears and 33'' tires and will be going to a 35'', Frame Plated, bumpers tire carrier winch, long arm, and center skid plate, so i have added some wait and i am sure the mileage has suffered. i have been around jeeps my whole life and have never seen a 4L with larger than stock tires get 19mpg ( just my experience ). Any ideas to improve the mileage I'm all ears. thanks all Link to comment Share on other sites More sharing options...
Eagle Posted August 17, 2018 Share Posted August 17, 2018 4 hours ago, STLHD said: Any ideas to improve the mileage I'm all ears. When was your oxygen sensor last replaced? Link to comment Share on other sites More sharing options...
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