I haven't had real good luck with non-OEM thermostats. You might either try a new OEM thermostat or swap the old one back in, to see if that changes anything.
I've been getting some advice from a local hotrodder and he suggested i look at the increase in CCs of the engine as well as Radiator Volume between the 1.8 and the 2.3 motors to help formulate an approach.
1. The 2.3 engine is 24% larger than the 1.8
2. The volume of the Aerio Radiator is roughly 28% larger than the Sidekick Sport Radiator (2 Row) .
3. I'm using a pair of 10" fans that are supposed to offer 1750CFM in pairs
4. I have a fan shroud for these fans, I used an aluminum baking pan and it is drawing air well.
So I need to increase the Radiator Size and air flow. I put in a V6 Vitara radiator, 2 row when the original was a 1 row. essentially doubling the volume of the original radiator but the temps are still high.
Even with the larger Radiator volume of the V6 Radiator it may not be enough to properly cool the J23 engine. I also have a 2" body lift so that may be changing airflow and coverage.
The next phase of my testing is to replace the 2 10" fans with a single 16" 3000 CFM fan to see what the temps give me.
I have ordered a 3 row Aluminum Radiator which will take a few weeks to arrive.
I've checked all the external pipe/tubes on this engine and put on a new water pump and Stant thermostat that I tested (pan of water) and the system is working as it should but
the data is leading me to the fact that my cooling system as a whole is too small.