CIarification, the OEM thermostat is working perfectly. While driving I see you it open and close by monitoring coolant temperature. Just it runs hotter than I like. Drops to 93C then climbs to 95C. Cycles between.
I wiould like a thermostat that is set to 180 - 183 degrees instead of the OEM 190 degree. Several suppliers show a lower temperature model. But they a one piece units. The correct thermostats are four parts and twist to lock into thermostat housing.
Can only find 190 thermostat so far.

 

The reason these cars have 195 thermostats is the engines are designed to run hot. Any thing below 230 will not harm them. Running cold is not good for modern engines. Ever tore down a 60s engine that had 160 to 180 thermostats, they are filthy from not cooking the moisture out. I just tore down a 60s nailhead Buick. Absolutly filthy even the oil pump screen was imprinted in the pan crud. If your car is not overheating leave it alone.

 

You will likely get a DTC P0128 but if emissions test is not required, then have at it.

As Terrific stated, the engine management was designed to operate at a predetermined temp.

It's your experiment, so go for it!

 

In my (now sold) sports car, the V6 went up to 135C. 90C is just the base for an engine. The upper limit is not 100C, due to the pressure valve.

 

Use OEM

 

Another thing that you lose is fuel economy. If the car isn't overheating or losing coolant; leave the thermostat alone.

 

Thanks!
It was easy to leave the 190 degree in there. Not having a lower temp to install.
I replaced expansion tank last week and figured it was a good time to swap if I ever was. Gates and Stant both show a 183 degree model a 2006 XJR.
But both most likely fit a x308. Flat, one piece unit. Not an insert model. BTW the Stant was stamped Motorad.
Never have seen over 95C. Even after freeway exit and stop and go traffic. But did not start monitoring coolant temperature until recently.