Originally Posted by carbob

I may need a bit of clarification about the big picture here. Is the balance the MAP thermostat is trying to achieve between higher temps that lead to better gas mileage and lower temps that lead to better emissions? That seems to make the most sense to me, having a hotter thermostat for overall good mileage but adding the DME managed control based on emission sensed feedback to improve emissions at times of poor emissions like high RPM’s by dissipating the heat. So I’ve read that failure of the thermostat means it is open as a fail safe, and I wonder if the failure of the MAP control also fails in the open position? The logical result would be a cooler engine than would otherwise happen with a good thermostat, which means better emissions, worse gas mileage, and maybe faster heater operation. Interestingly my mileage has been creeping lower, now about 15.3 overall on the dash computer, my temp gauge starts moving quickly after start and a very fast heater output on startup. I used to have mileage in the 16mpg range.