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Discussion Starter #1
Brand new to the great Jeep site here and I need some help please. I did a "search" first, but I guess I am not using the right terms to get a hit on my question. Some background and I apologize for my long, first OP here....

My 03 Jeep GCL 4WD, Auto, Select-Trac with the 4.0 with 94K miles seems to be running rich (black sooty tailpipe) and the VIC showing the gas mileage has now dropped to 15.1 city/highway from it's 15.8 mpg over the passed year.

So I looked for possible reasons in my 2000 Jeep GC, 2.5 inch thick factory service manual. It stated that the forward O2 sensor on the exhaust header pipe, controls the fuel/air mix, with the PCM being the boss. (bought the manual when I had a 2000 WJ, so trying to make do as these FSM's are $$$$ )

Now the question. My FSM and all internet searches shows my Jeep with it's Federal emissions should have a total of two O2 sensors, one pre cat and one post cat.

But it does NOT have a post cat sensor, the Jeep only has one O2 sensor on the exhaust header pipe. There is NO opening on its still OEM cat outlet pipe that has been sealed over.

There is NO wiring plug or harness even close that I can find, that may have been stowed away or hidden that could have went to a post cat sensor. And NO CEL's are showing.

So do I have a deformed Jeep that slipped passed the factory's final inspections ? How does my Jeep know it is getting the correct fuel/air mix, with no outlet exhaust data ? I am :confused: Thanks, Greg
 

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The beast from Brazil
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Welcome to the garage.
This is indeed a strange duck in the pond. The pre-cat sensor should be able to control the fuel trim, but I have not heard before of this setup with only a pre-cat sensor.
It was working fine before, so I would check the O2 sensor with a scanner and see if the voltage changes frequently between 0.2V and 0.8V. Also check what the fuel trim is doing: the STFT (short term fuel trim) and LTFT (long term fuel trim). Normal fuel trim is a single digit figure. If it becomes above the +10% (STFT + LTFT added together), then there probably is something wrong. The O2 sensor can be the problem, but also a (too) low fuel pressure, a vacuum leak in the intake or a leaking exhaust header gasket.
 

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Discussion Starter #3
Thanks Frank for the good reply,:thumbsup:

I only have a cheap Actron OB2 scan tool that just reads CEL codes only. I am not a good enough mechanic to warrant paying $100 + for the better program scanners to get the millivoltage of the sensor you have suggested.

I looked at the header O2 sensor this morning and it appears to be an aftermarket replacement (wrong color plug). So while knowing the WJ's are picky about the brands of O2 sensors used on them, I think I will get a NGK unit and put on it .... and see what happens. I'll report back on the outcome.
 

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The beast from Brazil
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But was the sensor changed lately?
Regarding a scanner, if you have a smart phone, then you can buy a cheap bluetooth or (a bit more expensive) wifi dongle and download the free "Torque"
app. With this app you can see many live data and reset fault codes.
 

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Discussion Starter #5 (Edited)
After just crawling under my Jeep with GOOD lighting and having it on ramps lifted, I just discovered I had a senior moment when I started this OP.

Back when I checked for a post cat sensor, I did so with a flashlight and not lifting it, so I could not get directly under the exhaust manifolds. (I am not thin by any means at 6'2" /238 pounds) So viewing at an angle while cramped up against the frame rail, I just saw one sensor. :eek::eek:

So a correction to my OP : My Jeep's 4.0 engine has the California emission's. There are two O2 sensors on it, one on each exhaust pipe just below the exhaust manifold. My FSM O2 sensor diagram shows the Jeep only uses the two sensors on the exhaust pipes, with no post cat sensor.

My sincerest apologies Frank for sending you on a wild goose chase while trying to help solve an issue that I did not have. :eek: I still need to track down the running rich issue. I just have a flip cell phone so I cannot use APPS. I may have to just break over and buy the $$$$$ scanner and maybe a vacuum gauge to check to see if I am getting a bad cat or restricted exhaust. . Thanks,
 

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I think auto zone will loan you a scanner. Call to make sure because although I have an excellent memory I can only retain it for about five minutes. :lol:
 
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