Overheating Subaru

My 1999 Subaru Legacy Outback sometimes overheats. If I’m drive at highway speed for a while (about 30 mins.) and then slow down later, say at a stoplight, the temperature indicator rises. When I start moving the needle stays up until I speed-up, then the temperature indcator goes down. Sometimes the coolant boils over and looks dark.

Is this a blown head gasket or bad water pump or what?

It sounds like a radiator fan not coming on to me. You likely have two fans on the radiator. One likely will come on anytime the AC is on and the other SHOULD come on when the coolant is hot. I suspect that other one is not coming on. When you are driving fast enough air comes through to keep it cool, when you slow down less air and you overheat.

You can try turning on the AC when it is heating up and see if that cools thing down. You can also stop when it is heating up, leave the car idle (AC OFF) and open the hood and see if a fan is running. If not, that’s it. New fan or sensor.

Joseph makes valid points about the cooling fans. It would be nice to know how many miles are on your Subaru.

The water pump is normally changed with the timing belt at the prescribed interval in the owner’s manual. We learned to our dismay that the water pump is apparently designed to fail at about the same mileage when you need to change the belt.

If the coolant looks “DARK” I would suggest you start with some cooling system maintanace.
New t~stat, radiator flush, fresh antifrezze.