Temp gauge fluctuation

[Dell]

Hi all
The past two weeks my temp gauge has been fluctuating it used to be rock steady an 90 but now it it goes up to about 80 within 10 minutes then drops to about 70 then goes up to 80 and keeps moving up and down between 70 and 85.
Last July I had New thermostat housing and I think that when you purchase it comes with sensor and stat,I have checked wiring connections has anyone any idea as to the problem C 200 CDI 2010

 

[grahamcol]

Could be the stat failing again.

 

[John Laidlaw]

Certainly behaving like a faulty stat

 

[Dell]

Thanks for replying
Yes that's my thoughts as well but I just strikes me as odd that temp gauge keeps going up and down faster than a tarts knickers on overtime.

 

[grahamcol]

Merc thermostats aren't very good. Both my W202 C280 and R170 SLK had new stats fitted soon after I bought them due to the stats tending to stick open and causing the engine temperature to be too cool. It's a very common problem.

 

[Dell]

Update
I have noticed that if I stop and turn off then immediately turn on and drive off the temp will then go to 90 after a couple of miles but I can drive forever and it will not go to 90.

 

[grahamcol]

Update
I have noticed that if I stop and turn off then immediately turn on and drive off the temp will then go to 90 after a couple of miles but I can drive forever and it will not go to 90.

In my experience they become stuck open when the engine gets really hot and will then only close properly after the engine has gone stone cold again.