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  #1  
Old January 20th, 2010, 17:22
stephenspann27 stephenspann27 is offline
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can a bad o2 cause rough idle, but no other symptoms?

My '87 XJ has erratic idle.. it will idle normal for a few seconds, then low.. a misfire a few times, then idle back up.. over and over. I've done quite a bit of trouble shooting, and replacing of parts.

Rather than list all the things I've replace or checked.. I'd like to just focus on whether the O2 could cause this problem.

I tried Ohming out the contacts on the 'o2 sensors plug and they all three read "open" I've read that at least the ground to heater wire should read "closed." I also read that you can damage the o2 by ohming it out because you are sending voltage through it.

In spite of my weird idle.. I get great MPG.. and I never get a misfire during normal operation or acceleration.. no misfires ever.. except for idle...

I know a bad o2 could cause the idle issue I have.. but I would just think I'd have other problems also..

I also know that the o2 only comes into play during closed loop. when the engine is cold, and I crank it.. it idles perfect for the first 10-30 seconds.. then the idle issues starts (maybe around when it goes in to closed loop) However, if the engine is hot, it will misfire as soon as I start it.

Last edited by stephenspann27; January 20th, 2010 at 17:26.
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  #2  
Old January 20th, 2010, 17:26
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Re: can a bad o2 cause rough idle, but no other symptoms?

I'd think it works or not and not just act up at idle.
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  #3  
Old January 20th, 2010, 17:30
stephenspann27 stephenspann27 is offline
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Re: can a bad o2 cause rough idle, but no other symptoms?

One more thing.. IAC is of course suspect in this situation, I've tried a J-yard one, and I now have a new one installed. When I was checking grounds, i tried to ohm out all of the contacts on the IAC plug (harness side) and all of the contacts read "open" shouldn't there be a ground among the contacts in the IAC plug?
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Old January 20th, 2010, 17:31
stephenspann27 stephenspann27 is offline
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Re: can a bad o2 cause rough idle, but no other symptoms?

One more thing.. IAC is of course suspect in this situation, I've tried a J-yard one, and I now have a new one installed. When I was checking grounds, i tried to ohm out all of the contacts on the IAC plug (harness side) and all of the contacts read "open" shouldn't there be a ground among the contacts in the IAC plug?
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Old January 20th, 2010, 17:42
stephenspann27 stephenspann27 is offline
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Re: can a bad o2 cause rough idle, but no other symptoms?

If the heating element is bad in the o2.. would that only cause rough idle until the engine gets to operating temp?
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  #6  
Old January 20th, 2010, 17:46
stephenspann27 stephenspann27 is offline
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Re: can a bad o2 cause rough idle, but no other symptoms?

I'm reading that in the three wire style O2.. one wire is ground, the other is heating, and the 3rd is the signal wire.. so if my ground wire is bad.. perhaps its grounding through the exhaust manifold.. which is causing the "signal" to be off. I"m going to go out and double check the ground wire.. and maybe jump the ground..
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Old January 20th, 2010, 18:21
stephenspann27 stephenspann27 is offline
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Re: can a bad o2 cause rough idle, but no other symptoms?

I have to take back what I said before about the 'o2 ground in the harness being bad, it does test good. it is the center pin btw..

I have also confirmed that there is 13 volts going to the heat wire on the o2, and there is about 4 volts going to the sensor wire...

However, I completely unplugged the o2 and drove around.. and it ran exactly the same as when its plugged in.. I plugged it in while it was running and there was no change in idle..

With my multimeter prove in the back of the o2 sensor plug and while reving the engine, i can see the voltage change.. I assume this means the sensor is good?

Last edited by stephenspann27; January 20th, 2010 at 18:44.
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Old January 20th, 2010, 19:36
stephenspann27 stephenspann27 is offline
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Re: can a bad o2 cause rough idle, but no other symptoms?

Sorry for all of the posts.. trying to get all the info out there before I get replies.

With my multimeter probe shoved into the o2 sensor's plug, I saw voltages ranging from 4.77 down to 4.44. Most of the general posts I've read about testing o2 sensors say that you should see voltages around .45......

I read in an old post from EcoMike where the heating element in an o2 sensor was bad, and the o2 sensor was only getting hot enough at high RPM to produce a value that the ECM would accept.

Last edited by stephenspann27; January 20th, 2010 at 19:50.
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  #9  
Old January 20th, 2010, 20:09
stephenspann27 stephenspann27 is offline
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Re: can a bad o2 cause rough idle, but no other symptoms?

I also found this FAQ by EcoMike with a lot of great info in it. http://www.naxja.org/forum/showthrea...nsor+t esting

Shouldn't I be able to Ohm out the ground/heater pins on the o2 sensor connector? I believe all three of the connections read open..

