Don't have schematics here. What was D27 used for before? It's not recommended in the ford camps to replace the NB with a pseudo NB signal from a WB. In general, it adds a conversion delay and the ford eecs sample with an expected delay as it switches it's LAMBSE's back and forth. If it doesn't detect the mixture yet switch cause of the delay, it continues to swing the LAMBSE further than needed in order to detect a switch happening. Guys are always told to weld in a dedicated bung especially when they seem to be chasing tuning ghosts. I would think the GM PCM's work similar.
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