Car heats up when going fast?

[Clipsy]

Hey everyone iam kinda new here and i've been searching these forums for a specific problem i have on my eclipse and i didnt found anything like it so i decided to make a new thread. My problem is that the temperature starts to rise in a slow but steady manner when iam going fast (usually 50-and up mph). I've checked the coolant levels and they are fine, i replaced the old radiator cap (which was causing my car to heat up WHEN NOT MOVING), water reserves were also full..i have no idea what could be the problem. The temperature starts to go back down as soon as i depress on the gas and slow down. I HAVE noticed however, that this happens when i have the A/C off, and when i have it ON it doesnt heat up, it has but it is rare when i have the A/C ON. Any one know what might be the problem? My car is a 2001 Eclipse RS only 79,000 miles.

[SilvrEclipse]

Welcome to AF.

This should be is the 2000+ section. I will move it for you.

Now for your problem, I would flush out your coolant system, go ahead and change the thermostat since they are cheap. Make sure the radiator fans are coming on. You also may want to add a bottle of water wetter to your coolant system. It can help drop the temps a couple degrees. Other than that make sure the radiator is getting plenty of air flow and nothing is blocking it that shouldn't be there.

[808drifter]

After replacing coolant be sure to burp the system.