weird idle problem

[Blackitalian]

I have a 87 turbo and i had my car hooked up to the ignition scope and its saying my car is idling at 700 rpm, but my needle is pointed at 400 rpm. sometimes the needle goes down to almost 0 rpms but runs perfectly fine and doesnt die at all. It never starts to putter or act like its going to die. Sometimes the needle will go real low and then quickly jump back up to 700 rpms. does anyone know what could be causing this?

[Ski300zx]

Sounds like the sending wire to the tach either is bad or is shorted. I dont have the wiring diagram handy, but it has to connect somewhere on the negative terminal of the coil, and then into the tachometer or ecu. not sure if the sending wire for the tach goes to the ecu or straight to the tach, ill check the wiring diagram at work and get back to you.

[Blackitalian]

thanks alot bro, ya i couldnt figure it out cuz it just does at it at random times but doesnt affect the driveability at all