[ChevyTech]

On this system the ECM looks at the TPS voltage when the key is turn on and remembers that voltage a zero percent throttle open. If you move the throttle stop screw so the TPS voltage goes higher then the ECM logic says the throttle is open so do not move the IAC. When you back the stop screw out after a engine start the voltage goes lower so the throttle is not detected as being open so the system will run the IAC. Every time you adjust the throttle stop screw the engine has to be turned off and restarted for the system re-determine the throttle position.

If you have a habit of holding down the throttle when you get in to start the truck and the ECM does not see a TPS reading with the gas pedal not being pushed it can cause all kinds of problems including the IAC issues.

The MAP is the same way in that the system looks at the reading when the key is turned on before the engine is cranked and uses that reading for the Barometric pressure reading.

Ignition switch failure is common on these trucks and I wonder if it is breaking connection as you turn the key to start it and losing the base reading from when the key is first turned on. Have you noticed any issues with the ignition switch?

The MAP reading is manifold vacuum, not ported vacuum.

The normal MAP reading varies depending on variables like altitude, engine condition, and weather so there is not a tight specification. The rule of thumb is 1 to 2 volts at idle. It should go above 4 volts with the key on and engine off.

There should be a desired RPM somewhere in the data.