'99 jetta water temp light flashing

Last couple of days, after starting, the water temp light is flashing and engine is cool and it is 50 degrees outside. Coolant level was a little low. No moisture in oil. No noticable coolant leaks. Added a little coolant and drove for three hours on interstate yesterday and light began flashing again after driving for two hours. Water temp is ok. Heater is working ok. I think the thermostat and water pump is ok. Anyone got any ideas?

If it is cool outside it is possible that it would take a long time for your thermostat to kick in. When that happens the coolant from your resivoir drops. If it drops too low then your light flashes because of the sensor in the resivoir. My advice is to pull over the next time immediately after it happens and check your coolant level. My guess is that it is dropping just past the minimum mark.

Make sure the coolant you use is red not green. If you mix the two you have a problem and need to flush your coolant for a while to get rid of the sluge.

Good luck