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4.0 bad gas mileage

1651 Views 27 Replies 10 Participants Last post by  GREASYHANDS
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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The downstream doesn't control fuel delivery, only the upstream does. I'd be willing to bet that's your problem.
i had to think about this again LOL. the cherokee only has one pair of o2 sensors (banks) sensor 1 is up and sensor 2 is down. TJ's (and other vehicals) may have 2 pairs (so bank 1 and 2) and sensor 1 (before cat) and 2 (after).

Just because you get a code that says bank 1 sensor 2 voltage high doesn't mean its a bad sensor. Best thing is like mentioned before, check the actual voltage with a scan tool of all your sensors. especial upstream 02, tps, map, IAT. See where the extra fuel is coming from.

90% of the time if you are not getting any other sensor codes, its the upstream O2 sensor. Its telling the motor the engine is lean and adding more fuel but will not trip a code.
I forgot that they started putting 4 sensors on some of the vehicles. I don't think that was the case in 98 though, unless they did it to XJs and not ZJs or something dumb (not ruling that out). I agree about the upstream sensor though, usually they drift a long ways before they throw a code, if they even throw a code at all. I had my upstream drift so far that my ZJ almost wouldn't run and it didn't throw a code.
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