Is my fuel expansion tank haunted?

[kronus]

Anyone familiar with how the fuel vapor expansion tank is plumbed in an E30? Something about mine is throwing the M50 out of wack...
With the purge valve connected to the throttle body, the engine will have an uneven idle and eventually stutter and throw a 1221 code. With it disconnected and the TB nipple blocked, it idles evenly and does not throw any codes.

I've tested the purge valve (closed, opens with presence of +12V), charcoal canister (air goes in and out of both top connections and bottom of canister), and the purge line (air pressure from engine bay into purge line will pressurize the tank).

At this point I'm not sure if something about the expansion tank is plumbed wrong, but the purge valve's effect on the motor is repeatable and consistent. I've replaced every motor accessory and have done a full top end rebuild, so I am confident it's not anything motor-side.

Body is a 91 318is. Motor is a late production M50TU.

[jeffnhiscars]

The tank sits in the rear passenger wheel well and I think it should be in vacuum rather than pressurized since the goal is to draw fuel vapors in for combustion. Have you checked your Gas fill cap and the vent lines around the tank to make sure Nothing is obstructed ?