Well, I just noticed this recently, ocassionally when I start my 280Z, it will idle at 650-700 instead of the 900-1000 that I have it set at, the engine kindof "rumbles" and creates a bouncing inside the cockpit... I was just curious as to what sort of symtoms these were, it is quite strange. It is EFI sensor related no doubt...

Since it's injected, I would think it's the air flow meter. My 300 does the same thing sometimes. You might want to check out the throttle valve. They can get gummed up and need to be cleaned.

Well, I just noticed this recently, ocassionally when I start my 280Z, it will idle at 650-700 instead of the 900-1000 that I have it set at, the engine kindof "rumbles" and creates a bouncing inside the cockpit... I was just curious as to what sort of symtoms these were, it is quite strange. It is EFI sensor related no doubt...

Could it be that some times the engine is cold and on a cold start . and another time it is warmed up and the idle is as set.