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PeterB16 12-08-2011 12:25 PM

O2 sensor bad?
Can an O2 sensor cause rough idle and hesitation?
I have been fighting a rough idle issue and just the other day threw a code P0432 (cat efficiency below threshold).
I reset computer and all was fine. Light went off, engine ran flawlessly.
Today is next day and rough idle is starting back and this afternoon (32F outside) there was hesitation when I quickly tap throttle at idle. Engine seems like it is missing under load, but no code.
Also, I am getting some sh*tty gas mileage. Averaging 270 miles per tank. 13.1MPG, mostly HWY mileage. 30 out of 40 of my daily miles are highway.

2004 Overland, 4.7L HO
75k Miles
New plugs (500miles)

01grand 12-08-2011 02:36 PM

Re: O2 sensor bad?
I see you say plugs are new, that was going to be my first suggestion. My next suggestion would be to check the air filter and change it if necessary. If that is good I would pull the intake tube and resonator box off the throttle body and give it a good cleaning. Wipe off the throttle plate and bore, make sure to open it by hand and clean the edges and back side. If it is really dirty I would recommend removing the throttle body completely and clean it thoroughly.

Also, remove the idle air contoller from the side of the throttle body and carefully clean off the end, making sure not to twist it. Also clean the idle air passage on the TB.

If an O2 sensor is really bad and sending a goofy fuel trim signal to the computer I think it could cause a rough idle and heasitation, and definitely poor fuel economy, but I'm not entirely sure on the idle part.

Frango100 12-08-2011 03:05 PM

Re: O2 sensor bad?
O2 sensors are only used when the fuel control system is in closed loop configuration. In closed loop, the O2 sensors give a feedback to the PCM and in that way the PCM can finetune the fuel injection, so that the 14.7 to 1 air/fuel mixture is achieved. When in idle for prolonged times, the system will also go into closed loop, but i won´t expect the engine running rough because of this.
Cleaning the TB and especially the Idle Air Control stepper motor, can solve rough idle issues, however normally does nothing for fuel economy.
It could be that a Cat is partly blocked, creating a higher back pressure and less flow. Could explain the bad gas milage as well. Also a low fuel pressure gives you a bad gas milage. Maybe you could measure the pressure at the rail to be sure thats ok.
If you have an OBDII scanner which is showing the O2 sensors voltages, you can see more or less if they work ok or not. The voltage of the 1/1 and 2/1 sensors should be fluctuating and NOT be at a steady voltage.

coronacz 04-17-2017 02:29 PM

Re: O2 sensor bad?
Hello and Happy Easter!
I am new in here, but not new to the Jeeps.
Couple days ago I bought 1999 WJ, 4.7, it has 186k miles on the clock.

Starts fine, runs cold and hot quite fine, around 600RPM +- It is gas powered (benzin) and also natural gas (LPG, propane, ...).

It has followning issue:
It takes 20liters/100km equals to cca 14 mpg. I used to have 2002 4.7 HO and it took around 15 l/100 km = 19mpg driving the same style. Something is definitely wrong there. I would like to do some brainsotrming what it can be.

I have BT OBD2 adaptor + Torque on my phone and I figure out following:
When I turn on key, engine off, the readings form both O2 sensors (federal) are close to 1, is it ok, that I see O2 readings as 1/1 and 1/2 in Torque? STFT at 0.

When I turn the engine on, O2 1/1 goes immediately down to 0 and stays there, STFT goes above the roof and stays there. I suspect this is my bad mileage. O2 1/2 (post-cat) goes slowly down to 0. Pretty much all the time O2 1/1 stays close to 0 and 02 1/2 sometimes goes up, but totaly randomly...
Once I shut the engine off both goes slowly up to close to 1.
DTC says P0171, if I delete the code, it takes some driving to get it back approx. 20 kms.

Do any of you have an idea what can go wrong there?
Regaring the diagnostic I have only very limited possibilities because I live and work as expat in Serbia. I do not know much Serbian and I do not know any Jeep friendly garage here yet.

So If you have any ideas I will be pleased if you can share.

Thank you.


