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Unread 12-16-2013, 08:43 PM   #1
ShadowTek
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Question Can a bad oxygen sensor cause fouled plugs?

I finally got around to changing my spark plugs today. The plug tips for cylinders 1-3 were completly black, but dry. The plugs for cylinders 4-6 were not blackened at all and seemed show normal wear. I think the plugs were about 40 to 50 thousand miles old.

Earlier this year I had a bad oxygen sensor that I didn't get around to fixing for a few months. I don't remember which one threw the code, but I eventually replaced all four of them.

Could riding around with a bad oxygen sensor have caused the fouling that I see with those plugs?

If not, what else could be affecting cylinders 1-3, but not 4-6?

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Unread 12-16-2013, 09:47 PM   #2
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o2 sensor, I think
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Unread 12-17-2013, 08:36 AM   #3
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Yes and no A downstream o2 could get a bad reading from a bad CAT. This would cause the car to run Rick/lean. Which leads to decreasing gas mileage, poor running, and fouled plugs. You will need to see if the cat is bad. look into having it checked out.
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Unread 12-18-2013, 12:28 PM   #4
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You're saying that the downstream sensor causes an altered air/fuel ratio, or that the clogged cat causes an altered AFR?

I didn't think that the downstream sensor affected mixture, only the upstream sensor?

Here are the plugs. From left to right, cylinders 6,5,4,3,2,1.

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Unread 12-18-2013, 01:01 PM   #5
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To my knowledge bad cat makes down steam o2 read wrong.
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Unread 12-18-2013, 01:07 PM   #6
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To my knowledge bad cat makes down steam o2 read wrong.
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Unread 12-18-2013, 02:21 PM   #7
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If you can, try swapping the o2 sensors from one bank to the other, see if the problem follows.
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Unread 12-18-2013, 02:23 PM   #8
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Or it could be bad injectors, try swapping #2 and #5.
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Unread 12-18-2013, 05:38 PM   #9
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Quote:
Originally Posted by ShadowTek View Post
You're saying that the downstream sensor causes an altered air/fuel ratio, or that the clogged cat causes an altered AFR?

I didn't think that the downstream sensor affected mixture, only the upstream sensor?

Here are the plugs. From left to right, cylinders 6,5,4,3,2,1.
Downstream O2 sensors in WJs can definitely alter the A/F ratio. You can read about it in the image of this page from the FSM.

http://i642.photobucket.com/albums/u.../O2sensors.jpg
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Unread 12-18-2013, 06:20 PM   #10
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That image won't load. Too many scripts.

I'll assume you're referring to the o2 sensor description page.

So if the purpose of the upstream sensor is to determine the "correct" air/fuel ratio, then the downstream sensor will force a different AFR to get the desired emmisions effect?

What happens if you unplug the downstream sensors? Can the upstream sensors not control the poper AFR without the downstream sensors?

This just seems like needless complexity.
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Unread 12-18-2013, 06:51 PM   #11
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Quote:
Originally Posted by jeep-drivin-YZ View Post
Yes and no A downstream o2 could get a bad reading from a bad CAT. This would cause the car to run Rick/lean. Which leads to decreasing gas mileage, poor running, and fouled plugs. You will need to see if the cat is bad. look into having it checked out.
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To my knowledge bad cat makes down steam o2 read wrong.
A failing O2 sensor will also make the O2 sensor read wrong. OP's problem doesn't point to the cat unless it's accompanied by a check engine light throwing P0432, cat efficiency bank 2. If a bad cat were throwing the readings off enough to foul the plugs that badly, there is no way it would not trip the engine light.

Quote:
Originally Posted by PA55 View Post
If you can, try swapping the o2 sensors from one bank to the other, see if the problem follows.
OP stated he already replaced all four O2 sensors.

