I recently replaced my entire cooling system and that included a Stant 180 degree thermostat.

While the truck runs very cool, usually 179F to 184F, I've noticed a considerable decline in mileage. Im wondering if maybe im running too cool and the vehicle ECM keeps the fuel too rich?

What is the sweet spot for operating temp?

 

188-193f imho

 

Sounds like you have a well calibrated T-stat.

For the mileage question, I would check real time fuel trims and O2 voltages (for '96 on). Don't know your truck year? Maybe try a warmer T-stat and see if mileage improves.

 

Thanks for the input. It’s a 98. Will check those trims today. If they’re out of whack, is that an O2 sensor issue?

 

I could simply need a engine ECU reset which will clear the adaptive values.

 

Bad O2 sensors can cause bad fuel trims, but there are other possible causes too (e.g. vacuum leaks, fuel pressure or injector issues, upstream exhaust leaks). That's why we check the O2 voltages in real time. If the voltages modulate, then the O2s are fine. If the voltages do not modulate, then we need to dig deeper.