Engine start good but will stall unless you pump the gas petal.

Hello, It is likely that your idle air control valve (IACV) is going bad, or getting clogged. An extremely thorough description of the IACV and repair procedures is outlined in the estimator tool. See the link here, it will provide you with a real estimate, and you can real all about the IACV, and I think you will agree that IACV early stages of failure replicate your situation exactly. There is a chance that a vacuum leak exists, a small one at that, but that will come to light through diagnostics. Let me know if you need anything else, or have follow up questions. When you go the the estimator link, search idle air control valve replacement: https://repairpal.com/estimator/buick/terraza