Frango100 04-17-2017 03:36 PM

Re: O2 sensor bad?
Welcome to the garage Karel.
Yes, it seems that your 1-1 O2 sensor is shot. The P0171 code means that you have a lean mixture. The PCM will compensate for that and first will increase the STFT and thereafter increase the LTFT. The PCM should try to keep the STFT as close to 0 as possible, by increasing or decreasing the LTFT.
Your jeep only has the 1-1 (pre-cat) and 1-2 sensors (after cat)? I would start checking the O2 sensor wiring and if ok, change the 1-1 sensor.
A lean mixture can be caused by a too low fuel pressure as well, so if you have the ability to measure this pressure at the fuel rail, you could rule this possibility out. It should read approx. 49 PSI. I had once a bad gas mileage and this improved a lot after changing the fuel filter/pressure regulator. I didn´t had the fuel pressure tester at that time, so don´t know what the actual pressure was. I didn´t get the lean code at least.
A bit more then a year ago I did get this same lean code, but I also had very low power output from the engine. I only had about 15 PSI of fuel pressure and needed to change the fuel pump.
My jeep just shut down last Saturday night, and this morning I found an almost 0 PSI fuel pressure at the rail, but the pump runs normally. Tomorrow I will see if the problem is the pump (again)or the filter/pressure regulator.

coronacz 04-17-2017 04:39 PM

Re: O2 sensor bad?
Thank you very much Frango.

Well I aslo considered fuel pressuere, but since the vehicle is also LPG (propane) fuelled (and its filter looks new and LPG injectors are almost new) I suppose it would have been very unlikely to have problem with pressure on both systems (gas and LPG). Therefore I do not suspect the problem in fuel pressure. But I have not got done any test.

Unfortunatelly I can not see LTFT on Torque (I only run Lite version, do you know if it is available in Pro? ), But definitely my STFT is above 30 all the time (except WOT).

According to the buildsheet my WJ is the one of Federal Emmision, I only found 1 O2 sensor befor cat and 1 O2 sensor after cat, in Torque theirs signal are shown as 01 1x1 and O2 1x2, is this correct?

I am not sure what I can check on wiring of O2 Sensors. I already checked and sprayed connection to the harness undeneath the car. But I do not know where it leads then. I mean where it is connected to PCM. Also, because of LPG, there is output signal from O2 1/1 fed also to LPG control unit. The wire of O2 1x1 sensor is stripped and led to LPG control unit.

Strange thing happened today. Afternoon, I drove it for some 20 kms and during that time everything seemed to work ok, the O2 signals, consumption, performance. Then I arrived to local offroad place, had some small talk there and ever since the problem persists once again... Strange...

Good luck Frango with your issue, please keep us posted.

Frango100 04-17-2017 05:19 PM

Re: O2 sensor bad?
Since the O2 sensor wire is stripped to be able to feed the LPG control unit, I would have a look at this point in the wire. Since the voltage is at 0V, there could be a short in between the wires or eventually to ground. Also look for shaved spots and/or melted isolation due to contact with hot exhaust parts. Since your problem now is intermittent, I won´t expect the O2 sensor itself to be the problem, but yes a wire problem.
And yes, the before cat sensor is called the 1-1 and the after cat sensor the 1-2.

coronacz 04-17-2017 05:39 PM

Re: O2 sensor bad?
Thanks for the answer.
I checked the wire to the LPG CU. It seems just to be just connected to O2 signal wire and then it leads to LPG CU with no interuption.
What else could possibly make the signal of O2 go to constant 0 after firing up the engine?

What makes me thing about the O2 sensor failure is the fact that it shows 1 when engine is stopped. When I turn it on it goes immediately to 0 and when I shout it off again it takes some time to go to 1.
What signal is there supposed to be on O2 sensor when engine is not running (even for a long time)?
0 (and Lean) means there is too much of 02 in the exhaust fumes? Right?
Then I do not understand why it shows 1 when engine is not running and the signal goes slowly back to 1 after I shut it down.

coronacz 05-29-2017 04:49 AM

Re: O2 sensor bad?
So, I am back with an update.
I replaced the precat O2 sensor, now the graphs and numbers from the OBD2 seem to be OK, LTFT around 0 or slight negative, less than 10% all the time. Consumption got better (around 18 liters of LPG/100km), but still it is now what I expect.
The engine still takes quite lot also when going down hill on road (engine breaking) - instant shows arround 5 liters or more. It used to show 1 - 2 on my previous 4,7 HO.
Can the clogged cat converter be a problem now? Can it got clogged running too rich when O2 was sending missleading info to the PCM and mix was too rich?

Frango100 05-29-2017 06:47 AM

Re: O2 sensor bad?
The cat will burn off excessive fuel vapors in the exhaust gasses, so if the mixture is too rich, the cat will burn it off. This can cause overheating of the cat and a meltdown of its internals, creating a restriction to the flow. To rule this out, an exhaust back pressure test would be necessary. See here on how to perform this: Exhaust back-pressure testing -

coronacz 05-29-2017 12:52 PM

Re: O2 sensor bad?
Thank you for the reply. Unfortunately I do not have a chance to do such test. What is the price of cheap cat in USA?
I can get this:
kat. benzín do 5000 ccm, keramická vlo?ka, vnit?. pr?m?r 58 mm (plochý)
for approx 70 USD in here.

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