Subaru Repair: intermittent overheat subaru, blown head gasket, combustion gases

Question
i have a 99 subaru impreza, 2.5l dohc. will idle all day long and not run hot, fans cycle like normal, gentle driving no overheat, but, pull a hill and wam! pegs the guage, overflows, pukes on the ground . let it idle it cools done, add alittle to the overflow bottle good till it pulls another hill. new timing belt , o.e. thermostat, new tensioner, waterpump new. no steam from tail pipe, no combustion gas in radiator.

Answer
First, when all of these things were replaced did you fill the cooling system the Subarea way? If not, you may have an air locked system and you will keep having problems, maybe until you have big problems. Second, your radiator may not be flowing well. It may flow ok when not loaded, but it may be a little restricted to flow when you load up the engine. Unfortunately, I know of no good way to check the radiator except to pull it and have it flow checked. And by flow check I mean have someone actually measure the output to determine if it is any good or not. Finally, as much as you wont like to hear this, I would not be surprised if you have a blown head gasket or two. These engines really are not good at dealing with any level of overheating. And since you are probably checking for combustion gases when then engine is not puking coolant on the ground, the gaskets may be sealing ok at that time but not when it is puking. Good luck.