Mitsubishi Repair: Cooling system 2004 Mitsubbishi Lancer Ralliart, radiator cap, fluid leaks

Question
The car has been running hot, but only during the summer 70 degrees and higher.  We replaced the thermostat, the fans work, and no fluid leaks.  The car can run for hours in traffic will run fine but when driving, either on a highway or around town it heats up just below the red, and i turn on the heat and stop giving it gas and goes right back down.  It started last summer and ran fine all winter and spring, and just started again.  Didn't want to change the water pump ($900) if it isn't that.  Any ideas?

Answer
Christopher: I can give you some things to check, but most of what I would tell you to check, you have already. Still you should check the radiator, the core of that and the grill restricted. You be filling up with a lot of bugs, or sometimes a bag will get caught up and cover the radiator so it is not getting enough air through it. When you take the cap off the radiator is the cap clean or does it have green kind of guming mess around it. I mean when is the last time you had the radiator check and back flushed. They say every two years for a car. The radiator its self could be bad and say try buying a new radiator cap sometimes those are bad so not doing their jobs. Are you running the right amount coolant in the system. Some people believe in the summer it is ok, to run just water in the system but no you should be running coolant all year around. The last thing the manual that I am reading from, says is that costly water pump. You can tell if that is bad or not, when the water pump gets bad it will start to leak though small holes in it. And so you should be able to find a small bit of coolant colored water under your car, if you looked. If you not finding this then I think the water pump is ok. Another thing to check would be the water pump belt make sure it is doing its job, not slipping or something as having a bad or old belt on that part. Hope one of these ideas helps you and God bless: Linda