Hi Chris,

I admit to helping Bruce debunk the myth that insists a dead tach needle on cranking is a useful diagnostic. On the other hand, a live tach needle (unless your engine shaking is so bad it knocks the needle off the stop) is a useful indication of coil switching, and hall-effect or crank sensor output.

Rather than make a number of tests on one car, I do this by knowing how a tach works and its likely limitations. The bouncing needle (whether yours bounces during cranking) could be affected by the tach's sensitivity individually, or how low the rpm can be to reliably move the needle, and the cranking voltage and cranking speed in your particular car that particular day.

Rex gets what it needs regardless of the fuel injection relay, and it is not a great idea to try to run Rex without the coil wire in place, especially Rex with its expensive and not so hearty coil. In much the same manner, a preheat hose cannot affect cold starting regardless of coincidence observed.
--
Art Benstein near Baltimore