My friend took her car to our local mechanic when her heater stopped working. They fixed the heater but there was foam in the coolant when they checked it. They told her it was a cracked block or head. Strange thing is, she isn't leaking oil, the coolant isn't low, and the car doesn't overheat. It seems to run great, but everyone says foam in the coolant is a very bad thing. Is there anything else that could be causing this?
That doesn't mean the engine has a crack. If the vehicle is running good, drive it. As far as the coolant, tell your friend to keep an eye on it to make sure it stays full and check the engine oil to make sure it isn't mixing with the coolant. As far as the bubbling, there could be an air pocket or air in the cooling system.
I always say, if it isn't broke, don't fix it. If the car runs good and there are no signs of a problem, drive it.