Hey everyone. I have a 1994 Suburban K1500 with 148,000 mi on her. I'm having an issue with the heat and engine temp. The temp is only coming up to about 140 deg. and I am only getting heat when I'm driving and the RPM is high. I replaced the thermostat but that did absolutely nothing. Engine coolant level is fine as well. Any help would be greatly appreciated. I've searched the forum but haven't found anyone with both of these problems together.
Thank you.

 

New t-stat may be bad, ie, not closing fully, heater core may be clogged.