(OP, if you're not cool with this off-topic pissing contest, let me know, and I'll delete my posts; I mean you and your thread no disrespect by entertaining this "discussion")
Roughly 195-210 not only because of the engine design itself but for a simpler, more fundamental reason--engine oil. Oil's optimal temp is a wider range, but 200 generally falls very comfortably in the middle of that range.
Run an engine too cool or slow it's warm-up too much can foul plugs, lower MPG, increase harmful emissions, and sustain more carbon build-up.
Again, regardless of all this, what's even the point of a 180? It's just a "door." It's open or closed. When it's open, it's just as open as a 195, and the 4.0 will still warm-up to about 200 no matter which "door" the coolant is passing through or what temp that door happens to open. The actual heat exchange is at the radiator. A bigger radiator and/or more airflow through it and/or more coolant flow are ways to alter how much and how fast heat is exchanged (i.e. lowering/raising the temp).
Given the attitude I can see brewing and that this IS an off-topic discussion, this is my last post on the subject in this thread.
To the OP, anybody that tells u that a 180 stat will not
yield lower coolant temps than a 195 stat, is flat wrong. A 180 stat will
yield lower coolant temps +/-95% (or most
) of the time. There are variables that come into play that determine minimum/maximum coolant temp.
Presuming we eliminate owner neglect and a cooling system that is in less than top notch condition, things like ambient temp, air flow, driving conditions, etc. are some of the variables. Let's take cold weather for a example of a variable. In cold weather, running the heater and using a 195 stat, actual coolant temp could be at little under 195 but a 195 stat is trying it's best to maintain 195 by remaining closed. Same scenario but running a 180 stat.......actual coolant temp will most likely be at or maybe even a little below 180. Again, a 180 stat is trying it's best to maintain 180 by remaining closed. Remember, a heater core is a small radiator and sheds lots of heat (if it's unplugged of course) so the stat may not need to open or opens very little and for only a moment.
There's another subject.....how the stat opens. The door example is a good one but the explanation falls short. The door (stat) has a infinite number of open positions between closed and wide open. The stat (door) also open/closes very very slowly. It virtually hovers between the closed/wide open positions, again in an infinite number of positions, depending on all of the variables present at whatever given moment/driving condition. Most stats are closed, or just ever so slightly beginning to open, at their rated number and most stats are fully open +/-15 degrees above their rated open temp. Again, a stat opens/closes very slowly and has an infinite number of open positions between full closed and wide open. One other little detail about a stat.....when wide open, it will allow more coolant to flow than the water pump can pump. It is not a restriction when wide open.
Let's not cloud up coolant temp scenarios with oil temps and fouled plugs but rather stick to the subject.....coolant temps. With regard to "what's the point of a 180" ask some performance guru's about the benefits of slightly lower coolants which usually allow for lower under hood air temps as well.
I don't believe that anywhere have I recommended/suggested using a lower temp stat. I simply said that the statement - "a 180 stat will make it harder for the computer to do it's job, they don't help cooling at all and Your engine will still reach the same operating temp with a 180 stat (vs a 195)" from posts #3 and #10 are false and misleading +/-95% of the time.
Oh yes, I touched briefly on cold operating conditions/variables so let me say just bit on hot operating conditions. The stock XJ rad has a very small frontal square inch surface area per cubic inch motor displacement (1.27 sq. in./cid, iirc) when compared to most other vehicles. Not a whole lot can be done about frontal surface area......it is what it is. If someone could mount a radiator from a F450 PowerStroke into the XJ, the only XJ overheat threads would be those pertaining to owner neglect.