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Originally Posted by PA55 View Post
Or it could be bad injectors, try swapping #2 and #5.
One injector per cylinder. The #5 injector cannot possibly foul the spark plugs in cylinders 4 and 6.
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Unread 12-18-2013, 07:02 PM   #12
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Originally Posted by ShadowTek View Post
I finally got around to changing my spark plugs today. The plug tips for cylinders 1-3 were completly black, but dry. The plugs for cylinders 4-6 were not blackened at all and seemed show normal wear. I think the plugs were about 40 to 50 thousand miles old.

Earlier this year I had a bad oxygen sensor that I didn't get around to fixing for a few months. I don't remember which one threw the code, but I eventually replaced all four of them.

Could riding around with a bad oxygen sensor have caused the fouling that I see with those plugs?

If not, what else could be affecting cylinders 1-3, but not 4-6?
To answer your original question here, yes. A bad O2 sensor is exactly what would throw off the A/F ratio on those three cylinders, causing the plugs to foul.

Disconnect your battery for an hour or so to reset the computer. The Jeep has been running with three fouled plugs and the computer has been compensating for that over time. Resetting the computer will erase those adjusted parameters and go back to original programming, so you won't get uneven plug wear or other issues from computer compensation.

When you hook the battery back up and start it, turn the key to on, not run, for three seconds to prime the fuel system, then turn it to start. Don't touch the gas pedal until the Jeep has idled for at least ten seconds. The computer learns all of its long term idle parameters from that first start up, and pressing the gas to get the engine to turn over badly skews the programming, usually causing stalls or rough idle.

I would pull the plugs again in a few thousand miles just to make sure they all appear consistent and aren't fouling.
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Unread 12-18-2013, 07:21 PM   #13
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Quote:
Originally Posted by ShadowTek View Post
That image won't load. Too many scripts.

I'll assume you're referring to the o2 sensor description page.

So if the purpose of the upstream sensor is to determine the "correct" air/fuel ratio, then the downstream sensor will force a different AFR to get the desired emmisions effect?

What happens if you unplug the downstream sensors? Can the upstream sensors not control the poper AFR without the downstream sensors?

This just seems like needless complexity.
Yes, that is an image of the O2 sensor page. WJs and other modern vehicles using multiple catalytic converters and multiple O2 sensors are engineered to meet the very clean emissions standards in California. The downstream O2 sensors allow fine tuning the A/F ratios so that converter outputs are extra clean even if the converters are not working at peak efficiency. The complexity may seem needless, but apparently it is necessary to meet government regulations.

Of course the success of this clean emissions scheme depends on having O2 sensors and catalytic converters that work reasonably well although not necessarily perfectly.
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Unread 12-18-2013, 07:31 PM   #14
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Originally Posted by the88thpianoman View Post
Disconnect your battery for an hour or so to reset the computer. The Jeep has been running with three fouled plugs and the computer has been compensating for that over time. Resetting the computer will erase those adjusted parameters and go back to original programming, so you won't get uneven plug wear or other issues from computer compensation.

When you hook the battery back up and start it, turn the key to on, not run, for three seconds to prime the fuel system, then turn it to start. Don't touch the gas pedal until the Jeep has idled for at least ten seconds. The computer learns all of its long term idle parameters from that first start up, and pressing the gas to get the engine to turn over badly skews the programming, usually causing stalls or rough idle.
lol I'm already there. I disconected the battery while I changed the plugs, I always wait a few seconds before I start until I hear the fuel pump quit, and I always let the car idle for about 10 seconds before I give it any gas so that things can warm up a little.

I guess my compulsions have some value after all.

Thanks.
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Unread 12-19-2013, 01:12 AM   #15
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Just because he changed o2 sensors doesn't mean that they all work, one faulty sensor would cause this, a wire may be shorting on the exhaust or even a plug not connected properly also it's not unknown for new sensors to be bad.

The reason I said swap only #2 and #5 injectors was to rule out faulty injectors he might have 3 bad injectors. If swapping #2 moves the fouling to #5 then he knows for sure that it is the injectors at fault.

If none of this has any effect then look at a faulty cat.

Jim
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