What a thermostat does is determine when the coolant begins to flow through the radiator and it also determines how quickly the engine will reach normal operating tempurature ....and that's about it. Warm up time on a smog controlled eninge is a relatively "dirty" event and the faster the engine gets up to operating tempurature, the faster all of the normal emmissions controls will function ...hence the 195deg t-stat. The temp rating of the t-stat is when it *begins* to open; they are not fully open until the coolant is quite a bit hotter. Will a 180 t-stat cause any problems in your XJ? Nope. Will your engine warm up just as fast? Yup. Will your engine *potentially* run cooler with a 180 t-stat? Yup.
In most vehicles the 195 t-stat isn't a problem but on the XJ the radiator and, more importantly, the flow of air *through* the radiator are somewhat compromised. The relatively small size of the radiator is limited by the design of the vehicle and the airflow, or lack thereof, through the radiator is a function of stuffing that *long* inline six into a space that should be occupied with a V-6 or I-4 engine. Take a look at how the area immediately behind the radiator is jammed with the engine, accessory drives, AC fan, etc ...all this stuff effectively blocks the airflow through the rad. Tack on a cooling system that's been neglected and you've set yourself up for a hot running engine. I'm sorry but IMHO, 210deg is just way too hot for an engine to run under normal conditions; and it doesn't take much to get the engine running hotter with the stock system. I think the AMC engineers made a big compromise on cooling so they could use their legacy six-banger motor (instead of those hideous GM 2.8L V-6's).
Here's my system:
2-core Readi-Rad "Heat Buster" radiator
Flow Kooler high flow water pump
180 deg flow balanced/hi-flow RobertShaw T-Stat
seperate mount tranny coooler.
4.0L six now runs about 25-30deg cooler (based on dash gauge) and will only reach 210 on the hottest days with the A/C on. Mileage hasn't suffered and it smog checks fine.
Mike
00XJ