Go figure, i dont drive my '91 vette very much [only 61,000 miles]. it  isgaraged, no sun, or rain. So, i get in, and start car for its weekly 'run-up' . It starts, idles fine. Seems like it's 800 rpm. BUT, when i look at the tach, it's reading 3000 !!!!!!!!!!! after a while, the 'Check guages' light comes on. What da heck ???

knn,
  Do a search on the forum or google for c4 corvette tach filter.  You probably need to replace it.

Mick