Hi. Welcome to the site.
Not a bad start, at least as far as first posts. The mask ID is a shorthand way to identify the program your computer is using. I believe yours is actually $0E, but don't get too worried are three different ones used in your pcm. WinALDL is free and plenty of people use it, but if your going to go beyond basic diagnostics you should consider TunerPro which does both logging and tuning. Either way, the datastream definition you're using should match the mask or you're liable to get incorrect data.
First question: Is the rebuilt engine a stock replacement with stock cam?
Second question: Why is your timing 4 degrees advanced? Is that spec? Most trucks are 0 with some early ones set at 6 deg? I don't remember any set at 4. Then again I have trouble remembering an anniversary even after 22 years.
Third question: Is your engine stalling? If you're concerned about an RPM drop but the engine isn't stalling, and it's a stock replacement, it's probably worth it to learn what idle speed the computer wants to see when the truck's in gear.
Fourth question: Does the PCM recognize that the vehicle is being shifted into gear?
Fifth question: Is the A/C OEM or is it an aftermarket installation?
The PCM only has a couple of methods for correcting for load at idle. Increasing spark advance is the fastest method and increasing IAC is the second fastest method. This signal, if present, indicates the need to rapidly increase timing. The ECM will typically increase spark advance slightly and increase IAC slightly when the selector is shifted out of "P" or "N." It will then begin attempting to achieve a desired idle speed. Loads are accounted for through the manifold pressure sensed by the MAP sensor, and sometimes by additional sensors. For example, if a power steering pressure switch is present that signal triggers a small increase in advance to make up for additional load. If system voltage changes, there are corrections in place which attempt to account for the change. Typically stock applications do not show dramatic rpm changes with a couple of volts difference but modified systems can be subject to larger variations. Ultimately, you'll want to get scanning software working to see what the ecm is doing. Switching to Tunerpro will allow you to share recordings of your data (called logs) so others can view them. Switching before you get comfortable with another software package will probably reduce your stress level.
HTH
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