Puzzled!!

[LegacyGT97]

Recently while on a long drive I noticed my heat gauge shoot up!
I left it down island with my parents and my dad, who is a mechanic of 20+ years, has been trying to sort out the problem for the last 2 weeks.
Its bizarre, you can drive the car for up to 100km straight with a constant speed without a problem, then out of no where the heat register will shoot up!
There is a ton of coolant in the system and the fans have now been wired to run constantly. Yet it still over heats. We've replaced the coolant sensor, water pump, multiple coolant hoses. We're stumped! Could it be the relay perhaps??

[kimokalihi]

Blown head gaskets?

[kimokalihi]

Try some subaru coolant conditioner. I just had an external head gasket leak and it fixed it in a day. No telling if it's going to last but so far it's been a week and no more leak. It's less than $3 at the dealership. You remove I think .125 liter of coolant from your system and shake that bottle up a LOT and then dump it in and go. I had left over gunk inside the bottle so I put some water in it and shook it up again and got the remainder of it out. Try it out, worth a shot.

[LegacyGT97]

The HG's went on my last engine, I've had this on in for little over a year now, it had 8x,xxxkms when I bought it and put it in.
Its weird that you can run the car straight for an hour and then the temperature shoots up. I'm thinking its not HG's as the problem would be there all the time, not intermittently. I'm not sure where else to look.

[Wiscon_Mark]

When you say, shoot up, do you mean you'll look down and notice it's gone up, or do you see the gauge go from normal temp to ridiculously hot in just a few seconds?

I'm having a problem with the latter, and the engine is not overheating - there's a problem with the temp sensor/gauge.

[Dead91silvia]

I just did HG's on my BD and the car could sit and run for a good long time, but then drive it and it would overheat. It could still be HG's. If you can, get a pressure test done. Another thing also, the gauge might be messed up some how...

[Huffer]

Temp sensor. And get an infrared thermometer to test the actual heat of the engine when the temp gauge reads "good" and then when it reads "too hot".