4g15's using EFI were/are very known to have the following weak points - distributor (which is mainly the coil we see fail) and ECU which can cause all kinds of gremlins.
CSM Shaun and I are poking at the ignition and related controls, the ECU.
We need to know the condition of spark when it won't start.
For reference, all the factory service manuals are located here: http://www.lilevo.com/mirage/
<-89-92 (4g15 mainly) and 89 Colt.Mirage Turbo (4g15/4g61t) are both located there and so are the specs if you are testing individual components.
So, when the ECU caps leak, which they WILL DO GUARANTEED (if never changed and acid damage stopped/repaired), is the acid will leak down the circuit board as the caps heat cycle and deposit a bad smelling and caustic chemical bath on the traces, chips, and whatever happens to get in it's way.
The ISC driver can be blown, the spark circuitry (as affected on my first car), and many more ghostly things can happen as a consequence.
There is an entire rebuilder industry focused on early Mitsu boards from 1989-1994ish time frame due to their capacitors being essentially defective. They were all made like this.
Now that you know a little history about what can go wrong, we would need a little input about the spark when the car does not start, and what the ECU looks like internally.
A spark tester can be setup readily on top of the motor and can probably be seen from the front seat as the motor cranks.
Next up, is removing the ECU from the right front kick panel. Remove trim, two 10mm bolts from the ground straps/case grounds, and find a way to dislodge the ECU pulling very sharply in a downward motion; be warned they are wedged in good and will have to back the same way!
The outside ECU case scews are very tight, so have a proper fitting phillips screwdriver and use your weight on them as you turn.
Once inside, you will find something like this:
GO HERE - http://vfaq.com/index-main.html
Select the DSM site
There are headings at the top - select ELECTRICAL
First post is Capacitor replacement - select and scroll down for pics.
Ha, Shaun and I just posted at the same time.