I believe my high engine temps are a result of a faulty thermostat. Even ran hot at 71F ambient temperature. Most times the A/C is off. I never realized the correlation between abnormally high engine temperatures and poor fuel economy b/c my mileage has been crap lately until I read the thermostat DIY. All fuel related maintenance items have been timely addressed. Could a bad thermostat cause my heater not to kick in, too? I replaced coolant 2 weeks ago and ran heater(on HI setting) to circulate fluid but air was cold with running compressor.And based on the fact that your old one was stuck open, I'd bet you'll see a decent mileage improvement.