Help! I've had an 80 Z28 just sitting in a garage for the past 18yrs. After a little work got it running and it runs smooth. Problem is the tach is at zero when idling but jumps to 3000 + when rev’d up, then drops back to zero. Bad tach or tach filter?? I do remember when trying to start the car years ago the tach spun around a couple of times and thought that was strange. Also fuel gauge always reads full, I dont believe I have a full tank after 18 yrs??? Any ideas on what could be causing the problems on these two gauges.