Just solved this exact same problem in my '89 (2.8L). <img src="/forums/images/graemlins/lol.gif" alt="" /> Things to check (in order of hassle/cost): First, check all the vacum hoses for leaks. Second, check to see if the idle air speed motor in the throttle body is dead. Third, check for leaking throttle body gasket. Fourth, check for leaking intake manifold gasket (NOTE: check the INSIDE of the manifold also - by this I mean toward the middle top of the motor). Fifth, check to see if the control assembly for the throttle body plates is loose. Part of this assembly is a shaft which goes through the wall of the throttle body toward the bottom. When I checked mine, it moved A LOT and the idle speed changed as I moved it! The bushing around the shaft was worn-out - acted just like a vacum leak. The bad news is that you can't get a rebuilt throttle body for a 2.8 ANYWHERE! You can rebuild it (kit is around $135.00) but this bushing is not part of the kit. The good news is that a throttle body for a 4.3L GM bolts right on with minor mods to the linkages. I did this to my '89 (used throttle body from a '90 GM cost $85.00 from a wrecking yard) and it runs like a striped-ass ape!