I'm having a similar issue with my Golf 1.6TD mk2 (for a long time, I assume). Just by reading your post I noticed I'm not in the right spot. My temperature needle sits just slightly before the 3rd mark, not in the middle (vertical) when warm. I replaced the 2 sensors (black), the radiator sensor and thermostat and it sits still in the same place. I measured temperature in the expansion tank and it reads 84-89ish. What sensors are you using? What temperature range should they be?
Need a little help here. My Golf 1.6TD is having some issues with the glow plug indicator and coolant temperature gauge.
When in cold start, the glow plug indicator (yellow) stays on for about 15s and after 45s off (engine running) it flashes 15 times. I notice that during this ~1min engine oscilate iddle rpm. After the flashing, it turns stable. When running normal (engine ~80ºC), the coolant temperature indicator is near the 3rd mark (on the overheating side), but the overheating alarm doesn't light. I check the coolant temperature, and it's normal. Cooling fan and it's (new) sensor are working fine.
I replaced the thermostat, the coolant and the 2 temperature sensors (on the front flange) but the issues keep on.
What are the recomended thermostat and coolant temperature sensors? References, please.
I'm a new member here and hope I can learn, help and share info whenever needed. I'm a happy owner fo a VW Golf mk2 1.6TD GTD from 1989. It's a white one, with 3 doors (large windows), large bumpers, Japan Racing JR9 wheels and 400.000km on the clock! Restoring since 2018 but it never ends!
but it never ends!