Small update: a friend of mine sent me this apparently there is a hidden plug underneath the plastic bit seen in the pictures above. I saw it on a couple other GM ecus from around this time period. Is this possibly a way to interface/program the system?
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The plastic bit is usually called a "knock filter." In older ecm's the knock filter contained circuitry to filter noise from the knock sensor so the ecm could focus on real knocking. The plug is standard to other GM ecm/pcm units using a similar knock filter design. I don't know of anyone using that plug to access the code.
It is not a data port, it is an analog filter plus an amplifier, and an analog-to-digital converter.
Same part is used on the 2.8/3.1/3.4L P66 PCM's. You have a P6-style computer, but it's got P66 style programming. Not currently well studied-but you might start with the flash routines for the 94/95 OBD 1.5 computers used for the 3.4L V6 and LT1 V8 computers.
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