Your 4-banger coolant and thus the heater core runs at the same temperature as the 6-banger does. Is your temperature gauge indicating the normal 195 (in really cold temps) to 210 degrees or did someone like a previous owner stupidly remove or replace the factory specified 195 degree thermostat with a 165 or 180 degree thermostat? Maybe you just need to replace your thermostat with a fresh 195 degree thermostat. If you replace it make sure NOT to buy any that are labeled "fail safe" which can just cause problems like the engine running too cool.Agree but it takes a long time. Maybe my 4 angry squirrels aren't enough.