Drop in Idle and/or stall when gas released

okay....I suspect the throttle position sensor has gone off~ https://www.google.com/#q=1989+Chevrolet+C%2FK+1500+throttle+positio n+sensor

Your idle air control valve is either dirty or faulty.

the problem is there are lots of possibilities. Dont go replaceing things randomly, have them tested. Possible thins are: TPS, IAC, vacuum leak, fuel pressure regulator, bad injector, worst case scenario the pcm has messed up. You have already taken care of basic tune up items so thankfully that is out of the equation. I have a love hate relationship with the gm TBI engines. I love the reliability but most problems have the same symptoms and its impossible to just look at it and replace one thing, You gotta test components

My 95 X-71 305 engine is doing just about the same thing I've replaced every sensor u can think of but as long. As your going down the hwy it runs good so null not one person on this site can give me any insight on what it is so good luck.

Having same issue. Replaced map sensor, cts, egr valve and accuator, basic tune up including rotor, cap, wires and plugs. Checked both sides of plugs after running for 2,000 miles. Both sides were chocolate brown. Cleaned iacv and it looked damm near new. I'm thinking it might be a bad wire harrness. Connectors are a little rough. Don't have a wire tester to check voltages.

try replacing the TBI gasket and go from there.

I had same problem check EGR valve and check for vacuum leaks with carb cleaner around base of throttle body listen for idle drop around vacuum lines.

same problem.. ran perfect after a basic tune up( pugs wires cap roar ect. variables for me... i got a new catalytic converter and a muffler( flow master and a magna flow... now it s idle is so low it stalls. and doesn't seem to have the power it used too.