the OBD 1 system was pretty simple compared to now.
i think it now comes down to checking all the vacuum lines and closely inspecting all electrical connections and wiring for insulation cracks that may have allowed the "green fuzz" to start growing in the wire strands. that stuff has caused me tons of grief over the years because at times it is so hard to detect. if the engine harness is somewhat stiff, it is easier to detect, but you may still have to look real close.