My 2002 Mercury Sable is overheating. Well actually when I'm on the freeway or going up a hill the temperature gauge starts rising to the top, when I turn the heater on full blast it goes back to normal. On top of this its running a little rough, hesitant, bad gas milage. Any ideas why its starting to overheat? Thanks

 

have you checked your thermostat? or replaced it? any coolant leaks? low coolant in the resivour? (having to add coolant also means coolant loss--not being funny here).

 

I have been thinking I need to replace the thermostat, it seems to make the most sense. The coolant is slightly low but hasnt been losing coolant at any alarming rate. Its actually still green and not brown. I think I will look into replacing my thermostat.

 

Does anyone else have any suggestions or ideas? I'm on very limited funds so I want to make sure I'm not wasting time and money on a thermostat. I know a thermostat is cheap but I really can't afford to screw this up. Thank you.

 

The thermostat is the most likely suspect. There is really no way to check without taking it out. If you want to be sure it is the thermostat without replacing it, find an old pan that you no longer will use for food, & put some water in it. Then put the thermostat in the water fully emerged, but suspended off the bottom of the pan with a piece of wire, & then with a candy thermometer, put the pan on the stove & see if the thermostat fully opens between 190 -212 degrees. If it doesn't, you know it is bad. Most likely if it is bad it will be stuck in one position.

Other possibilities is a clogged radiator, but I doubt it with clean coolant.

Is the fan coming on when hot. Again not likely as you are moving on the freeway.

A bad head gasket. Being you mentioned it is running a little rough, this is a possibility, especially if the hesitation goes away after about a minute or two after starting the engine cold.