My 92 tracker has been getting progressively worse with its running rough in a narrow rpm band when the engine is cold. When I start driving with a cold engine and the rpm's get near 3k before a shift (seems worst going from 2nd to 3 rd) there is a very noticeable hesitation, and the engine starts running rough. It only does it once or twice and goes away as the engine warms.  I've replaced the fuel filter, new plugs, cap, rotor, wires, air filter and pulled the injector to clean it.  No change to the problem!  Runs great the entire rest of the time!  Any thoughts about the O2 sensor?
Thanks Joe

Sounds like a good possibility is it could be running lean during the warm-up period. How about checking the coolant temp or air temp sensors CLT/IAT? Both of these should be adding calculated amounts of fuel during warm-up.

You should be able to find the testing methods in the FSM.

My 92 tracker has been getting progressively worse with its running rough in a narrow rpm band when the engine is cold. When I start driving with a cold engine and the rpm's get near 3k before a shift (seems worst going from 2nd to 3 rd) there is a very noticeable hesitation, and the engine starts running rough. It only does it once or twice and goes away as the engine warms.  I've replaced the fuel filter, new plugs, cap, rotor, wires, air filter and pulled the injector to clean it.  No change to the problem!  Runs great the entire rest of the time!  Any thoughts about the O2 sensor?
Thanks Joe

Mine does the same thing ('95 16V), and I have recently replaced all the things you have, too (except the injectors). Which engine do you have?

- Andy

Any updates?

Although Im no expert but I highly doubt its the injectors. I would be suspicious on fuel delivery somewhere if the hesitation was consistent but you say its only on a cold engine.

I think the 2 keys here are "during warm-up" and "upper rpms".  That makes me want to focus on the computer not calculating amounts of fuel correctly based on sensors during that time.

Theres a possibility of the O2. It heats up it does give fuel input as well. I would try the other 2 sensors first.  Obviously I think an ohm meter check would be the best place to start.  With 2 of you, it sounds like a more common problem that others may benefit from reading in the archives though.