Right, I've been having a heating problem with my car for about a year. I can compensate by turning my dash heater on all the way, and while that's not a problem during winter (I live in Alaska), it's no fun during the summer. Besides, it's not fixing anything. This is getting to be a problem because sometimes the coolant's getting so hot that it's filling my overflow tank from completely empty to over the "full" line. I had a chat with a mechanic I know from the Subaru shop, and he said I was low on coolant after he had a look. He also said that I probably had a leak. That was six weeks ago, and since, I've probably managed to get a total of another 8oz of fluid into the system (it's full), and I haven't managed to find any leaks at all.
I'll go over the bullet points here:
-Thermostat is fine
-Radiator fan works
-There's nothing blocking my grill
-There's no indication of any head gasket problems
-No leaks in the line
-The faster I'm going, the more it heats up. Driving on a street at 35MPH doesn't noticeably heat things up, but when I'm cruising down the highway, the temp starts rising.
-The line is circulating (The dash heater wouldn't work otherwise)
So, the only thing I can think of is that I've got blockage in my radiator, either from a clog or an air bubble. I'm still open to other possibilities before I try to flush the system and end up throwing away a ton of antifreeze, although I'd like to take care of this before I have to start worrying about the head gaskets.