It sounds like your fuel injector stopped working. You might try checking to see if the fuel injector itself is working (or clogged). Does it BUZZ (TBI Injector) when the key is turned to start the engine?(you can disconnect the starter solenoid wire [black w/yellow stripe on the starter itself] to determine if it is working without the engine actually turning over, [the ECM does not care if the engine is actually turning over or not].) Be sure you are hearing the TBI buzzing, and not an other emissions component buzzing!!
If the engine runs with a squirt from some starter fluid(and only from the starter fluid), then it would seem that the ignition and starter do not seem to be the problem (spark is being delivered on time). The injector is either clogged, not working, or NOT the problem!! Assuming the fuel delivery system (ei. fuel pump, etc.) is working properly!!
At 123 miles since I converted to FI (I also am using an '91 tracker 8v TBI, and ECM.), the the fuel injector stopped just as you described. I have used the touble shooting chart in the FSM and have not been able to pin point the problem. In the trouble shooting section, the problem seems to be short in the ECM's Blue w/black stripe wire. (I hope this is not a permanant or destructive problem) This problem did show up on the ECM checklist on page 6E2-C1-2 in the Driveability and Emissions section of the 1991 Geo Tracker Service Supplement [the additional stand alone, supplement]...I used a volt meter that has an auto scale meter to try to troubleshoot. Unfortunately, the volt meter has an auto scale function that is too slow to see momentrary changes in voltage. (USE the trouble shooting light as described in the FSM or you will miss important info.) Once I used the light instead of the volt meter, I was able to determine a short in this wire (somewere in the blue w/black wire which connects to most of the ECM functions.) I'm gonna pull the wiring harness and try to determine were the short is!!!!
If your ECM is lost then it will cost big $$$$ to correct this!(ECM replacement or repair). I have two ECM's (one being a rebuilt ECM w/warranty) and it does not work (engine runs) w/ either ECM. WHEN I figure this out, I will post the results. I'm sure more people than I have experienced this problems!!
Whazuki -> a squirrel squashing, deer smacking, driving machine!! [img]/wwwthreads_images/icons/wink.gif[/img]