How do I "confirm" that I have a bad head gasket w/ my overheating '98 Legacy GT?

Drive it like you stole it. Go through a few cycles of quick acceleration to get the rpms up. Watch the temp needle on the dash. If you see it rise like its overheating then drop back down quick, that's when a bubble of exhaust is causing flow to stop in the water jacket.

 

 

When this is going on (once it starts it usually will go for awhile) pop the hood and look in the overflow reservoir of the radiator. You should see bubbles. When the bubbles stop, the needle on the temp gauge goes back to normal.

 

When the repair shop didn't believe that it was a gasket problem, I would take it out and get it to behave badly in about 10 miles. Warm it up and race it. That was a pattern for my 98 OBW anyway.

bubbles/gunk in the radiator overflow tank is the biggest sign, though that often doesn't start until the leak is starting to become a problem.

 

for the bubbles to start on mine, it would take about 20 minutes on the highway before it would start. mine only overheated once, then i started researching it, checked for bubbles, and tada - found the head gasket was gone.

 

sucks. don't it?