I have a 2000 Forester with 270K miles. It runs fine.. no issues and no bad or steamy exhaust.

I just replaced the radiator. At that project I also replace the top and bottom radiator hoses , the radiator thermostat and put a new gasket on the water pump. I also put in a new radiator cap.

It seems that I did what I've always done on old US cars for the last 20 years.

So, Now the Subaru is having an odd issue.

The overflow tank gets full after a drive and there are air bubbles also coming out too into the reservoir.

When the engine has sat for an hour and cools off... I take the cap off and look inside the radiator. The fluid is low because I can see the aluminum on the inside.

The next part is I start the engine with the cap off and add a little fluid and let the engine run and watch the radiator fluid.

I add a little more fluid to basically fill the radiator.

At this point with the cap off and engine running I can see air bubbles being pushed through the top hose because the air bubbles come over to the opened cap hole ... The temp gauge is showing normal temp as it points to the middle of the gauge...no over heating or under cooling.

There is no oil in the radiator fluid and the engine oil is not foamy... and the trans oil is normal... no issues. Plus the heater is under the dash and it works and is not leaking.

I can't figure out where the air bubble are coming from and what to do to make it stop.

Am I missing something?