Yep, it is a matter of balancing competing issues or forces, that we optimise to minimize the bad, and maximize the good. At some point, temp wise, too cold lets water build up in the oil, leading to damage to the oil and to the metal. At another extreme the oil and metal is damaged by high temps. The older engines do better with 180 F. The newest OBD-II seem to prefer 190 F. In fact some of the OBD-II hardware kills gas mileage at lower temps, while the Renix is OK at 160 F.
I have noticed that even the Renix gets a little better gas mileage at 220 F than 200 F, or 200 versus 180 F, apparently due to the hotter intake manifold thinning out the air at letting the engine run leaner per stroke at idle and cruise. But I have also confirmed that Renix will go closed loop at 100 F while OBD-II will NOT!
Point #2, is that the answer (optimal temp) does vary with use of dyno oil versus synthetic, and Renix, versus OBD-II controls. One can not make universal cart blanche statements, unless you want to get nailed here by old timers who know better.:scottm:
IIRC, someone a while back confirmed that OBD-II XJs do not like 160 F T-stats, or a lack of a good, minimal CTS temp reading at the ECU in that range of 180 F or higher, IIRC. Seems they stay open loop at much higher temps than renix before going closed loop, but I don't know the exact critical temp, may be in the 96 FSM.
I have ran 180* stats for about 10yrs now (been driving for 40yrs!). Always past smog for me. But, then again, I always replace my cats, keep an oil change along with a good tune up.
