I have a problem with my Legacy R spec-B. More specifically - with the coolant system.

While driving normally or even pushing it hard, coolant temps are rock solid with the needle pegged just below half-way. When bringing the car to idle after a 20min+ drive, it will sit happily until about 5 minutes in when the temperature needle will slowly creep up towards the red. If I do nothing it gets to the very end of the white bar before stopping. If I give the car a 10-15 second sustained 2000rpm the temps pushes up slightly, then levels off quickly.

Originally when it started doing this, the expansion tank level was very high and coolant was not being taken back into radiator once cooled. A new radiator cap seems to have partially solved this, although on a spirited drive it has pushed the coolant out again which has not been withdrawn on cool down, leaving the top of the rad empty.

With the new radiator cap on, I now have a new symptom which is rapid bubbling coming from the expansion tank tube on vehicle shut down. This tube does not bubble at all when the engine is at revs, and only gives one bubble every 2 seconds or so when idling.

I have done a combustion gas sniff test, and it passed. I have replaced the radiator cap which has helped somewhat. I have bled the entire coolant system for a good 30 minutes using a proper coolant funnel with the heaters on full whack - bubbles stopped after about 10 mins, but I kept going just to make sure. No more bubbles were created even as I ran it through the rev range and kept a sustained 5000/6000 revs for around 20 seconds. There are no visible leaks or spray marks anywhere in the engine bay or around the hoses. The carpets/dash on the passenger side show no signs of being wet or having been wet previously. The heater works consistently when the car is up to temp, it doesn’t lull even at idle. Both fans come on correctly just as the coolant temp begins it’s slow march up. Regular driving and before the temps creep up at idle they don’t come on at all. Rad was replaced with a brand new unit a month or so ago, with new coolant and Subaru Coolant Conditioner.

I am assuming that the radiator is boiling when it heats soaks with no flow when the engine is killed, which is producing the rapid bubbling in the expansion tank. The condition is completely manageable by keeping on the revs when at a standstill, and moving the contents of the expansion tank back to the rad once it’s cooled (if it hasn’t done it by itself).

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What I want to know is; is there anyway that air could be entering the system when the system is hot and at pressure (1.1 bar)? Because logically the only way for this to happen is a head gasket combustion leak, as it’s the only thing that has enough pressure to force gases in to the system. But it’s passed a sniff test and the coolant is still crystal clear. No soot, no black bits. Oil is clear, oil cap is clean also.

If it’s not air entering the system, then could an immediately heat soaking radiator boil the coolant when it’s under pressure? Bearing in mind that when it’s rapidly pushing out air bubbles once I’ve switched it off, the fans still don’t kick in and the temps stay constant.

Any thoughts?

Alfetta for your time