NEED HELP 91 BUICK REGAL

Check for vacuum leaks and clean or replace Idle Air Control valve or Mass Air Flow sensor depending on which one your vehicle has.

There are several things that could be wrong. Places like Auto Zone can do a scan and give you an idea of what might be wrong. The 3800 engine used during that time could have a cam sensor problem there is a magnet on the end of the cam that has been know to fall out, the engine then goes into a limp home mode and runs bad, poor gas mileage. Replacing the magnet solves the problem. Those engines also tend to have ignition coil controller problems, look of "goo" coming out of the crack under the coils.