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Engine pops and sputters warm, ok with O2 sensor pulled


gutbusterman
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Ok guys. Last year in November, in stop and go traffic, my Pathfinder started popping and sputtering in the intake and wouldn't go over 10 mph. Idled fine, and even seemed to rev ok in neutral. But as soon as I put any load on it, it would pop and sputter. I disconnected the O2 sensor (because it was easiest) and it ran pretty good thought not 100%. I drove it occasionally through the winter like that, and the most noticeable thing was the exhaust was really stinky (probably cooking the cat). I finally had time in May to look into it, so I connected the sensor to see what happens and it ran perfect. So I left it and kept driving it. Everything seemed fine until today when the same thing happened!! It runs perfect cold, but as soon as it warms up a little (probably until closed loop) then it falls on it's face. Unplug the O2, and drives kinda normal. So, it is temperature related somehow. Yes, the light is on, always... I haven't pulled codes yet. Im not sold it's the O2 sensor.

Thoughts?

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Sounds like a bad O2 to me. If you want to be sure, though, there's a test procedure in the service manual for the O2 sensor. EF&EC section, looks like it starts around page 36. Like checking codes except you want mode 1, not mode 3. The O2 can only tell the computer "Rich" or "Lean," so when it's running closed loop, the computer adjusts the fuel mix to bounce back and forth between the two. In mode 1, the green LED flashes with the O2 sensor reading. If the light remains on or off when the truck should be running closed loop, the sensor is probably shot.

 

If the computer knows the O2 is bad (or unplugged) it'll just throw a code and run on open loop all the time. Sounds like that's what it did all winter. The trouble is when the sensor is wrong but the computer doesn't know it. The sensor fails showing lean, the computer adds fuel to try and change lean to rich, and when the sensor still reads lean, the computer just keeps adding fuel, so the engine runs like crap. Because the issue is intermittent, you might try checking the wiring between the plug and the sensor before condemning the sensor, on the off chance something is wrong there (which might explain why the issue is intermittent). The guy who fixed the exhaust on my '95 cut the O2 wires and spliced them back with some kinda hokey connector that didn't hold on for long.

 

If/when you do end up replacing the O2, I've read that it's easiest to do with the exhaust hot. I haven't tried that, but it sure was a PITA when I did it with the exhaust cold, so the next one I do I'm warming up first!

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