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4.0 bad gas mileage

1653 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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Coolant temp sensor could throw the light if it malfunctioned completely, I would check the codes again, or maybe try to see if you can get somebody to look at streaming data.
I don't know if Autozone has that capability or not.
My service material says for '98 the downstream sensor is bank 1, sensor 2 (1/2 HO2S). The upstream sensor is bank 1 sensor 1 (1/1 HO2S).

It may still be a problem with the upstream sensor, but that code is for the downstream.
If there aren't any exhaust leaks between the engine and the upstream o2 sensor then its probably the sensor.
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