I'm a Ford guy but the fords & GM trucks are a lot alike. You'd have to see what they call it in chevy language but there's something called the curb idle valve on a ford. It gets plugged up with crap and will give you a poor idle. You might want to pop the distributor cap and check the spindizzy to see if any of the contacts are badly burnt. Maybe some condensation in there too ?
If all else fails, and you can't think of anything else to check, pull the ECM and look for any burnt spots on the board.
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