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  #1  
Old February 6th, 2019, 04:16
in_motion in_motion is offline
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P0171 and P0172; Lean and rich at the same time?

96 Country, 4.0, AW4, 2wd

I'm chasing down what I think is either a vacuum leak or exhaust restriction. When I push the throttle, I get sputtering and misfires unless I floor it above 2k rpm, then it will go. After I let off, we're back to limping along. When the CEL comes on, everything drives just fine, I just get ~10mpg.

I've done a visual/"wiggle" check on vacuum lines and haven't found anything amiss. The only two places that might have an issue are the line going into the heater valve (looks like a tape fix there) and the cruise control line going to the vac canister in the bumper (which I fixed with no issue till now).

I multimetered the CTS, MAP, and IAC and they all checked out. I also checked the TPS and it is getting power to pin 1 but no response from pin 2.

So besides getting a vacuum gauge to properly test for leaks and replacing the TPS, what else should I be looking for?

Thanks in advance!
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Old February 6th, 2019, 04:29
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Tim_MN Tim_MN is offline
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Re: P0171 and P0172; Lean and rich at the same time?

Suspect a faulty OBD sensor. Probably a faulty Coolant Temp Sensor, a faulty O2 Sensor, and the typical cracked exhaust manifold.


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Originally Posted by in_motion View Post
When I push the throttle, I get sputtering and misfires unless I floor it above 2k rpm, then it will go.
Typical CTS symptoms.

Quote:
Originally Posted by in_motion View Post
I just get ~10mpg.
Should make one suspect a faulty O2 sensor, damaged sensors wires, blown fuse, and/or a leaky exhaust manifold.


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Last edited by Tim_MN; February 12th, 2019 at 17:05.
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  #3  
Old February 6th, 2019, 06:28
in_motion in_motion is offline
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Re: P0171 and P0172; Lean and rich at the same time?

I did replace the post cat o2 sensor, which is how the CEL started turning off. Would the pre cat sensor cause poor fuel economy? Sorry for all the noob questions, this is my first foray into diagnosing engine electrical problems.
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Old February 6th, 2019, 07:33
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Re: P0171 and P0172; Lean and rich at the same time?

The rear O2 only monitors cat function. The front O2 monitors combustion and the PCM will adjust the fuel mixture based upon faulty data if the O2 sensor is malfunctioning or the exhaust manifold is leaking.

Be sure to buy NTK brand O2 sensors, Bosch O2's and Jeep 4.0L do not play well together.



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Last edited by Tim_MN; February 12th, 2019 at 17:05.
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  #5  
Old February 9th, 2019, 10:15
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Ecomike Ecomike is offline
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Re: P0171 and P0172; Lean and rich at the same time?

Quote:
Originally Posted by in_motion View Post
I did replace the post cat o2 sensor, which is how the CEL started turning off. Would the pre cat sensor cause poor fuel economy? Sorry for all the noob questions, this is my first foray into diagnosing engine electrical problems.
Check for an exhaust manifold leak/crack and test the O2 sensor and wiring, grounds and power feed, and output signal. Repair and replace what is bad.

Its bad O2 sensor data (front O2 sensor, not the post Cat sensor). Now you need to find out why. Damaged wires, bad O2 sensor, cracked exhaust, or all of the above?
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  #6  
Old February 11th, 2019, 06:13
sailorjim86 sailorjim86 is offline
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Re: P0171 and P0172; Lean and rich at the same time?

If your getting bad signal from the pre-cat O2 sensor (sensor itself or wiring from it to PCM) you definitely want to get on it. As said above, the post-cat O2 sensor only monitors for cat efficiency and will just throw the CEL. The pre-cat sensor is crucial to F/A ratio/fuel mapping and if it's bad you likely run way too rich, which will burn up/ruin the cat.
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Old February 12th, 2019, 04:54
in_motion in_motion is offline
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Re: P0171 and P0172; Lean and rich at the same time?

Awesome, thanks for the replies. I'll most likely just get a new upstream o2 and CTS. Can't test them currently as the whole front end of my motor is in pieces :-P
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Old February 12th, 2019, 07:16
CJR CJR is offline
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Re: P0171 and P0172; Lean and rich at the same time?

I'd also carefully check for cracks in the intake manifold. I bought a XJ that the PO had cleverly used aluminium epoxy to repair the intake manifold crack in order to sell the car. Eventually,after a lot of looking, my son and I finally found the "repaired crack" .

Best regards,

CJR
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Old February 12th, 2019, 16:38
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Re: P0171 and P0172; Lean and rich at the same time?

They can be tested, loose, indoors, even a new one before its installed can be tested. Check the online data tables for the temp and resistance spec for the CTS, and you tube or on right ups online on using a torch and butane and Digital volt ohm meter to test the O2 sensor.

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Originally Posted by in_motion View Post
Awesome, thanks for the replies. I'll most likely just get a new upstream o2 and CTS. Can't test them currently as the whole front end of my motor is in pieces :-P
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  #10  
Old March 8th, 2019, 07:07
in_motion in_motion is offline
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Re: P0171 and P0172; Lean and rich at the same time?

Just want to tie this one off - too many threads out there with no conclusions. Replaced the upstream o2 and CTS (yeah, at the same time. no time to do it all scientific like) and boom, back to normal. Runs smooth as butter, fuel economy is up to the mid-teens, and the only dash light that is on is the "low washer" light.

Thanks so much for all of the help!
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  #11  
Old March 8th, 2019, 09:02
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kastein kastein is offline
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Re: P0171 and P0172; Lean and rich at the same time?

Glad to hear it! I was about to pile on the "check for exhaust manifold cracks" party because I've had that cause your issue before, since air leaks into the exhaust between exhaust pulses and results in a spuriously high O2 content that the O2 sensor dutifully picks up (even if it's working fine! Many good O2 sensors have been replaced because of misdiagnoses from this) and the ECU decides it must need more fuel because there's still oxygen in the exhaust. It snowballs like that and you end up getting 8-10mpg and a P0170 or P0171.

Glad it was just the sensors though, for your sake.
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