I have an 09 outback with 140,000 miles and did HG at 119,00 miles. I was driving to Montana the other day and while driving at 85-90 mph on cruise control for an extended period my car would purge a lot of coolant into the overflow and spray out the top of the overflow cap while driving. Luckily it's winter and I didn't overheat too bad but the temp guage was a few ticks higher than normal til I pulled over and topped it off. Since doing HG we haven't had any trouble until this recent episode. The car has only ever done this at 85 mph on cruise for an hour or more.

 

I've tested for exhaust gas in the coolant and it came up clean. I'm not sure if this is a HG issue or what? I put a new timing kit, thermostat, and water pump in at time of HG. Not sure why it does this?

sounds like headgasket but let's get some more information.  replaced headgaskets tend to differ slighly in their failure modes from OEM installed.

 

how did you test for exhaust gas in the coolant?

 

when the temp starts to tick up - does it do it for extended periods of time or is it an instantaneous thing - quickly then subsides?  or is it slowly creeping up over a long period of time?

 

how many times did it do this and was this the first time it ever happened?

 

did it only overheat on steep mountains grades or randomly?

 

when the headgaskets were replaced were the heads resurfaced and which headgasket did you use?  

I checked for gases using a block test which breaths the rad fumes and turns the liquid yellow of gas is present.

 

When the temp ticks up its because the engine suddenly blew half gallon of coolant out the overflow. It then only goes down if I replenish coolant.

 

This was the first time it has happened but it happens twice in two days and both times I was on mostly flat ground going 85-90 on cruise control across Montana. When I got to the mountain passes between Washington and Montana my car was fine but I was also going at much lower speeds though.

 

Heads were not resurfaced as I can't recall ever overheating my car but am wishing I would have done it now...I used a very nice three layered metal Felpro gasket.

yep, headgaskets.  or probably...we can hope I'm wrong.  

 

they're consuming coolant internally and eventually the system level gets too low, coolant flow gets compromised more and more, until the car will start to overheat and push coolant out.

 

they pretty regularly pass those block tests, so i wouldn't consider that very telling.

 

resurfacing heads is wise just to get a proper finish on the metal, not just for "warping" or flatness. i don't even check for that because they're problematic heads to begin with so i might as well make sure i get a proper finish if easily possible. 

 

OEM Tstat, Rad cap, check fans, radiator....but i doubt it'll be those things.