When my sisters car sits idle the temp guage stays in the middle. When i drive the car at a high rpm the temp guage then jumps up to the H. Then when i slow down or stop the car the guage starts to move down toward the middle again slowly. When the guage needle is on the H the engine or radiator is not hot at all and there is no smoke or leak and i see no signs of the car actualy over heating. But the heater (only one time this happened) blew out cold air. I already changed the thermostat. im more or less thinking it could be the thermostat. If any one has a clue on what it may be please let me know the car is a 97 Subaru legacy GT. Thanks

Do the electric cooling fans engage?

Check you coolant levels and also check for any air blocks in the coolant system. Although it looks more like a gauge error, it is to be better on the safer side before your car gets into a catastrophic heating problem. Moreover I would advice you to check your radiator fins, just run your fingers gently through the fins and make sure they are sturdy. If you have not changed your radiator since you bought the car, the fins are probably worn out an would crumble off if you run your fingers through them. It would also be helpful if you can tell something about the weather condition (avg, warm, cold?) at your place, this could affect your heating system in many cases.

Check coolant levels. make sure system is burped properly. So it Stays normal at idle but heats up as you drive. You will see this with older vehicles when the radiator is stopping up. This would also account for the heater blowing cold. (Heater core stopping up). Have you ever used that stop leak junk? If so that would also cause radiator and core to get stopped up. Or at least would cause them to become partially stopped up. Have you done any other work to it lately?

earth to mars? any life up there~?

the cooling fans do engage, did use stop leak how would i fix this situation do i have to replace the radiator or heater core or flush out the cooling system would that help?

You can try flushing it out of your radiator but with the age of your vehicle it might not get it all out. If heater blowing hot now I would not worry about it so much unless it stops again. Why did you use the stop leak? Did you ever fix the leak properly? If not now would be a good time to fix it.

she used stop leak for the oil leak i have it was a few months ago and it didnt work . this started happening after she got a gasket changed on her exhaust system the heat changes to cold only a few times but the gauge is always doing this

So the stop leak was poured in with the oil and not the coolant?

correct

Ok then that would not be causing this issue. Was she getting oil in her coolant?

no

http://www.subaruoutback.org/forums/66-problems-maintenance/37821- random-overheating.html --- check this out. Seems that model legacy has issues with head gaskets going bad and acting as yours is.

the radiator was replaced about 8 months ago

Ok from what I have been reading on other sites about your vehicle its probably a leaking head gasket causing this issue. Have you noticed air bubbles in radiator? There is a test for exhaust gasses in your coolant. Read the link above I feel you will find some helpful info there.

I do not feel you have a radiator issue. That is as long as its completely full of coolant.

http://www.subaruoutback.org/forums/66-problems-maintenance/35881- 97-overheating-fluid-leaking.html

http://www.subaruoutback.org/forums/111-gen-1-1995-1999/26851- overheating-problems-ongoing-never-ending.html

http://legacygt.com/forums/showthread.php/1997-lgt-overheating- problems-105800.html?s=c6bd759d6b5c3fe9adf751f199182501&

I would almost bet you have a bad head gasket.

Hey I am having the same issue? runs fine forever at idle or low speed as soon as I put it up to 3k rpm my car jumps the the High side of the temperature gauge. I have no clue what the problem could be. Although there is a cold spot in my radiator right by the thermostat and the hose running from the radiator towards the thermostat is not hot at running temperature. Any suggestions? could this be a water pump? sounds like the same issue. Also Ive been running water in the radiator only, so maybe I could have corroded the water pump?