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Old 11-06-2008, 05:40 PM   #1
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Question O2 sensor question?

Hello, I am a proud new owner of a 97' Wrangler SE but I have a lot to learn about this vehicle and I was hoping I could find some help. As soon as I purchased this Jeep and made the 100 mile drive home I knew I issues with gas mileage, I'm only getting around 9 mpg!! I started by bringing it to Advanced Autoparts and having them do a computer reading on it, they found a bad O2 sensor so I purchased that part as well as new spark plugs hoping this would do the trick. I had them turn off the check engine light but it came back on after 7 miles and after a return visit to Advanced Autoparts they said the computer is still kicking out a bad O2 sensor code, it is also saying that the 1st piston is misfiring and its running lean. Through research I just found out that there is another O2 sensor right after the catalytic converter so I am thinking about changing that one as well. My issue is that my exhaust is spitting out a considerable about of carbon discharge, putting my hand up to it for 30 seconds turned it black even while idling. If I do choose to change the second O2 sensor is there anything I should do about the discharge from the exhaust or should the O2 replacement help fix this? Sorry about the length of this question but I wanted to give up all the info I had, any help would be greatly appreciated as I would love to get this thing OFF road!
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Old 11-06-2008, 06:06 PM   #2
Knows a couple things...

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The 02 sensor after the catalytic converter (aka 'downstream' 02 sensor) only checks the cat for proper operation. It has nothing to do with checking the air-fuel mixture as the other 02 sensor(aka 'upstream' 02 sensor) does so it will have nothing to do with poor fuel economy.

Carbon coming from the exhaust could be a sign of the engine being full of carbon from having been driven too conservatively. There are several ways to get rid of carbon, a couple of which are free and pretty easy so I wouldn't worry about the carbon too much.

I can't say why the #1 cylinder is misfiring, could be a bad valve, valve spring, or ignition related... spark plug, distributor cap, rotor, ignition wiring, etc.

Good luck with it.

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Old 11-06-2008, 06:32 PM   #3
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Ehh, thanks for the feedback.
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Old 11-06-2008, 06:51 PM   #4
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Interesting but not uncommon.

Let's go back to basics:
If the #1 cylinder is lean or misfires, it puts too much O2 into the exhaust, due to all the Oxygen isn't getting burned in that cylinder.

Now the exhaust has too much O2 in it, the O2 sensor "sees" it's has too much total O2 in the exhaust, it interprets that as the entire system is too lean. So it richens ALL the cylinders, till the O2 drops to about right. It richens ALL of them, not just the #1. Now #1 is right, but all the rest are rich.
Too rich makes that carbon you felt. It's incomplete burning, if you measured it, CO would be high.

The computer gets it's information from the O2 sensor, which is "pushing" the mixture toward rich, so the computer sets a code lean, even though the actual exhaust is rich.

I'd start by looking for a vacuum leak near the #1 cylinder, or even an exhaust leak ahead of the O2 sensor. Ie. Hoses, injector seal etc. Carefully spray a burnable Carb cleaner around suspected leak areas. Many carb cleaners are water based and won't burn. Dribble some on the floor and light it, if it burns it'll work. When you hit the leak, the burnable cleaner gets sucked in momentarily, you'll feel a slight RPM change. Propane works even better.

It could be something as simple as a plug wire - easy way is to swap it with a known good one - not one with another cylinder. New doesn't always mean good.

Let us know.

Sometimes ya gotta think backwards, a vacuum leak, or even an exhaust leak can make it run rich!
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