Hard to start when cold

Engine Performance problem 6 cyl Two Wheel Drive Automatic 210000 miles

I've a 99 Eddie Bauer, 4.0l E code, 4x2. Bought it new. At about 190K is started to have a cold start problem. Engine cranks, catches but either barely idles and stops or revs really high and stops. After several attempts and using the accelator to rev the engine up/down, and working it it for maybe 2min, it will level out nicely, no misses, no rough idles, and will from that point on, start on command, just like new. This problem seems to appear after the engine is cooled down. The colder the engine and ambient temp, the worse it is and the longer it is before it corrects it is. Check engine code is always 171. Right bank too lean. I have changed both right side O2 sensors (have ones for left side but not installed), replaced MAF sensor, EGR vacuum control solenoid, tested/cleaned EGR, replaced EGR vacuum sensor, replaced plugs and wires, checked for leaking vacuum, and replaced engine coolant sensor (behind thermostat housing). I have read there is a temperature control sensor that feeds to the ECM (TCS) that could be the problem but cannot locate on engine. Anyway, am looking for ideas.

Again, this problem only happens when the engine is cold. I noticed it happening this past summer, but for a much shorter period of time with the warm weather. This winter it is unbearable! I am the original (new car) owner.

Thanks very much