Intermittent P0171; seems to be temperature related.

My car occasionally throws a P0171 code. I have an ELM327 OBD-II dongle that I use with the Torque Pro app for Android to monitor conditions. The Mass Air Flow sensor has been replaced, and the flow rate seems to be in spec. The O2 sensors seemed to be responding normally, but I went ahead and replaced them, since they were 14 years old. I also replaced the fuel injectors, just in case they were clogged (they weren't). I have owned the car since new, and occasionally run a bottle of fuel system cleaner through the tank to hopefully minimize any potential for buildup or clogging in the fuel delivery system.

After observing my short-term and long-term fuel trims for a while, I have noticed that things are much better in warm weather than in cold weather. It was running lean last winter, then it ran fine all spring/summer/fall. Now that it's winter again, it's running lean again. Even in cold weather things get better after the car has warmed up for a while. It seems to be more tied to air temperature than engine temperature. The fuel trim does not drop as soon as the engine is warmed up, but will drop after a while once the engine bay has warmed sufficiently. The car also has a rough idle that is only present when cold, and will go away when warm.

This leads me to suspect a small vacuum leak that gets bigger when the materials are cold, but at higher temperature the thermal expansion closes (or reduces) the leak.

I have sprayed starter fluid or throttle-body/intake cleaner around the engine bay with the car running to see if I could find a vacuum leak. I did the same with steam from a household steamer. I also hooked up the steam machine to an intake tube (with the car off) to see if I could spot a leak somewhere. So far, I haven't been able to find anything.

If it is a vacuum leak, maybe I need a mechanic to do a real smoke test that will be more effective at finding the leak.

Or maybe it's something else that I haven't thought of.

What do you think?

Thanks for your advice!