I have noticed the check engine light goes off a lot on these little buggers. MY Girlfriends 97 Tracker, my buddys 99 Vitara with the 1.6, and my 96 Tracker all have the check engine light go off here and there. HERE is n easy tip.
Disconnect your battery overnight.
You should only need to disconnect your battery for 2 hours.

On the 97 and 96 Tracker it is probably a problem with the EGR valve. You can go to pretty much any auto parts store and borrow their OBDII computer and see what the error message is. I'm not sure if AWK has a troubleshooting section on his faq, but I ended up tracing the problem down to a bad EGR Relay Solenoid Valve on the back half of my motor. Now that I know the procedure, it is pretty easy to test for.
Start up the engine when cold and reach around the EGR valve and squeeze it. Your engine should die, or try to die. This means the EGR valve is ok. If so, then work your way down the vacuum system using the diagram under the hood. Just pull each line off and use a vacuum test tool, or just give it a good suck. The EGR valve will probably open up. The EGR Relay Solenoid Valve should be closed when the engine is cold. If your wondering, just pull the power and see if it has 12 volts. If it does and you can still pull air through it, the relay is busted and needs to be replaced. Hawk Suzuki is probably the best bet to get the parts, as you can just tell him if it is the front or rear one.

As for the 91, no idea. I would try replacing the 02 sensor, if not you might have to take it to a shop.