The other day my 152,000 miles Montero Ltd 3.8L started to seriously jerk when in idle and at low RPM's, until over around 1200 RPM after which it ran normally. This would only happen when transmission was engaged either in Reverse or Drive, no code for transmission issues. When the engine had no load, Neutral or Parking, the engine will not jerk or stutter, it would work normally. I took it to Autozone to check and Code P0431 came up, the dreaded catalytic Bank 2 issue. This code had come up once before, but along with a Small Vacuum Leak Code, that I forgot the code number, which usually means the Gas Cap or a rear O2 sensor or a small leak on a vacuum hose. I changed the rear O2 sensor for Bank 2, and that solved the issue. At that time there was no operational issue with the SUV, except for poor gas mileage.

Back to the new malfunction, since P0431 came up all by itself, and since it is not the first time, and given the mileage and the poor quality of fuel here (my wife fuels wherever is the cheapest and convenient; usually 87 Octane), I decided to empty the pre-cats, and depending on how the Monty operates, then go ahead and order a new Pre-Cat "Y" Assembly ($289.00 at Rock Auto). I installed back the "Y" with the Cats free flowing, I installed spacers for the rear O2 sensors, and reset the CEL.

I started the vehicle, started nicely, changed gears to Reverse, and it jerked and stalled. Did the same thing in Drive, with same results. I though that it might be the trans, but it was totally rebuilt some time ago by the same guy that does most racing trans here, and it has worked like a charm; trans fluid is clean, no bubbling, very little shavings (very normal) when I drained the fluid to be in the safe side; so, pretty much sure the issue is not the trans.

When the problem started at higher speeds you could tell the Monty was somewhat more sluggish than usual, and maybe the fuel mix was richer than it is supposed to be, as after awhile at Idle and in Neutral a slight exhaust gas smell could be perceived. After I installed the empty Cats back, and after starting the vehicle a few times I perceived the same smell, and at one time to start the vehicle I had to slightly depress the gas pedal as it appeared to be slightly flooded, meaning a rich fuel mix.

So, I no longer have a CEL on, as it was reset by me, and I cannot drive the Monty as it stalls when the engine is loaded. Which means I will probably not get a CEL for quite awhile. If I cannot fix tomorrow, I will take to a shop on Friday.

Might it be that the P0431 was correct, and that the rear or front O2 Bank 2 sensor needs to be replaced? Or, might it be something else? Any suggestions? Has anyone had this problem, and knows the solution?

 

Rear O2 sensor has absolutely no effect on drivability of the vehicle. The fuel mixture is controlled only by the front O2 sensors on both banks. Not sure why you decided to empty out the cats. All you had to do is to install the spacers on rear O2s and that's it.
As to the drivability issue - is it possible that you plugged front and rear O2 sensors incorrectly (swapped them)? If I recall correctly two sensor leads terminate at the same spot near transmission bracket. That would mess things up and completely confuse the PCM. Possibility #2 is an exhaust leak that is introducing air to the front O2s. That would result in PCM calling up for more fuel delivery and sub optimal performance.
The best way to go about this is to get (borrow) a scanner that can read live O2 data and see what is going on with your fuel trims. I can't stress enough - DO NOT REPLACE ANYTHING, UNLESS YOU CONFIRM IT IS BAD. That O2 sensor you replaced way back - most likely was totally OK.

 

Hunter, always very helpful...Thanks I took it to my trusted shop, but they are baffled by what it is doing. They believe it maybe the trans, but when the Montero trans goes bad the gear indicator starts to light up like a Xmas tree and starts to give warning codes; this is not happening at all. Also the trans works like a charm, once you put it in 1st gear (D) and are able to accelerate beyond the Montero idle issues the trans works like a charm. The shop is leaning towards a problem with the trans valve body, personally, I'm on the fence about that.

I emptied the cats as they were extremely dirty, and I wasn't going to loose much by doing that, since my plans were to order a new "Y" and get it over with. When I emptied the cats my suspicion was correct, it had so much deposits and build-up, plus metal from the "cleaning" elements were broken-off, which was the rattling that I was hearing.

As for installation of the O2 sensors, they were properly installed. To get the cat "Y" off I unplugged the Bank 1 and 2 rear sensors, and unplugged the Bank 2 front sensor. Bank 1 front sensor is not screwed to the cat "Y" it is screwed to an exhaust pipe that connects the Bank 1 manifold to the cat "Y". In order to take down the "Y" only the rear Bank 2 sensor has to be unscrewed, unless you rather unbolt the front driveshaft to have space; this is more work than unscrewing a sensor.

Bank 1 and 2 rear O2 sensors are color coded at the plug. Bank 1 has gray plug, while Bank 2 has a black plug. Then, the wire for the front O2 sensor of Bank 2 has a heatshield as it is routed right next to the Bank 2 manifold. In order to mess up what goes where you have to be a 5 year old. Also, the bracket for rear O2 plugs, located at the trans, has Bank 1 plug on the top, and Bank 2 plug below it.

As for leaks, that is one of the first things I did, check for any leaks, everything was tight and sealed properly.

Who knows it might have been the trans valve body all along, but I highly doubt it. It was replaced when the trans was rebuilt, there are no trans issues light coming up, and the only engine code that showed when the problem started was P0431.

Since, the CEL was reset, no codes are showing up, and since the problem cannot be duplicated when the engine has no load from the trans, it won't light back up at neutral. So, the shop was going to run it with the scanner connected to see if they could find the issue.

 

Baffling. It sounds like you have eliminated all engine related issues that could cause the jerking and stuttering. I guess that leaves the issue with transmission. It is possible that the valve body that was installed during transmission rebuild was defective. It is rear, but it happens. Running through the drive cycles with the scanner connected is a great idea. Hopefully something will show up. If not, it will confirm the issue is not with the engine, but with transmission.