I have an 89 IROC 305 5-speed, the tach is eratic when driving, you know the RPMz do not match the speed. The tach readz like 4000 @ idle which I know is around 700/750. And when the car is off the tach reads all the way maxed. Where is my problem? it has done this for a while but has gotten worse and worse until like now where it just reads maxed.

Thanx...............Bombero

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1989 IROC-Z Convertible, 305 TPI 5-Speed 97,000 milez. all stock inside and out, No Modz, very clean ride!

 

my 84 z28 is doing the same thing.....but all at the same time the engine lights quitting coming on when you turn the key to the on position but as soon as you start the car the choke light comes on and stays on........also my cruise quit working at the same time but while driving and turn on cruise it pegs the tach out.........

Have you noticed when car is running and you use your turn signal does the needle bounce any........

I've been told it's a bad ground somewhere....I'm still looking if you have any luck at all please send some info my way and I'll do the same