Engine Temp / Heater Core

I noticed my temperature guage is not coming up to normal temp. It was usually right in the middle constantly. Now it fluctuates and will not come up to the middle. It gets almost there, then the thermostat kicks in and drops a bit. I have replaced the thermostat to no avail. It does blow out heat in the cab but not as hot as I think it should. I'm thinking maybe the heater core is plugged up or the blend door (after reading some posts on here). I am going to try to flush the core and possibly the radiator and see if that fixes the problem. Any more tips to try would be helpful!

What I see most of the times with a temp gauge that indicates lower than normal, temp gauge that seems to move more than normal AND low heat output is low coolant level. I would make sure the vehicle is cold and then check the coolant level before going any further.

Did you replace with a 190 or 180 t-stat? When I replaced mine Advance Auto listed both for the 4.0L.

I have got bad new t-stats years ago. So ever since then I test them before installing.

I hang them in water on the stove and with a thermometer. I check the temp when full open. Since I started testing I found one that was way off, store brand.

I try to buy stant brand. Never had a bad one.

EddieS'04, which is the correct one? 180 or 190

The oem was marked 190........