my 88 golf is overheating , cannot determine cause
Find a cooling system pressure tester, buy rent borrow, if you must, pay, one that goes on the radiator cap and pumps pressure in and look for leaks
And I don't know if it has a serpentine belt with spring loaded tensioner or an adjustable belt(s) but in either case make sure belt tension is right
It's the sensor/thermostat. Replace it.
Thermostat and/or the coolant pump impeller. Pump impeller is made of plastic and after the thermostat dies the pump gets so hot it breaks into a bunch of pieces. Belt slippage is most likely your tensioner being over or under torqued. Is the belt drifting left and right at all on the wheel or do the grooves align perfect?
make sure aint a air lock other wise flush it through with a hose change water pump and thermo stat. pump is only £30 and a stat only £10