Another thermostat and temperature question

[n_hudson]

The temp gauge on the dash is like a yoyo. The car warms up normally, balances out half way (normal operating temp) then will drop to 70 then back up... its like a yoyo. 

I had the thermostat replaced and its still doing it. (I assume the sensor was replaced too). 

It doesn't matter what type of driving I do, stop start, motorway... its the same either way. I don't notice a difference in HVAC temperature when its dropped or hovering at normal.

I found a few things on google, suggesting air in the system and to bleed it, but Im guessing the 1.6 tdci is a self bleeder?

Thanks for any suggestions peeps... 

[unofix]

11 minutes ago, n_hudson said:

(I assume the sensor was replaced too)

Never assume always ask - it makes an A*s out of you and me, as my old boss would say.

Is the electric cooling fan switching on to soon or staying on to long ? The tempature on my Mk4 hardly ever gets to the mid position. I would say with a high degree of confidence that there is no air in the system. The cooling system on your car is fitted with an expasion tank, any air would find its way there and the coolant level would drop and need topped up.