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  #25  
Old 04-12-2011, 05:32 PM
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Re: Oxygen Sensors and Their Role in Engine Management--A Primer

Hi guys, just a quick chime-in, most of the slow response codes dont just come from a lack of heat issue, but more overly so, due to a leak in the weld joint of the cats. Slow responsees ar caused by air leaks. Back flow air through the exhaust, and be sure to temporarily plug weep holes, use a soapy water mixture around the welds to find the leak. that is the first point I'd like to bring out. As for the cat efficiency codes, the downstream O2 sensors in sit to low in the flow and need to raised up slightly to correct the stoichiometric readings. This is a small correction, that usually requires the welding of a new O2 sensor bung.
As for throwing all 4 O2 sensor codes, that is generally an indication of a failed high-side driver in the PCM.

FWIW & IMOHO, CMR tuning can make some corrections, but generally just turn off the downstreams.


MM
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  #26  
Old 04-12-2011, 05:44 PM
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Re: Oxygen Sensors and Their Role in Engine Management--A Primer

^And the Magnumman strikes again!


He diagnosed the same issue with my vehicle.
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  #27  
Old 04-12-2011, 05:49 PM
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Re: Oxygen Sensors and Their Role in Engine Management--A Primer

great info MM +rep!
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  #28  
Old 04-16-2011, 06:55 PM
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Re: Oxygen Sensors and Their Role in Engine Management--A Primer

Quote:
Originally Posted by magnumman426 View Post
Hi guys, just a quick chime-in, most of the slow response codes dont just come from a lack of heat issue, but more overly so, due to a leak in the weld joint of the cats. Slow responsees ar caused by air leaks. Back flow air through the exhaust, and be sure to temporarily plug weep holes, use a soapy water mixture around the welds to find the leak. that is the first point I'd like to bring out. As for the cat efficiency codes, the downstream O2 sensors in sit to low in the flow and need to raised up slightly to correct the stoichiometric readings. This is a small correction, that usually requires the welding of a new O2 sensor bung.
As for throwing all 4 O2 sensor codes, that is generally an indication of a failed high-side driver in the PCM.

FWIW & IMOHO, CMR tuning can make some corrections, but generally just turn off the downstreams.


MM
I found a TSB for the '08's on this: http://www.wkjeeps.com/tsb/tsb_wk_2300108a.pdf .

This correlates with Service Manager's suspicion that the acceptable limits for O2 performance need to opened up. Noise spikes would be interpreted as outside limits. He needs the Jeep to check whether or not it has been done. Can't do it from records.

The symptoms in the TSB seem to replicate those in my HEMI.

Downstream O2 sensors are tucked up out of the air flow and are on top of exhaust pipe. Both manifolds, both cats, and all 4 O2 sensors are new. Exhaust system was leak-tested.
[I thought that downstream sensors evaluated cat performance, not A/F ratio. That is the function of the upstream sensors.]

Symptoms persist.
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  #29  
Old 08-20-2011, 03:05 PM
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Re: Oxygen Sensors and Their Role in Engine Management--A Primer

Does bad O2 sensors cause slight bucking/surging in the rpm?
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  #30  
Old 08-20-2011, 06:27 PM
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Re: Oxygen Sensors and Their Role in Engine Management--A Primer

Possibly! Normally, when an 02 sensor fails, it tends to make the engine run richer, adversely affecting economy and performance to a substantial degree. Idle and acceleration quality will also deteriorate. One thing for sure, if you suspect they're bad, change them. If you continue to run with bad 02 sensors, you put the catalytic converter(s) at risk of failure too. Then you're really into some $$$. How many miles are on your 3.7? Has any maintenance or mods. recently been done? Have any of the other sensors/valves ever been changed....crank position, cam position, MAP sensor, EGR Valve, PCV Valve? Can you describe the symptoms.....speed/rpm range(s), engine hot/cold, accelerating/constant speed, etc.? When was the last complete tune up? When was the transmission last serviced? And finally, any codes?
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  #31  
Old 08-20-2011, 06:58 PM
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Re: Oxygen Sensors and Their Role in Engine Management--A Primer

All maintenance is up to date. I changed the crank sensor today. I had terrible misfiring issue and the computer told me that those misfire codes were false. I also had the crank sensor code.

But....long before the codes, my Jeep has also had a slight buck/surge around 40mph @ 1500 to 2000 rpm.
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  #32  
Old 11-25-2011, 02:51 PM
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Re: Oxygen Sensors and Their Role in Engine Management--A Primer

Quote:
Originally Posted by lll2for3lll View Post
All maintenance is up to date. I changed the crank sensor today. I had terrible misfiring issue and the computer told me that those misfire codes were false. I also had the crank sensor code.

But....long before the codes, my Jeep has also had a slight buck/surge around 40mph @ 1500 to 2000 rpm.
Mine has the same bucking feeling...ironically it is the same year and engine size...you ever figure out the problem?
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  #33  
Old 12-13-2011, 10:02 PM
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Re: Oxygen Sensors and Their Role in Engine Management--A Primer

I've been off on another trip, so haven't checked in lately. lll2for3lll, did you get your issue resolved? If not, you may want to look at the EGR Valve if you're having a low speed surge. This is a symptom on the 5.7's (probably the 3.7's and 4.7's too) when the EGR Valve is going bad. Also, did you change the cam position sensor too, or just the crank position sensor?

Earlier exhaust leaks were discussed in this thread. I now have first hand experience with them. On the 2000+ mile trip I just got back from, I drove about 3/4 of it with a pretty good exhaust leak at the header flange of the No. 1 cylinder. I first knew something wasn't right when the MDS started working erratically, didn't stay on as much as it normally did, and sort of "popped" like a cork every time it engaged. Also, the fuel economy started deteriorating. I didn't get any codes throughout either. IMO, what happens is additional oxygen is introduced to the exhaust flow from the leak point and the O2 sensor picks this up as a lean condition and adds fuel to the entire (driver's side in this case) cylinder bank. Long and short term fuel trim are also noticeably different between the right and left cylinder banks. I figure I lost anywhere from ~2-4 mpg in economy, which really irritated me, since it was the first time I've had my WK down to sea level.
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  #34  
Old 12-14-2011, 07:36 PM
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Re: Oxygen Sensors and Their Role in Engine Management--A Primer

Fuel trims between banks is a great way to pick up on a possible mechanical issue.
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  #35  
Old 01-11-2012, 09:35 AM
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Re: Oxygen Sensors and Their Role in Engine Management--A Primer

Wow great write up. I guess i fall right in to the category to change out my sensors.
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Old 01-27-2012, 07:15 PM
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Re: Oxygen Sensors and Their Role in Engine Management--A Primer

I picked up a $20 ELM327 OBD2-Bluetooth adapter. I loaded up some apps on my Droid2 Android phone and can now see the values you mention. Torque has nice graphs and has a great widget for showing DTC test status (current and since reset). This is a handy way to see what tests have or haven't been performed (useful for emissions-related codes).

Thanks for explaining which ones are upstream vs downstream. From some limited testing this afternoon I noted the same sort of wild fluctuations with the upstream sensors and relatively stable downstream. Just how much fluctuation or what values should I be seeing on the upstream ones?

I've not had any DTC codes for the O2 sensors. I've got 61k on it so I'm probably due for replacement. But I'd like to know just what values I should see during regular operation.

I drive it like I'm fleeing a back robbery so I've never expected mileage to be great, but it has been taunting me with 10-11mpg on the EVIC and that seems lower than it ought to be, even with Bullit-like driving habits. So just what other values should I be examining to determine low mileage causes?
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