My car was overheating on hot days and cylinder 1 was misfiring. I had to add coolant multiple times. I finally added radiator stop leak and the car died while driving two days later.
I have seen this a number of times on this vehicle. There have been quite a few concerns with blown head gaskets on Subaru engines, especially this one. Adding stop leak is not going to fix it and in some cases, make it worse. If you have a cylinder 1 misfire then most of the time that would mean that coolant is entering that cylinder. I would verify this with a bore camera on that allows me to see the coolant entering that cylinder. EIther way this engine will need some serious repair or even replacement. If you are not comfortable with this type of inspection then I would enlist the assistance of a certified mechanic, who can diagnose your car's overheating issue.
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