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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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i thought bank 1 is the upstream sensor.
Its a possibility im not 100% sure its the downstream. But the PO said it was the downstream one. And a mechanic also told me the same thing
Thanks for the input. Does anyone know for sure which O2 is bank 1 sensor 2?
Yea ill will be checking the codes again tomorrow. I thing autozone just has a cheap actron scanner too.
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
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.
Yes bank 1 sensor 2 is the downstream. I just had it scanned again and that code is gone so the sensor was bad. But now i have a code that says bank 1 system too lean. Any ideas? Still think the upstream sensor is bad?
Its not. Its a new pace setter full length
I just put in some 5.0 mustang injectors and it still feels the same so i dont think its an injector problem. I just dont want to go spend another $60 on an O2 and that not be the problem
Just ordered an O2 off of ebay. Its the same brand that rock auto has but i got it for 31.49 shipped. I will also try re torquing the intake/exhaust bolts later too
New upstream O2 sensor finally came in this morning. Installed it and reset CEL. Been driving it all day and its doing a million times better on gas. Light hasnt come on either. Thanks everyone for your input.
Yup its all good now. Cant wait to go to R&V and Silver Lake
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