question ?? 420SEL

thanks for the input concerning the smoking problem. i'll be having bot the modulator valve and valve guide-seals checked tommorrow (if it were the valve guides wouldn't it be using oil.
today the starter smoked and i had to be towed in for repairs. another question though concerning the cruise control. it appears that the control circuit works and holds the engine rpm at around 2000 to 2200 rpm but it will not latch in mechanically and regulate the speed of the car. scenario goes like this, at around 60 mph i set the cruise switch, it does not give any indication of working so i shift to neutral and the rpm is holding at 2000 rpm. i click the cruise to off and the rpm drops back to 900 or so, pull it back in gear and resume speed, it has wprked in the recent past and might be a vacuam leak but i don't know how this system work exactly, any suggestions as to where to start.

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