Hello all. I'm new to this forum and Subaru, so here's my story. I recently bought a 2002 Outback 2.5 AT (base) from my grandfather. He wanted to go down to one car, and we discovered it had head gasket issues, so to save him the headache of working it out and selling it, I bought it from him for cheaper.
I had the head gaskets replaced by a professional mechanic, who also replaced the timing belt, water pump, thermostat, etc. while he was in there. The car has been running fine, except recently I noticed the temp gauge becoming slightly elevated when on longer or higher-speed trips. On short, 10 minute or so city drives, it goes up to the dead middle and stays there, right where it should be. But when I go on slightly longer drives, or when I drive at 50/60+ mph, the needle goes up to the third dash and sticks for a little while. If I keep driving for a while, it slowly returns, but then goes back up, and keeps repeating this. If I stop the car and let it idle for a few minutes (fans turn on just fine), it will return the temperature down.
I brought it to my mechanic, he said he could find nothing wrong, but replaced the thermostat as a matter of course. It still does the same thing. Keep in mind, this is in winter, with 20 degree F or lower temps. Heat works fine, cooling fans turn on, and it hasn't gone past the 3rd notch.
I'm thinking that either he installed a higher temperature thermostat or my gauge is not accurate. I haven't had a chance to speak to him to ask him if he did install a different temp t-stat. If he did, like an aftermarket one, would it be worth the time and money to have him install an oem one? And how would I test my gauge? Any insights would be appreciated. Thanks!