Another Bad Gas Mileage Thread

87 4.0 aw4 with a 88 motor and trans running great 31 bfg mts and 3.55s. Exaust, advanced timing and a CAI pulling air from the cowl chamber. Just got all my sensors in check after a bad o2 I suppose foulded from the bad coolant temp which must have been making it run rich as hell. Still getting about 10 mpg city with a little wheeling every once in a while and a couple wot races with the local ricers. So I suppose I could maybe do a little better and thats better than whatever I was getting before but I drive real conservative for the most part. I feel I can get 15 and am trying to figure out where I'm loosin it. Not much left to replace except injectors especially if they are the 215000 mile originals. Can worn injectors be a source of running rich? I'd like the upgrade them anyway. Then I'll get to the adjustable map eventually. What are your opinions on gas mileage? I know its one of the hardest performance aspects to deal with and improve

the tire size is not correct for the speedo gear, how are you calculating mileage? if you arent accounting for that then your mileage numbers will be lower then actual.

jeeptorino68 said:

the tire size is not correct for the speedo gear, how are you calculating mileage? if you arent accounting for that then your mileage numbers will be lower then actual.

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yea by like 10 miles at most. If it was all highway miles at 60 where the speedo is off by say 4mph at most it would be about 12 and at city speeds at least half that.

Just had the same on wifes 89, replaced O2 and bad coolant temp that was sending false temps to ECM. MPG went from 10-11 to 14. Replaced ECM, MPG back up to 20. Bone stock 89 aw4.

Mike Mike said:

Just had the same on wifes 89, replaced O2 and bad coolant temp that was sending false temps to ECM. MPG went from 10-11 to 14. Replaced ECM, MPG back up to 20. Bone stock 89 aw4.

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The computer itself was causing it to run rich? I've actuallly got a spare one but what was the signal that the computer wasn't functioning properly? Computers are just a rare thing to fail so I'm not assuming mines bad or anything like someone would likely accuse for efi problems. Just something that would be nice to test.

The coolant temp sensor was sending low temp to ECM ( was showing temp never over 170f) So it stayed in open loop. even with the new coolant temp sending correct data, ECM wouldn't respond and go closed loop. A friend who owns a shop did the work. I'll check with him to see what the tip off was. the bad ECM was a Standard brand that was just installed in Dec. The jeep never seemed to run right with that ECM. Maybe a defect from factory.