[BMWZ42023]

Hello good folks

My Z4 E89 sDrive20i 2017 89k miles Has developed an issue with a strange twist

So a good month ago now on a journey the temperature got high and I got the amber warning on the screen. I stoped and I heard the fan going full speed. I check bimmerlink for a code telling me "Engine cooling system: Motor speed, coolant pump outside tolerance". I restarted the car and went a long my journey with no problems.

Fast forward to now and after a number of test runs, trying to learn and troubleshot what it could be.

Doing the same test journey from a completely cold start the water pump speed will hover around 194 rpm and will stay that way until around 14 minutes in the journey doing mostly 55 MPH. The car gets fully warmed up around 12 minutes with the coolant temperature hovering around 220F and oil temperature hovering around 200F. once around the 14 minutes hits the water pump speed just stops as in O rpm. Then soon after the coolant temperature will go upto 255F. I pull over stop and turn the engine off. Start it back up to carry on for a few more miles to then turn around and head back home. After restarting the car I took note the water pump speed would settle down to 175 rpm and stayed that way until back home.

After more test runs on the same roads from a cold start. I've concluded there is an issue but this issue only happens from a completely cold start to when just after the car gets fully warmed up. And hear is the twist, I can stop the issue from even happening by simply switching the engine off and back on again when car gets to around full temperature which stops the water pump from stopping in the first place.

Can anybody think what it could be? I really hope it's not the water pump as it seems fine.