I think the answer to the title of my thread is "yes, if the heater is bad" The question is.. how can I test the heater? I know the voltage and ground are there to run the heater..
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  #10  
Old January 20th, 2010, 20:25
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Re: can a bad o2 cause rough idle, but no other symptoms?

Quote:
Originally Posted by stephenspann27 View Post
My '87 XJ has erratic idle.. it will idle normal for a few seconds, then low.. a misfire a few times, then idle back up.. over and over. I've done quite a bit of trouble shooting, and replacing of parts.

Rather than list all the things I've replace or checked.. I'd like to just focus on whether the O2 could cause this problem.

I tried Ohming out the contacts on the 'o2 sensors plug and they all three read "open" I've read that at least the ground to heater wire should read "closed." I also read that you can damage the o2 by ohming it out because you are sending voltage through it.

In spite of my weird idle.. I get great MPG.. and I never get a misfire during normal operation or acceleration.. no misfires ever.. except for idle...

I know a bad o2 could cause the idle issue I have.. but I would just think I'd have other problems also..

I also know that the o2 only comes into play during closed loop. when the engine is cold, and I crank it.. it idles perfect for the first 10-30 seconds.. then the idle issues starts (maybe around when it goes in to closed loop) However, if the engine is hot, it will misfire as soon as I start it.
That is correct. One should be a ground for both circuits. The resistance at room temp across the heater (12 volt in, and ground) should be 8 ohms as I recall. You have a bad O2 sensor. And yes it can cause a rough idle, by switching the ecu from closed to open loop unexpectedly. It will also hurt gas mileage, and shorten life of cat converter.
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  #11  
Old January 20th, 2010, 20:28
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Re: can a bad o2 cause rough idle, but no other symptoms?

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Originally Posted by stephenspann27 View Post
If the heating element is bad in the o2.. would that only cause rough idle until the engine gets to operating temp?
No. There is not a high enough exhaust temp at idle to keep the O2 sensor in the required temp zone for it to work properly at an extended idle (say over 10-20 seconds), thus the need for the internal heater.
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  #12  
Old January 20th, 2010, 20:30
stephenspann27 stephenspann27 is offline
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Re: can a bad o2 cause rough idle, but no other symptoms?

I think i'm onto something then, I just verified that there is no connection between the center "ground" pin on the o2 and the "heater" pin on the o2..

i should be able to get a resistance value between those two pins if the heater was good right?
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  #13  
Old January 20th, 2010, 20:32
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Re: can a bad o2 cause rough idle, but no other symptoms?

Quote:
Originally Posted by stephenspann27 View Post
I have to take back what I said before about the 'o2 ground in the harness being bad, it does test good. it is the center pin btw..

I have also confirmed that there is 13 volts going to the heat wire on the o2, and there is about 4 volts going to the sensor wire...

However, I completely unplugged the o2 and drove around.. and it ran exactly the same as when its plugged in.. I plugged it in while it was running and there was no change in idle..

With my multimeter prove in the back of the o2 sensor plug and while reving the engine, i can see the voltage change.. I assume this means the sensor is good?
Sounds like you have proven that the idle problem is not caused by the O2 sensor system, but is elsewhere. As I said below though, the O2 sensor needs to be replaced as the heater is burned out if you do not get something close to 8 ohms.
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  #14  
Old January 20th, 2010, 20:34
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Re: can a bad o2 cause rough idle, but no other symptoms?

Quote:
Originally Posted by stephenspann27 View Post
Sorry for all of the posts.. trying to get all the info out there before I get replies.

With my multimeter probe shoved into the o2 sensor's plug, I saw voltages ranging from 4.77 down to 4.44. Most of the general posts I've read about testing o2 sensors say that you should see voltages around .45......

I read in an old post from EcoMike where the heating element in an o2 sensor was bad, and the o2 sensor was only getting hot enough at high RPM to produce a value that the ECM would accept.
For the Renix, it is different, not .45 volts. The Renix is 2.45 volts at 2000 rpm, +/- .2 volts for a good system. At idle it will oscillate from about 1-4 volts once a second, apx.
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Old January 20th, 2010, 20:38
stephenspann27 stephenspann27 is offline
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Re: can a bad o2 cause rough idle, but no other symptoms?

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Originally Posted by Ecomike View Post
Sounds like you have proven that the idle problem is not caused by the O2 sensor system, but is elsewhere. As I said below though, the O2 sensor needs to be replaced as the heater is burned out if you do not get something close to 8 ohms.
Well i think the idle problem could be caused by the o2 if the heater element is bad.. right?
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