S204 'overheating'

[SpikyMikey]

So, having not used my car for a couple of weeks (S204 2013 petrol with the M274 lump), I took it out today in advance of a run round the M25 tomorrow.
About 10 minutes into my trip to fill up, the temperature gauge suddenly shot up to 120 degrees and the rad fan is running like a jet engine.
There's no high temp. warning light on and the car is running fine otherwise. The temperature gauge sits at the normal 80 degrees at idle but as soon as the engine is under load the needle fluctuates randomly between 80 and the readline.
Coolant level is OK (or it was before I set out; I'll check it again later when things have cooled off a bit). No obvious leaks however.
Any bright ideas from the Mercedes collective?

 

[Benzowner]

What water pump is fitted? I had an S class which did similar things and it turned out to be the plastic veins inside the water pump had disintegrated and were effectively not moving the water around the system.

 

[SpikyMikey]

What water pump is fitted? I had an S class which did similar things and it turned out to be the plastic veins inside the water pump had disintegrated and were effectively not moving the water around the system.

Thanks; I'll flag that up to my garage when it goes in. Hopefully it's the pump and not the thermostat which apparently is a swine of a job on the M274.

 

[Petrol Pete]

Don't forget your Aircon is 'connected' to the engine cooling fan , the condenser is right alongside the engine cooling radiator. Is your aircon working properly ? If the temperature of the condenser rises the fan will kick in. Worth checking.

 

[SpikyMikey]

Don't forget your Aircon is 'connected' to the engine cooling fan , the condenser is right alongside the engine cooling radiator. Is your aircon working properly ? If the temperature of the condenser rises the fan will kick in. Worth checking.

aircon is icy cold and on auto seems to regulate its output correctly so I think its OK, but thanks for the thought

 

[SpikyMikey]

Thanks; I'll flag that up to my garage when it goes in. Hopefully it's the pump and not the thermostat which apparently is a swine of a job on the M274.

Sooooo, car went in yesterday to have the problem diagnosed. They've just called; the thermostat is cream-crackered. New bits on order from MB. The tech at the garage confirmed it's a pain of a job.
Surgeon on standby to remove whichever arm and/or leg is necessary

 

[SpikyMikey]

Sooooo, car went in yesterday to have the problem diagnosed. They've just called; the thermostat is cream-crackered. New bits on order from MB. The tech at the garage confirmed it's a pain of a job.
Surgeon on standby to remove whichever arm and/or leg is necessary

Update: Benzowner wins the prize. Water pump is also 'cattle-trucked'