I have a 1990 chevy 4x4 reg cab im have problems with my gas mileage just started happing about a month ago when it got really cold somebody told me to change the o2 sensor and then somebody tole me to change the coolant temperature sensor i dont know what to do going crazy, people are telling me differnent things any help .

 

from your description I would say the thermostat /coolant temp sensor. And definately not the O2 sensor. the cold won't affect that unless it's a coincidence.

 

How long has it been since your last tune up? Dose your engine light come on? I am going to assume you don't have any other problems since you didn't say anything but your gas mileage. You also said it's cold. When you start a gas engine in cold weather you have to run a rich mixture to get enough of the gas to vaporize to make it burn. That is done on old cars by a manual choke, then an automatic. If you have a factory carb you should have an atuomatic choke. Throttle bodies and fuel injection don't have a choke, but the computer does control that. When a car is warm and running correctly it will run around a 14 to 1 air to fuel,when it is cold it will go to around a 9 to 1 to get started and warmed up. So if it is cold and your choke is on and you don't let it warm up and only drive a short distance your engine will not warm up completely, and your choke will not shut off compeletly, thus you use more gas to keep it running.