[MBZ] Idle speed changing

[Peter Merle]

My '79 non turbo 300D idle speed sometimes surges for no apparent reason
. It in fact happenen once when I was watching the engine and there was
nothing to explain the cause - eg aircon/linkage play/ fan come on  etc
. I can only think than something in the transmission is causing a load
change on the engine? Any ideas ??
Peter