I am putting a new-to-me daily beater on the road that I picked up recently - 2006 Toyota Camry 2.4 with 285k miles (automatic). I have spent my time going through the worn-out original suspension on it and frankly have not driven it around much. There was a P0301 code (cylinder misfire # 3) when I got it and no check engine light. One time when I was driving it around the block I did get a CEL and an actual misfire. Plugs and coils were changed by the PO. I did put a valve cover gasket it on it after that to address a pretty good oil leak. I took it on its first drive today (about 50 miles round trip), no check engine light and it drove good but gas consumption seemed a little higher than I expected (qualitatively, I didn't check actual MPG). I plugged my OBD tool it, had a pending code for P0301. I observed fuel trims at idle and driving and it drifts nicely between +1.5-4.5% short term fuel trim without any instantaneous changes with sudden throttle changes. The LTFT is pegged at +25% explaining the higher than expected fuel consumption. I have to say this is one of the first cars I have worked on in a long time that had perfect STFT and bad LTFT. I cleared the codes to reset fuel trims, but the LTFT sticks at max instead of going to zero and building back up. I watched the voltage on sensor 2, and it is 0.1 at idle but goes up to around 0.5 while driving at speed. I have yet to smoke test it, will do tomorrow. At a little loss as to what combination of problems could potentially render perfect STFT and atrocious LTFT.
I'd look for a vacuum leak. Perhaps intake manifold.
Maybe somewhere else
In your shoes I'd "smoke test" this car
But understand, you have to resolve that P0301 code before doing anything
I smoke tested it today, no leaks from anything. I have an App-based OBD2 monitor so I watched it while driving. First and foremost, the P0301 never returned in the 150 miles I drove it from last night to tonight. The STFT was in the perfect range most all the time, except for an occasional hiccup I saw where the STFT went to around 20% (and the LTFT went to 0%) for a few seconds while driving then went back to it's "normal". Fuel economy is better than I thought, App shows it averaging around 28MPG. I am wondering if there is a chaffed wire somewhere, or front 02 bad and it is actually running lean (but I would expect that it would be "knocking" if the engine was running that lean) or the rear o2 is bad and out of range. I will have to pick the car up on my lift and check everything out this weekend.