New Overheating Thread

[Nomake Wan]

Okay, I'm gonna make a new thread to attack this one, again. For reference, here's the old thread:

http://www.subaru-svx.net/forum/showthread.php?t=40723

Anyway, I just got done doing a trip up to San Francisco and back from Orange. It's about 400-something miles, with almost 6 hours straight of driving. The drive back is what really gave the most clues to my problem.

First, the car has no problems around town, ever. Even on the freeway, it just has no issues around where I live. Even if I stomp on it, it still doesn't push the heat.

Now, the trip from Berkeley back? This was actually a great indicator that something odd is going on. It was 4 hours of straight driving, on almost completely flat roads. Here's a map:

Google Map

Now, we stopped to get gas before we hit the Grapevine. It was when we came to a stop at the gas station that the car began to overheat. It had been fine this whole time, but as soon as we stopped, it heated up. I kicked the heater in, the temps dropped a bit, and I turned the car off. We got gas, and by the time we were finished at the station, it was back to normal operating temp. We hit the grapevine.

Predictably, the high load on the grapevine caused another overheat the very first time we had to slow down because of some idiot moving into the fast lane and doing 50. I then moved over into the slow lane and limped us up the hill, the temps staying about 3/4 of the way up the gauge. As soon as we got downhill, the problem vanished.

The final time was when we hit gridlock traffic on the 5 South. There was a major accident, and everyone was stopping to look at it, which had completely stopped traffic for miles and miles. Again, as soon as we slowed down, the temps crept up. I put the heater on, and it seemed to help a little, but not much. What actually helped the most was putting the shifter into '1' so that the revs stayed up. We never moved faster than 10 MPH for the whole time, so keeping the revs around 1500-2000 seemed to keep the temps manageable.

Now, my analysis was that there didn't seem to be enough capacity in the system, since turning on the heater adds a circuit to the cooling path and thus increases the volume of the cooling system. My dad's analysis, though, after driving the car around Orange and such, was a little different. He thinks there's something caught up in a pathway somewhere that's blocking flow; not enough to completely block coolant, but enough that after running for extended periods, the heat is too much. And that turning on the heater isn't really adding volume as much as it is dropping pressure in the system, which is jolting whatever it is that's stuck. He says it feels like something 'thermostatic'.

The Subaru dealer did a test on the cooling system and concluded that it was not a head gasket (block test) and that they actually couldn't get it to overheat (supposedly the car was driven about 100 miles). Aside from the block test, I don't know if they did anything else. I assume not. I also did block tests on my own which all came up negative. My dad also doesn't feel like it's a head gasket.

Thermostat is new but hasn't been tested for operation (I would assume it works since I don't overheat around town). Water pump is original; new one will be going in soon, but somehow I don't think that's it. Radiator has about 15000 miles on it; there are no obstructions on it; fins look clean. The fans both kick in properly. The passenger's footwell isn't damp, so I'm assuming that my heater core is fine as well.

Where else should I look, or what tests should I have done? Let's see what this new information brings.