2000 Mercury Mystique 6 cyl Automatic 125000 miles
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Just recently, my problems started with the A/C losing its cool after about 5-10 minutes of use. I did notice a single drop of cold water fall on my foot while driving before the A/C issues began, which was unusual. After that, I noticed the car start to stutter and hesitate slightly when taking off, but it has no problem with power when pushing on the accelerator. Shortly after, I noticed that the Heater does not blow hot air anymore. One last thing. When idle or at a stop light, the temperature gauge rises to Hot, but when accelerating, the temp gauge falls back towards normal. All of this happened within a 3 day period. I've recently had the thermostat and therm. Housing replaced and the all the belts were checked shortly before that. What's going on?

Check your Water Pump. My sister's car (same car) did the same thing, when the water pump was removed it revealed that the impellers inside the pump had completely rusted away which really limited how much coolant the water pump was circulating. New water pump and now it works like a champ.

I agree, the water pump is may very likely be the problem and is usually caused by running too much water in the system and not enough coolant. This will result in the impeller rusting away. Just adding water instead of coolant is a common habit people have when the level in the radiator or reservoir is low. And another thing you might check is if the electric fan is coming on when the engine gets hot and when the ac is on. If the fan isn't coming on it may be the fan motor or a coolant temp. sensor that the computer uses to know when to turn the fans on. Good luck