The next step after inspecting for leaky vacuum hoses, etc. is to read out the engine error code(s). The instructions at the following link explain a simple means to do it (your car most likely is one of those covered; make sure your diagnostic connector matches the shape and number of connections shown to be sure).
FAQ How to read older GM codes for free
One clarification on the page: when the author writes " Ground terminals A and B" in step 1, that means connecting terminal A (Ground) to terminal B (Diagnostic enable). The easiest means to do so is to bend a paper clip into a U-shape so one leg of the U goes into the A pocket of the connector and the other goes into the B pocket. You can also use the points of a needle-nose pliers, but the paper clip is more convenient. Be very careful not to connect any other terminals together, and don't forget to remove the paper clip before trying to run the engine. (When I had a 1988 Buick, I kept a U-shaped paper clip in the glove compartment just for this use.)
Once you know what the error code means (there's a link on the page to a table, you'll have a general idea of where to look for the problem. If you want to know more about what the code indicates, please provide the information in the comments or post your question again with the error code (sometimes the same code is caused by one several different parts, each of which is too expensive to replace on a "this might be it" basis).
Read more at: https://www.obd-codes.com/faq/read-gm-2-digit-obd-codes-free.php