I own a 1990 VW Golf 1.8. It’s been acting up recently, over heating and spewing out back-flow through the radiator
overflow.
I changed the thermostat, but how do I detect whether the problem is a blown head gasket, plugged radiator, or defect in the water pump. All time consuming and difficult to get to!
What color is the coolant? If it is not orange, that may be your problem. I believe the 1990 models were using G-12 coolant and using other coolants could cause problems. That could include a plugged or corroded radiator. Was the water pump replaced the last time timing belt was replaced? How long has it been since the last timing belt replacement?