I have a 91 Z 5.0 TPI that had been sitting for a few months. When I start the car it idles well, but the tach reads 2000 RPM, and I know that it is not running that fast, maybe 1000 RPM. The tach worked well when I parked it (had to replace the B&M shifter cable), but now reads too high (it seems to be a progressive error: off by ~1000 at idle and ~1500 at higher revs). In addition, the car starts to sputter after about 10 minutes of idling and then dies. I assume that the injectors might be clogged, since the car sat for 3 months (in Florida), and that could explain the sputtering (as could needing new plugs), but the tach issue combined with the other problem makes me think of the ignition system (module or distributor). I am not getting any trouble codes from the computer. Any help would be greatly appreciated.

Also, how can I adjust the computer to compensate for the engine modifications that I have, originally I assumed that the computer would compensate for increased fuel and exhaust flow, but now I'm not sure. I've got headers and a 3" exhaust through Flowmasters, a crane cam, ported the intake manifold, and various other bolt-ons including upgrading the ignition. Thanks in advance for your help.

 

Mine did that last year after sitting in the garage for a couple of months , i just started to sputter and then it died after maybe 15 minutes. I had to wait several hours before i could start it again.
I changed the distributorcap and the problem was gone.