Coolant oddity

[Gloscs]

Just changed the water pump and temp sensors on my 2017 TSi 2.0 petrol. After refilling with coolant I ran the car for 3 mins at 2000 rpm, then left the car running for about 15 minutes and noticed that there was no coolant returning to the expansion tank and the fans had not come on. The temp was steady at 90 according to the gauge. Having used the car since and the temp gauge is all over the place. Anyone have any ideas? Thanks

[1.4_tsi]

I suppose there must be significant air locks in the system. You definitely need to run the engine until it reaches full temperature, ie when the radiator fan comes on.

VW use a pump system which creates a complete vacuum in the coolant system, then allows the system to fill by suction. In the absence of that, for some engines, VW recommend pressuring the system up to 1.5 bar to remove air bubbles prior to warming the engine up after filling

[Gloscs]

Hi 1.4_tsi,
Thanks for replying. I came to the same conclusion eventually, after watching a few YouTube videos. I removed the expansion cap and squeezed the water hose beneath the air inlet a number of times for about 5 minutes, eventually there were air bubbles coming up into the tank. I disconnected the return hose and upon starting the car coolant flowed. Reconnected and running up to temperature the hose got hot and the fans came on at 120C. I was surprised at how much effort was required to shift the air, maybe filling more slowly may help.
Regards