Volvo: engine revs up and down, vacuum hoses, vacuum leak

Question
B230f question
my 1992 940 non-turbo had a rough idle then started only idling down to about 1600 rpms then up to 2000 rpms and down to 1600 then up to 2000 etc. at first this happened once in a while. now it does the reving even while traveling down the road.  replaced spark plugs, distributor cap and rotor and this seemed to help with the rough idle. not much other work don in a while.
any ideas ?

Answer
It sounds like a dirty or clogged throttle body.  They get filled with soot over the years and reduce air flow like a clogged artery, a lot of times causing the engine to surge like you described.  I would definetly take a look at it and clean if neccesary.  Also, surging can be caused by a vacuum leak, so it might be time to carefully inspect all vacuum hoses under the hood.  Good luck!