If the temp is NOT staying at least above the 1/4 mark ( not the 40 mark) then your car is NOT holding the heat it should be holding. Usually that is almost certain that your stat is the problem. It seems like the stat is stuck open or that it is floating and not sealing properly allowing just enough fluid to get by and keep the engine colder than it should be.


So until you can get the temp to stay at that 1/4 to half mark I wouldn't bother with the heater core. Your heater core is not getting fluid that is hot enough to keep putting out the warm air.

Neither the Bonneville or the TranSport ever make the 1/4 mark unless approaching 200F. 180F is just like the picture above shows, just under the 1/4 mark.
Guage is decidedly non-linear. Bottom line seems to be 100F, 1/4 is 190-200, 1/2 (straight up) is 220 and full is 260F (think red line is 250F). GM seems to feel that everything is OK at 220-230F.

Anything over 150F should produce decent heat but GM guages are so notoriously inaccurate that the only way to know what the ECM thinks the coolant temperature is with a scan tool.

Image is of 181F (is night so not clearest but if I use a flood or flash, the OTC 2000 display is invisible)