Help Overcooling 1.8 v50 2008 B4184s11

[ST420]

Hi

First of all the car was taking ages to warm up to running temp and would lose temp as I was driving all the way to cold so assumed thermostat faulty.

Replaced thermostat and coolant, bleed and all good a day or 2 later I noticed I would lose half engine running temp so I wondered if pump or coolant temp sensor after trying bleed procedure again.

So I looked at the pump looks brand new and all works as should and replaced coolant temp sensor. Still the problem is there

Today I noticed that if I the heater fan is off the car will run at normal running temp whilst driving and all seems well but if I turn it half power I'll drop one notch down from running temp and full power 2 notches down so about a 45 degrees to the left of what it should be turn it off and will build back up to running temp.

I'm wondering if this model has a coolant bypass/shut off valve?

also if my coolant cap could be at fault as I noticed no hiss from expansion when opening hot so not building pressure?

also never hear the rad fan kick in either but thought that would lead to over heating not over cooling?

any help appreciated

thank you

[mcfe]

Firstly, log the car with some diagnostic tool to be sure the problem really is the engine being cool or a faulty instrument cluster.

Secondly, have you replaced the coolant temp sensor? And third, does the engine fan turn on when this engine temp drop occurs?

[ST420]

Ok will try diag to rule out cluster but I doubt it as it does run up to temp when stationary, yes I have replaced the coolant temp sensor and the fan never comes on I've also tried disconnecting the fan to rule out coming on whilst driving sill have same results