fuel efficiency issue

[92xjlaredo]

alright i need some help here and im coming to you guys first. i recently did a tune up on my jeep, consisting of plugs, wires, distributor cap and rotor, fuel filter and oil change. now prior to all this i was getting around 18mpg on the highway, which is around average considering what was wrong with it.....it was misfiring on cylinder 6 because the plug was broken off completely and i had no idea, all the plugs that were in it were autolite plugs, gapped at around .070 when i pulled them out and as crusty and white as snake shit, and in general jeeps tend to run best on champion copper plus plugs, the distributor cap was in decent shape and the rotor was okay. the fuel filter was so rotted that it was seeping out the inlet nozzle, and the rubber hose connecting it to the fuel line was dry rotted and cracked. so the fluid in the filter was as brown as the rust on the outside.....now that i have proper plugs and gapping (.035) the damn thing is getting around 13mpg, other things i know of that are wrong with it are an exhaust leak, a seeping fuel tank on the seam (usually i dont fill it over half because of that) and what i think the culprit of the fuel economy is an O2 sensor that ive never changed, and i have no idea how many miles are on it....thats my next step to swap that O2 sensor for a new one, my question to you guys: is there something else i might not know of thats common to cause this behaviour in my jeep. btw its a 95 XJ with the 4.0 and automatic transmission np242. thanks a bunch for any help

[JoeNitro]

i have no idea why your MPG would have gone down. dont think that the O2 sensor would have much to do with it but... i could be wrong.

[skippy]

hm since the O2 sensor monitors the air/fuel ratio and adjusts it, it might be it, but i'm not sure..especially since the jeep's a 95, i'm not too familiar with pre-OBD II.
..but that still doesn't really make sense why your mpgs would all of a sudden go down after a tune-up like that.

[92xjlaredo]

well....i cant figure it out either, and the rotor on the distributor wasnt a 95, the mighty six has a yellow E on the side of the block, so i had the auto parts guy look up one for a 97 or newer cherokee, and bingo! fit perfectly, so its not the original motor either, could be anywhere between 97 and 01. and im also not getting a check engine light which i believe is monitored by the computer and also watches the O2 sensor. i also didnt think that the fuel leak would cause it either because if it were leaking enough to cause 13 mpg youd think thered be a puddle on the ground after i get out of work and leave it parked for the day

[netrachamp1853]

you didnt happen to cross any plug wires did you?

[92xjlaredo]

not completely sure, gonna go double check that RIGHT now, nearly positive i didnt though....that would be most retarded.....

[skippy]

i know this is somewhat off-topic, but wouldn't a fuel leak like that cause the MIL light to come on? with an evap code?

[92xjlaredo]

no plug wires are crossed, i figured not because it runs very smooth. and as far as an evap code im not sure....however i did notice something earlier when i checked the plugs. the computer still has a 1995 date, and as far as i knew they made some changes to the newer cherokees (97 or newer) im wondering if the computer cant properly readjust the timing because its programmed for a 95 and it doesnt know what to do.