Mitsubishi Repair: heating, coolant temperature, air vent

Question
I have a 1997 Eclipse 2.0 L GS, Manual.  When I let my car idle for 5- 10 min my car warms up and the heater works, but as I start driving the tempature slowly begins to drop and the heater starts blowing cold to luke warm air. what could the problem be? Thanks- Eric

Answer
Eric,
That is a common problem and there are two main causes.  One, you should replace your thermostat.  When the car is not in motion, less cold air passes over the radiator, and the air will feel warm blowing out of the vents.  If the thermostat valve is stuck open, the coolant temperature will not be regulated, and the will continuously cool down when the car is in motion.  Thermostats are cheap and easily replaceable.  It should be replaced every 6-8 years.  

Second, you may have a fresh air vent stuck open.  If a vent is open to the outside air, that will definitely cool the air down that is being blown into your car.  As a quick test, switch your vent to use Recycled air instead of Fresh air.  You may notice warmer air coming from the vents after that.  The downside is that your windows will fog up more commonly in the winter time when recycled air is used.  But most likely, your thermostat needs replacing.  
Good luck!