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Unread 06-05-2013, 09:06 AM   #1
Kingr98
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1998 ZJ 
 
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Black Exhaust Pipe

I noticed that the inside and the last inch of the outside of my exhaust pipe is black. I think that I remember my dad teaching me that means the engine is running "rich".

Is this a bad thing? Is this an O2 sensor thing?

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Unread 06-05-2013, 01:21 PM   #2
PolkaPower
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Yea it's not good. Usually the 02 sensors but there are other things that can cause it too like an out of spec coolant temp sensor telling the computer that the coolant isn't hot enough so it dumps more fuel into the motor. Bad for MPG and bad for the cat. Leaky injectors can also cause that but that is much less likely than the 02's being bad.

When was the last time they were changed? Should be done every 50-60 thousand miles. I change mine earlier. Get Mopar or NTK. NTK makes the Mopar sensors. Change them both at once if that's your plan. Disco the battery when doing it.
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Unread 06-05-2013, 02:47 PM   #3
Jeeples
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Are you getting less than normal fuel economy? Unless you've noticed poor MPG's as of late, I would suggest cleaning up the tip a bit with some steel wool or a scotch brite pad first and see how long it takes for the tip to turn black again.

At this point your vehicle is 15 years old and it may just be normal carbon build up on the exhaust tip. Of course if you are unsure of the age of your O2 sensors, it couldn't hurt to replace them anyway. FWIW, Rock Auto had the best price on them when I replaced mine this past winter.
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Unread 06-05-2013, 03:52 PM   #4
Kingr98
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Thanks for the responses. As far as I know, the O2 sensors have not been changed. I bought the Jeep with 89K miles on it and have not had them changed.

I am actually getting pretty good mileage. On my last 500 mile round trip, I got around 21.5 mpg. Around town, I stay around 18 mpg.

My Cat is already rattling bad, so it is up next to replace. That will probably be a good time to replace the sensors since I will already be under there.
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Unread 06-05-2013, 07:06 PM   #5
coralman
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Check for cracks in the exhaust manifold.
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Unread 06-05-2013, 07:11 PM   #6
Kingr98
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OK Coralman, I am definitly no mechanic, but I can't figure out how cracks in the exhaust manifold cause black exhaust pipe tips?? That being said, I will check.
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Unread 06-05-2013, 07:20 PM   #7
Virmagicus
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The black on the inside is completely normal, it's carbon... Does your exhaust have an aftermarket tip or is it at an unusual angle? Do you notice any black smoke while driving?
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Unread 06-05-2013, 07:36 PM   #8
coralman
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Introduction of oxygen into the exhaust, read by the 02, as a lean condition, which causes the pcm to add fuel. Increased stank,lol out the exhaust pipe. Other than that what V said.
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Unread 06-05-2013, 11:44 PM   #9
tucsonzj
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after 20 years, I expect my exhaust to have some black in it. I think this is being over thought. You're burning gas, then after it is ignited is pushed out throw your exhaust.

If you have no codes, are still getting same MPG's as before and nothing is running unusual, there's not black smoke coming out of your exhaust, leave well enough alone. Just my .02
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