I just bought a 1990 Camaro RS auto with the 305. The tachometer reads a 5000rpm idle and stops working when driving. Could this be a faulty iac valve or just a short? Thanks

 

Welcome to the high tach club! The tach on my old 91 read double, and when doing research at the time I found that many 3rd gens are way off like that. Basically, if you have a 3rd gen with an accurate tach, you have a rare one. One reason may be a resistor within the tach itself is bad, another could be with the ignition module inside the distributor (where the tach gets the signal). The IAC would have nothing to do with the high reading.