6 mpg, wow, I was getting at least 8 mpg with mine just parked in the driveway idling for hours, LOL, while I ran tests.
I think the problems many others have had with different non-oem thermostats has had more to do with a wide range of different flow rates through the different thermostats (different maximum opening sizes), than it has to do with the opening temperature. As I recall the Renix has a 180 F OEM, dealer optional, thermosat available.
In the summer mine gets up to 190 F with the AC running and never over 200 F on a 100 F day with the AC on. In the winter it runs right at 172 F. I have seen no mileage difference between the winter & summer or between the 200 F summer and 172 F winter operation on mine. But I suspect I have the right size throat, opening, on my non-oem 165 F thermostat.
I have verified my temperatures with multiple (3) external temperature measurement devices including infrared and a thermocouple style probe & meter. For whatever its worth, I installed a new plastic pressure bottle and cap, fan clutch, fan shroud, and all copper/brass radiator (2 or 3 row, I forget which) and the notorious, much malined, 165 F non-oem thermostat shortly after buying my jeep (87, 4x4, wagoneer), and it has never overheated, and I got 16 mpg peak mileage during a winter trip to the hills in Austin from Houston, and that was with a defective TPS sensor keeping the tranny from ever getting into its peak efficiency, peak torque range (Note TPS was just replaced, and I am working an AC upgrade, so I do not have the new mileage yet, but I am expecting to see it finally hit 20 mpg.
I have a suggestion on your mileage problem. Check the third, larger diameter wire going to the O2 sensor and see if it is getting the proper 12 to 14 volts it needs to heat the O2 sensor. IF it is not, it might explain why your mileage and overheating problem came and went with a hotter thermostat. Also check the resistance on that third wire to ground on the disconected O2 sensor itself. If the O2 sensor internal heater goes bad (near zero ohms, or infinate resistance) it will give out O2 data, but it is slow, bad data (biased) and can cause overheating and poor mileage under different operating conditions making it look like a thermostat issue!
scottmcneal said:
Ecomike, if i put a anything lower than stock on my 89 it get like 6 miles to the gal, then when on open road not freeway, it will over heat.. I did some reading about t-stats an went with a oem 195 an now it runs good an doesn't over heat...open desert out here