Coolant thermostat reaching 100 while driving up hills

[Headon92]

Hi, I have a mk5 golf 1.6 tdi, I recently noticed the fan coming on every now and again when I come to a stop.
While driving recently I noticed the coolant thermostat on the dashboard sometimes climbs from 90 up to 100 while driving up long hills, going anywhere from 55 to 70mph. I have taken it to a garage and the mechanic ran some tests and did a compression test? he was under the belief that it was the water pump which has now been replaced but I am still in the same situation. Could anyone shed some light on what could cause this?

Thank you

[OEM+ DUB]

Hi there

If the waterpump has been replaced, for me the next likely culprit to look at is the thermostat (the one that controls the flow of coolant around the engine only when cold and then allows it to circulate to the radiator when hot).  Also, have you heard your radiator fan running?  Worth checking whether this is operational as if failed this will not provide additional cooling air over the radiator to bring temps down when in traffic.

Given what you say about coolant temperature on the gauge going up when climbing a hill, i.e. on the move, I'd be leaning towards the thermostat.

Hope this helps.

[Danthefourwheeler]

I just got my thermostat replaced a short while ago as a precaution, temperature runs perfectly, nice warm air coming out of vents, its worth a look

Sent from my Pixel 5 using Tapatalk

[mjmallia]

There is also an inline thermostat near the front of the alternator..........and another one if you have DSG above the box