Thanks for the warning re: living in an inspection State. I'm near the end of the road on the top of the North American Continent, and there is no vehicle emissions requirements here, other than noise pollution, lol, meaning--sans muffler.
Cold start emissions would affect sensor readings as per ECU commands as per lean/rich fuel and air, spark timing, right? I do not entirely grok the 'gain much', or 'less' points raised. I think you are correct as per the air box butterfly to blend, or not to blend is probably 'switched out' when warmed up. In the winter, on my model 17, 258, carbed, I'd tie down the flapper to maintain as much warm air I could get down the carb to prevent it from freezing up solid, i.e., caked internally, and externally with frost, and bits of ice.(only minus four right now). XJ is new to me, and am concerned that there will be raw fuel leak down into crankcase oil, or some O2, or some _____ other slow death malfunctions, that will stress out the engine, and implied, and stated components, lol, as determined by some French computer during the first few miles of driving from dead cold.
My intention is to dis the air/filter box, replace with a home built battery tray, reroute the air intake hose/filter through the cowling, but do not see an easy way to maintain an effective pre-heat system in that mod., in order to keep the OEM efficiency'balance' as determined by the springs, vacuum, and solid state engineers.