Here's what I just went through on my 89. Maybe it will help. I have a 3+yr old GDI 3row radiator, newer clutch on my fan, a 99 aux fan, new heater valve (only sat in my rollaway for 2 years...) and ZJ overflow bottle, just converting to an open system about 2 weeks ago. At that time, I also replaced the t-stat.....just because.
After the conversion, it ran too hot, crazy because before the conversion to an open system it rarely got above 210. I also had the water pump start to leak after a 40 minute drive in 95+ heat, running at 220+ degrees.............aarrrrrrrr! New pump goes in the next day.
So I suspect the new t-stat as faulty and buy another Stant Super-stat, this time testing the opening point on the stove using boiling water and a probe thermometer. I discover that neither Stant Super-stat is opening at 195......both barely move at 210, which is as hot as I can get my stove.
I snag another t-stat, this time going with a regular one, not a "Super" model. I notice right off that the standard t-stat spring is something that I can open with my thumbs, I can't budge the spring on the "Super" models.........hmmmm? Maybe Superman could..........?
So I boil test the new t-stat and it opens at 195-200, just like it should.
I installed it and my Jeep is cooling nicely, just like it should. 95+ degrees, AC blasting, stop and go traffic, temp gauge never hits 210.
BTW, I highly recommend the conversion from the closed system to the open system, having already replaced the radiator with a GDI closed type, I did mine by having a local radiator shop build me an inline filler with a pressure cap neck, which I installed in the upper radiator hose. Replacing the old heater valve that always allows hot water to circulate through the heater core made a big difference on the AC performance, once the air inside my black Jeep is cooled down after sitting in the hot sun, I can actually cruise with the AC controls set on Med. and Cool instead of High and Max like I used to, this alone is worth the swap. As for the t-stat problem, this really leaves me torqued. I'm returning both Super models to Moe, Manny and Jack's tomorrow for a full refund. I thought that by purchasing a "premium" unit, I was getting better performance, but this particular experience proved otherwise, hope this is an isolated case, YMMV. Good luck.