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4.0 bad gas mileage

1653 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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whyme86
90Notch306 said:
No hesitation at all. It runs great just has some black smoke when im on it and only gets 9mpg. Plugs, wires, cap, and rotor have all been done recently. Fluids are also all clean. It is a mechanicly sound truck except for this problem
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the problem is your running the wrong fuel in it if its blowing black smoke its prob a diesel :teehee:
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