astonishing! Even in hot areas a thermostat should be fitted, to regulate the coolant temperature. Driving without thermostat may only be a compromise as the thermostat does not open (completely) so, that the engine overheats.
I don´t believe, that the (quick) boiling depends on the ratio of coolant fluid and water.
It rather might be a result of inadequate bleeding.
Another very bad reason for the coolant boiling could be a burnt cylinder head gasket.
