m30 throttle stutter/hesitation

[Lurker27]

When quickly applying throttle from idle, the engine stutters for a second, then revs up to correct itself. It takes a good 3-5 seconds to fully return to idle. Gradually increasing the throttle results in seemingly normal engine function. The stomp test throws a 1215 code, so i'm thinking bad AFM or a vacuum leak that i have yet to identify.

-e30 m30 auto-manual swap
-idle is solid at 750-800rpm
-checked for vacuum leaks with propane torch - no obvious leaks
- cleaned AFM, ICV
- currently has only downpipes
- no intake before AFM
- plugs have been cleaned but are slightly fouled

any help would be greatly appreciated!

[Lurker27]

Update: high fuel pressure caused by blocked return line - can I just run a new line that dumps into the fuel tank or is there black magic in the positioning of the outlet at the bottom?