The 180 degree thermostat will make the engine run cooler and in turn a cooler engine will produce more horse power due to a richer fuel to air mixure, hense the lower mileage. This was a gimmee back in the day when I was playing with older 5.0L mustangs but there is a trade off. Some swear by the 180 degree T stat and I do agree that it give a slight performance upgrade but keep in mind that the 01 D runs on a closed loop system meaning that the computer monitors exhaust gas for unburned fuel and will lean out the mixure to be compliant, while the intake air charge is telling it to increase fuel to warm up the engine. Less air, hence the old term, choke. Some folks claim they dont get a check engine, others do?Im no computer guy. FYI Fowled plugs, milky oil, carbon, areall by products of engines that dont warm up properly, so just watch your D. If you were at 160 I think this may be an issue. Not at 180. I ran 195 in Maine and 180 in AZ.I'll leave that for the performance gurus on the forum. Most ODB II systems need a 195, for emissions. Keep that in mind if your state has a sniffer test. As for the post about a quality T stat, I agree. Post again if you cant get it burped. The cap is at the highest point so you should be able to do it with out to much fuss.

 

[8D]Sorry been crazy, out with that freakin FAILSAFE 180 and in with new 195. She burped herself, opened up and topped of the coolant. Thanks Guys. Sorry it took so long to repost solution. Going to the zone to figure out check engine light but PJ2 isprobably right and its been running rich when the wife has ran it.