4.0 bad gas mileage

1653 Views 27 Replies 10 Participants Last post by GREASYHANDS, Jun 5, 2010

90Notch306

Discussion Starter · May 28, 2010

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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1988JeepXj

· #8 · May 28, 2010

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.

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1988JeepXj

· #13 · May 28, 2010

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.

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1988JeepXj

· #21 · May 28, 2010

If there aren't any exhaust leaks between the engine and the upstream o2 sensor then its probably the sensor.

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