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PeterB16 12-08-2011 01:25 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 03:36 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 04:05 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.

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