Oxygen sensor replacement

I've been driving my 89 Chevy S Blazer without knowing they need an oxygen sensor every couple thousand miles. Ah, well.
4.3 engine. I finally found the sensor on the passenger side of the engine,
between spark plugs 2 and 4. Truck was bought about 120,000 miles and presently has 265,000 miles. Might be time for a replacement sensor.
Question: Is it better to change the sensor on a cold engine, or on a hot engine? And any practical tips from someone has done one?
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Christopher A. Young
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but should be changed at about 50,000 or 75,000 miles. Do it on a cold engine because it's much kinder to your hands and arms (no burns). The threads on the sensor are already coated wirh anti-seize so just remove the old one and install the new one. Also, DO NOT overtighten the sensor (if you do, you may strip the threads in the manifold or exhaust pipe).
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% There are two classes of pedestrians in these days of reckless motor traffic - the quick and the dead. ~ Lord Dewar 1933 ~
Climbing into a hot car is like buckling on a pistol. It is the great equalizer. ~ Henry G. Felsen 1964 ~
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Any clue where the oxy sensor is? I thought I found it on the side of the motor between spark plulgs 2 and 4, but that's more likely a coolant sensor. I've looked all up and down, and can't find the silly thing.
The problem is that the truck has 265,000 miles on it (4.3 engine) and runs rough and stalls at idle when cold. Maybe I should give more information?
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exhaust pipe connects to the manifold. Try looking on the driver's side but your vehicle may have it on either the driver's side, passenger side or both (some vehicles / engines use two O2 sensors). I'm not certain about the number of sensors or exact location but you'll probably find it easier and quicker if you look from underneath the vehicle. Just follow the exhaust up to the engine and it's gonna be on there somewhere.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% There are two classes of pedestrians in these days of reckless motor traffic - the quick and the dead. ~ Lord Dewar 1933 ~
Climbing into a hot car is like buckling on a pistol. It is the great equalizer. ~ Henry G. Felsen 1964 ~
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I looked, and a fellow from the auto parts place looked. The 89 is a transition year. Throttle port injection, so it's neither up (carb) nor down (cylidner injection).
The Haynes book said that under "runs rough at cold idle" to look for vacuum leaks, tighten down the sorta-carb and then PCV valve. And maybe EGR. I got the PCV at the auto parts today $2.99, and the parts guy showed me the EGR, which is not as much a PIA as a steering pump. Or an alternator bracket. Done both of those this year.
Hint: If you neglect to replace the small half inch long bolt on the back of your alternator, you will spend four hours or so replacing the broken alternator bracket. Yes, I know the Dodges you have worked on all your life don't have a back bolt. But Chevys do. DAMHIKT.
Thanks ever so much for the wisdom.
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I would actually do it on a 'slightly warm' engine, or use a torch to heat the area of the sensor. Doing it stone cold could damage the threads.
On this vehicle the sensor is located in the exhaust pipe beside the transmission.
Steve

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Odds are it's not the oxygen sensor.
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You might want to check your EGR valve, that sounds more like your problem than O2 sensor at least that's what mine was doing.

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Another online mechanic suggested EGR. Are they a total pain to change out? What's the process t here for changing one?
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Seems to be two rusty bolts, and a vacuum tube. Anyone else correct me, or share some wisdom I'm missing?
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