the tach works but it not right saying 3k at idle and the oil psi guage dont work at all.what could it be?

 

The factory tachs on Camaros suffered from this problem(not sure about Firebireds).Check all the connections first.But the tach driver board failure was the most likely problem(actually resistors on the board).Best bet is to find another tach unless you can ID the bad component.Do a search as there are many threads on this topic.
On the oil pressure gauge you could have a loose or bad connection at the sending unit or the sending unit itself could be faulty.

 

i forgot to search my fault. but thanks

 

on the oil my 91 z28 would work somewhat then 0 reading then 10, 25 then nothing. i purchased a new sending unit got underneath, checked the wires they were ok, pulled the old one out and replaced,,,,wow 30-40 psi works like new. thanks bruce