Hello I have just bought a 2020 S-Max with the 2litre Diesel engine. 

 

I have noticed that if I stop for a while with the engine running say waiting for the Mrs to pop into the shop. The engine temperature drops from the middle back towards the blue. If I'm stationary long enough it will drop back into the blue. 

Is this normal?

 

Many thanks 

Diesels can considerably cool down in cold weather, when stationary

If it starts doing it while driving then suspect a stuck open thermostat

A stuck open thermostat could be it, or your aero grill is stuck open and you were pointing into the wind. How far had you driven and was it from a cold engine start? If you have the interior heating on, it's effectively acting like a fan on the main radiator, so yes, you can cool down the engine that way when it's really cold.

It's a trick owners use in older cars with big engines that like to overheat when idling in hot weather (at lights or in heavy traffic etc). The main fan might be on full trying to cool the engine coolant down, but if the system isn't particularly great or built for that sort of temperature/climate, the temp gauge will continue to rise. So by turning on the interior heating on full heat and fan speed (during the summer with windows down), you are adding a secondary radiator to the cooling circuit. It works, but you'll sweat buckets doing it! 

I have an SMAX 2017 2.tdi 150bhp. How long are you idling for and is it within 5 miles of starting off from cold? If so, your oil and coolant temps won't have reached 50°C yet, but your dash temp will tell you everything is at normal operating temps.

If you then idle for 20mins rather than drive under load and you have the interior heating on at a fairly high fan speed, the temp gauge will re-adjust down to the real temp as you are moving heat from the engines coolant into the car. 

In these last few cold UK days, I managed near on 22 miles before my oil hit 90°C and coolant just before. I have an OBDLink MX plugged in permanently to my OBD port and run the "Car Scanner Pro" app to monitor temps, battery state of charge and DPD soot levels (to name but a few). What the car doesn't tell you on its dash (because you don't need to know say Ford), this app and others can.

If your car is fine on longer journeys with no temp issues, you are probably fine. If not, get an OBDLink OBD reader and monitor the temps yourself. 

Hope that helps