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4.0 bad gas mileage

1652 Views 27 Replies 10 Participants Last post by  GREASYHANDS, 
90Notch306
I just picked up a 98 XJ and the gas mileage is horrible. Put $20 in it and went 70 miles. CEL was on and i had it scanned. Bank 1 sensor 2 O2 sensor high voltage. Replaced the downstream O2 and everything was good for 10 miles now the light is back on. The truck runs good though. Before and after i changed the sensor. Seems rich though. Has some black smoke out the exhaust when i step on it. Ive been searching around for the last couple of hours about this. Everything i read is about it chugging though. Mine runs and idles fine. Will the coolant temp sensor throw a CEL?? Any other ideas??
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NSmist
Quite possibly your front sensor. Are you getting any kind of hesitation when you hit the gas?

While your at it, when's the last time it was tuned up? What do your fluids look like as well as you're ignition parts? Those can play a good part into mileage too.
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