R107 500sl temp gauge query

[ianh2]

Hi all.
I have a 1985 500sl.
I've noted the temp gauge sitting halfway between 80 and the next,unmarked,line (100 degrees?).
I can sometimes see the needle fluctuate, and rise or fall, though not always.
It moves around in that 'quarter'of the gauge, but doesn't go past the 100 degree line.
Whilst the engine was running, after a 60 mile journey,I removed the 'radiator'cap, and was able to remove it easily. I thought I would it would be under pressure and feel tight,but there was hardly any tightness at all.
My concern is - Should I be concerned?
Is it an airlock?
Is it the symptom of a failed thermostat?
If not, is it normal?
I assumed that the temp gauge would sit at 80, but I can't recall if it ever did!
Any and all advice welcome....

 

[Xtractorfan]

All the things u have mentioned could cause the problem i would suspect an airlock...does the heater ever go cold .. and have the system pressure tested and refilled and fit a new thermostat

 

[ianh2]

thanks.
The heater is red hot on demand.
i'll have a look at the price of a new radiator cap and thermostat,.

Should she normally sit at 80 degrees then?