Thanks, guys, for all your replies!
I think I’m starting to understand how the cooling/heating system works in my Merc, just need to get used to it.
@TimC300 Thanks for the diagram, it makes more sense now.

 

Another observation…
Today it was −10°C during the day. I started driving without any warm-up and with the heater turned off, and the engine reached operating temperature in just 5 minutes, which is great.
As soon as it reached operating temperature, I turned the heater on and the gauge almost immediately dropped from 90 to 60 degrees. After that, it took another 10 minutes for it to climb back to 90.

 

Turning the heat on opens the heater valve located on the back of the drivers cyl head. The coolant temp sensor is also in this area.








Coolant then flows to the heater core in the dash, also flows to the windshield wash reservoir heat exchanger if the car has that option.

 

Interesting… Does this mean that once the heater is turned on and coolant starts flowing through the heater circuit, the gauge no longer shows the actual engine temperature?

 

No. Just that coolant is directed to extended elements that permit heat from that coolant to be rejected, thus lowering the overall temperature of the coolant. The engine coolant sensor is registering this change accurately.
The heater matrix rejects engine coolant heat to the interior of the car in order to raise cabin ambient temperature, and the screen wash matrix rejects engine coolant heat to the liquid in the screen wash reservoir.

 

It depends on what the coolant temp is. We would look back at the info posted above. If the coolant temp is 80-115 then the computer will configure the dash to show 90. The point to showing the temp at a constant 90, or any temp, is so the driver isnt constantly looking at the temp gauge going up and down if it did show actual temp.




Yesterday I started driving and it was 37 F outside. I had the heat off and it took 7 minutes for the dash to show 90 C. It stayed at 90 C so I turned the heat on, after a few seconds, 5-10 seconds maybe, the temp started going down but did not go under 80 C, then it slowly went back to 90 C but it took a few minutes. Im not positive but i think the coolant temp at the sensor did drop below 80 C so the computer calculated that and was in the process of lowering the dash display to show actual coolant temp, but then the coolant temp got hotter after mixing with the warm coolant and the computer reconfigured it to then show the 90 C.

I actually like the feature of the dash showing a constant temp if the coolant is within working range. I worry too much and if the temp gauge were going up and down id be focused on it more. As it is I recently got a nail in a tire and put a plug in it so im always watching the tire pressure screen. The plug is holding great.






I mapped out how the coolant flows in my M272. 948 engine with heated windshield washer fluid. Hot coolant from the engine exits the rear drivers side head at the shut off valve, hot coolant flows to the heater core and to the windshield fluid reservoir, the coolant (now colder) then flows to the small electric coolant pump near the coolant reservoir, from the pump it goes back into the engine connected to the radiator hose. I never paid attention until now.

The coolant pump is not connected to the coolant reservoir at all. At least not on mine. Coolant pump just has the three hoses.





Some more info. The heater valve is normally open by a spring. When actuated/energized the valve is shut closed. So if the valve were to stop working it should be stuck in the open position. I think the electric coolant pump would need to be on for coolant to actually flow though.