My guess would be that he was able to see various FI parameters
functioning in "real time".  E.g. see what the oxygen sensor, AFM,
injection timing etc. were doing as say, the throttle was opened.

Possibly there are other signals available that are designed solely for
diagnosis work that he could access through his computer that told him
where to look for what was wrong.

I'm no expert but I am very interested in this aspect.  Perhaps you
could ask him next time you see him?

-jeff

Bill Marshall wrote:
> If the digijet doesn't store engine codes, then how did a mechanic use a
> computer and program to find the particular problem on my '85?  I watched him do
> it.  In fact, the first question he asked me when we came in was, "Has the
> battery been disconnected?"
>
> Perhaps they are not engine codes in the same way modern cars have, but none
> the less the info is still there.  He pinpointed the problem in minutes.
>
> Bill Marshall
> 85 GL Tiico "Pandora's Box"
> Aurora, IL
>