Yes, check this page: http://www.muscularmustangs.com/test1.php
'90 can output codes through CEL
. Look at the diagram in the middle of the page; all you hook up is the "SIG RET" to the "STI" with a small piece of wire or a paper clip - then put the key on 'Run'.
First test will be KOEO
(hard faults), separator pulse, then CM (codes in memory it's seen) codes will be output.
Might check the fuel pressure to make sure that is in spec, that can have you in circles if it is too low or too high. Also make sure you are getting spark. If it starts on a little bit of starting fluid, most definitely fuel problem.