Weird overheating issue

[tchalikias]

Hello all, it's been a while since I've posted here, alas I'm back with a problem with my trusty '09 200.

Today as I was returning home after work, I got the dreaded 'engine overheating' notification on the dash, and indeed, engine coolant temp was approaching dangerously the upper end of the gauge.

I immediately pulled over and put the cabin heater on full blast with the windows down. The temperature started to drop, and I was able to return home.

The facts:

-no fault codes stored
-I can hear the fan working overtime (on its high speed mode)
-coolant level is at max. strangely, the coolant tank was barely warm to the touch. I know you're not supposed to do that, in case you get sprayed with boiling engine coolant, but I went ahead and carefully undid the coolant reservoir lid, obviously there was pressure and some coolant leaked out, but again, it wasn't hot at all! Certainly not 60-80 degrees, as it should be.

I'm gonna be taking it to a garage tomorrow, but this has me a bit stumped - I expected either to find zero coolant in the reservoir, or for it to be boiling, either due to fan failure or a leak somewhere in the system... I know the water pump is driven by the timing belt, however can the impeller just fail and stop moving coolant around? All I've ever heard regarding water pump failure is leaking (no leaks that I can see, coolant reservoir at max).

 

[RSRowe]

It’s far more likely to be a stuck/faulty thermostat.

 

[tchalikias]

Apparently my brain has turned to mush after a full day's work.. indeed the thermostat seems to be the most likely culprit. For some reason, I thought that since the cabin heater works, the thermostat must be working, i.e. allowing coolant to circulate... but then I remembered that the thermostat only blocks coolant flow towards the radiator, and not towards the heater core! Duh.

 

[Foxspeed]

It’s far more likely to be a stuck/faulty thermostat.

most likely this and the stats are an issue on them... but usually over cool the engine

 

[tchalikias]

To be honest there have been a few instances during the past couple of years where the engine seemed to run too cool (i.e. temperature gauge not in its 'regular' middle position, but lower). But in those cases had I chalked it up to low outside temperature and highway driving. Perhaps the stat was on its way out and decided to give up the ghost in the closed position? We'll see after I can secure a good quality replacement part. Until then, it's gonna stay parked.