During the summer a couple months back I replaced my radiator with a mishimoto aluminum radiator cause it was getting 90 to 100 degrees everyday and sometimes it would get past 210 almost into overheating territory if I sat for too long. I replaced it and my temp went back to normal and would stay good but now that it's getting colder out my jeep doesn't want to get up to tempature it will get halfway between the second mark on the temp gauge if it's in the 70s or if it's colder it stays at the SEC nd mark or just after it. Is it just the radiator is too efficient at cooling and just put a price of cardboard behind my grille or do y'all think my thermostat is defective.

 

I would say your thermostat. I have never had an overheating problem, only not coming up to temp. I tried a stant, gates, and the cheap and expensive Motorad, they all worked for a while then it would start not getting to temp again. This time I went to Stealership and paid 55 bucks for "MOPAR" it arrived and is stamped Motorad part # 8350 1426 AD. It is different than the motorad from Advance as it has the weep hole,and made in Israel vs China. I have been running it for a week now dash stays just touching 210 and drops a little when stat opens, I am running a REM II reader and can see the temp of the water as it enters the block from the rad from the coolant temp sensor. Sitting it stabilizes at 187, if I give it a little RPM the electric fan kicks at 198 and shuts down at 193 (mind you this is off the reader the dash guage is reading just under 210 stable a little over 210 fan on and just under 210 after it kicks off). I have been averaging about 6 months on stats we'll see how this one pans out.

 

I'd say either your thermostat or possibility your coolant temp sensor is going bad.