Termostato, refrigerante para VOLKSWAGEN Golf III 1.9 TD 75cv a bajo precio (spanish or french)
Yours is a 87ºC, and your thermostat needs replacement because is not closing.
VW dsahboard coolant temp indicatro says 90ºC if real temp from coolant temp sensor says 70 to 102ºC, so if yours says 80 fuel sensor is getting maybe 65ºC.
MAybe you need also a replacement for the coolant temps sensor
Sensor temp. refrigerante para VOLKSWAGEN Golf III 1.9 TD 75cv a bajo precio
Ask your repair shop to change thermostat and coolant sensor and put VW G12 coolant (G12+ it is a good option). IT's organic, not water with chemical that reacts when you mix different cheap coolants.
I'm using a 12€ per 5L like this:
IADA · Productos
Technical data:
http://www.iada.es/pdf/producto/ft_glycogel_50.pdf
Safety data:
http://www.iada.es/pdf/producto/fds_...o_amarillo.pdf
It should be liquid from -37ºC to 145ºC.