scotch
+1y
I don't think a bad coil could would be getting you up to 1,500 RPMs. That would require air. So I think you have a vacuum leak, bad IAC, or possibly a computer as it regulates one side of the IAC valve.
Did you check all the vacuum lines around solenoids? Also check them at the intake manifold as that is the hottest part of the line and probably the most likely place for them to become brittle and crack or break. Check around intake manifold gaskets. Also throttle body gasket and around the intake pipe.
Could be a bad IAC valve. One side is automatically controlled by a spring so that once the engine gets up to operating temperature, it is fully closed (on that side of the valve.) However, the other side is constantly adjusted by the ECM based on conditions. So if the valve goes bad, or the solenoid/motor that controls it, then that could cause erratic idling also. Since it did change somewhat when you unplugged it, it's possible that the IAC valve is bad. Try checking the resistance of the connecter to the IAC. At around 73 degrees, it should indicate 7.7-9.3 ohms. It might vary a little more than that if you are significantly warmer or colder, but it probably won't be dramatic.
Once last thing to check is to pull the computer and take a look at it. It's located behind the passenger kick panel. Open the top of the case and take a look at the circuit board. Look for the two capacitors that are about the diameter of a AA battery but only 1/4 as tall. There are only two that look like that - thought they might be different colors. If you seen anything on the circuit board around them, then they are leaking. Sometimes it's real obvious and sometimes it's so light than you could easily miss it. The area under and around these to capacitors should be as clean and shiny as the rest of the board.
Wouldn't hurt to check the plugs. Having the wrong plugs can cause some weird problems. It's not uncommon for owners of B2200 EFI truck to get sold the wrong part because the parts clerk sells them parts for a carbureted truck.