Idle change overnight (no idle fluctuations)

My 1973 Citroen DS 23 IE (D-Jetronic) is running well. But one problem I am trying to resolve is that I would set idle properly according to specs (after warm up, etc...) and the car would idle fine.

Next day, or sometimes later in the same day, the idle will change by a few hundred RPM. There is no fluctuation in idle up and down as in the symptom of a bad auxiliary air valve (AAV), rather, idle is steady, but varies up or down over time, and remains steady at this particular RPM (in semi-automatic Citroens, there is a low and high idle, both go up or down by the same delta).

Could this still be an AAV issue, or? 

Any pointers will be greatly imprecated!

Samuel
  

Hi Samuel,

could well be a sticky AAV yes. And that will create big problems on DS semi-automatic if idle is not exact.

Thank you Dr DJet ... much appreciated!

I disassembled my spare AAV and will clean and test. So, far it looks like the wax thermostat cartridge is defective.

Best
Samuel
   
 

Hi Samuel,

well a defective thermo element is the normal case. It is dangerous to pull out upper part of AAV. That is fragile and aluminium cast can break easily. So be happy that you managed without breaking anything and without damage on fitting.