Hi, my 94 G20 van with a TBI 350 has just started idling rough once the motor warms up.
It's normally quite smooth when in open loop but has always had a slight fluctuation in manifold vacuum when idling in closed loop of about .5".
Just in the last couple of days the van has started to drop 1" of vacuum at idle and then occasionally stall, the van seems fine driving around, at this stage the problem appears to be just at idle and only once it's warmed up and in closed loop.

For testing purposes what's the quickest way to disable closed loop at idle within a $OD bin?