My 98 Forester operates great as long as I drill a 3/16" hole in the thermostat. Over a year now with no coolant added, but as soon as I install the proper tstat, it pushes coolant into the overflow and becoming coolant starved, then overheats. It will idle for hours probably with tstat in place and stay at normal temp, but I cannot drive it without problems.
I could have issues with work done, but to date:
New head gaskets - valve/head work
Replaced water pump, thermostat, radiator cap, radiator.
Bled air from system each time
I have good flow through the radiator and heater core, and the water pump seems to be pumping properly. I just need heat going into cooler months, and the hole in the tstat will not allow that. This also was the result right after the head gasket work about a year ago, so that is when I drilled the tstat. Before head gasket repair I tried several block sealants but did my best to clean that out when we did the head work.
I just started a trial and error with another thermostat and drilled two smaller holes first, tried that and still overheated. Plulled it out and drilled one more and that seems to work fine allowing some heat but not enough pressure to push coolant out. Not a fix, just a cover up to get through the winter, I would still like any ideas.
When it cooled overnight, it did draw what coolant it needed from the overflow so that makes me think there is no breach in the system??
testing for hydrocarbons in the coolant will answer the head gasket issue. I had one car with a slight intolerance in the block that caused a overheat after gaskets were done. The other line of thought is that the radiator is getting clogged up. Have seen that after head gasket repairs too.