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Even if you did have compressed gasses leaking into the coolant, it doesn't seem like it would be "foamy".
Have you put in fresh anti freeze and water? I'm guessing that your mix might be somehow contaminated with something??? like SOAP!?!
Quite the mystery. Anxious to hear the cause.
- Did you recently flush the cooling system? The post on contamination may be the clue.
- Air pockets can cause what they call cavitation where the fins on the waterpump "whip" up the coolant into a foam. Boats can have this problem where the water can actually boil by the prop.
- If it continues I would drain the system, install the backflush kit, drain and flush the heck out of it. If you use a readiator flush solution, make sure the water coming out of the fill neck is CLEAR. Then refill with fresh coolant.