I know the early 90's 4 Runner had problems with the exhaust valves and cylinder head gaskets. A compression check and cylinder leak-down test should tell you the condition of your engine.
Water is clear clean coolant green.
Foam is clear clean white. I notice no leaks. However, I am always having to add water to the radiator. Much steam from rear tail pip in the mornings. I have been smelling the radiator water.