Is your car idling high? After driving the car for 30 minutes, pull off the road and pull the dipstick out and smell the oil. If the oil smells like gas, you have an enrichment problem. It could be the aforementioned thermostat or an coolant temp sensor. If the coolant temp sensor is sending a crap reading to the computer, it is doing the same thing as the bad thermostat. If the computer always thinks it's cold, then it's going 5o keep the idle air control valve open and run rich, which wold give you bad gas mileage.
Once again, easier to replace the temp sensor than to try to figure out whats worng with it. Usually pretty cheap.
Now, I am not one to advocate throwing parts at a car, so if you want to do it the right way, find the coolant temp sensor and an ohmeter. Wait until the temp sensor is cold and take a resistance reading. I don't know what the spec is of the top of my head, but they should either be in the Haynes Shop manual or the factory shop manual. Drive the car until the car reaches operating temp and take another reading immediately (don't burn yourself!!!) then see if the resistance changes. If it doesn't meet specs, you know for a fact you have a bad sensor.
Let us know how you do!
