What idle speed control did the early efi systems in 1991 use?

Question

what was common

Answer 1

Well, as far as early Throttle Body Injection systems (in contrast to Ported systems), an ISC (Idle Speed Control) motor was originally the most common method. This was actually a 256 position "stepper" motor, which physically manipulated the throttle linkage, and through that the throttle plate, as opposed to the modern IAC (Idle Air Control) valve, which controls an indirect air bypass built into the throttle body. The IAC was however used on some later TBI systems.

Of course at that point in time especially, which system a car had varied depending on which motor the vehicle had. MPI was originally more often found on more performance oriented packages. TBI historically speaking, was mostly a transitional system that made use of existing (or lightly modified) 2 barrel Intake Manifolds and such, lessening the need for massive re-tooling on a short term basis, as well as being simpler in general for the electronics available at the time.

Additional Details

Yes, I clarified my post to reflect that. If you wanted a definitive answer, maybe you should have been more definitive in your question, as to model and engine. Clayton was indeed wrong, in the sense that he implied there was never any other method. Any time a person makes a blanket statement about an entire model year, they're almost bound to be at least partly wrong.  :-)

Answer 2

prob. the same as they do now, a valve located on the throttle body