1992 jimmy check engine light came on and is now running very rough

Change fuel filter, oxy sensor, and add can of SEAFOAM to tank of med-premium gas. sounds like EGS(ethanol gas syndrome). It causes problems with emissions.fuel, and internal engine. it dumps goop nd water in the systems. it will mess up an oxy sensor and converter quick. and it seems to affect GM products the worst. try what I said, and see if that will help. when they read the code. it will probably be emissions problems(like a PO300, etc)

Too old to be complicated ,look for split or damaged vacumn pipe on the inlet manifold ,also a thin pipe runs to the ignition coil module and this controls the auto advance unit .This will throw the light on and cause it to run a bit rough