Code 41/91

KOEP? Do you mean KOEO? You must be reading the Continuous Memory codes, i.e. 41-C.

There are 3 categories of codes:
O - KOEO, Key On Engine Off
R - KOER, Key On Engine Running
C - CM, Continuous Memory

Disconnecting the battery will clear CM codes. My code reader can do it, too.

No gasket. I was able to test for blockage by attaching a length of hose where the diverter valve hose connected to the hard line. Then blow through it. Thought about pulling that hard line, way too much work for nothing.

I went through the same Wild Goose Chase as you. I did find a bad TAD valve, but it solved nothing. I read your other thread, my ordeal is not unlike yours. I've got a thread going too, it's a big one.

Mine wasn't setting any codes at all, until I replaced the distributor yesterday, needed the PIP, only, but distributor wasn't rebuildable. New dist (PIP), has same road symptoms & other new erratic ones, however, it set Code 14-C, bad PIP. Must get replacement distributor today. Obviously, they don't test these things before leaving the rebuild factory. Cardone is the rebuilder. Don't really want to spend $300 for a distributor: Ford, MSD, Accel, etc.

Go back to Old School Basics: Air, Fuel, Spark. Need all 3 to make Fire.
However, in your case, since it starts & dies, & you've replaced many parts, I'm going to say it's the damn PIP, just like mine. The PIP (Profile Ignition Pickup) is responsible for way more than it should be: engine rpm, crank position, cylinder identification. See Probst p.71. Could also be the TFI, p.120. The TFI is effectively an amplifier for the PIP. Don't expect OBD-I to be that precise, it was the first attempt at engine control management. I suspect it just doesn't have enough time time to get the code for the other side before the engine dies.

Most mechanics replace both the PIP & TFI. I have 2 TFI's, get same results with either one, which indicates they are good.

I have found, that if it's related to electrical in any way, there are few experts. I've seen engineer's eyes glaze over when trying to explain electrical issues. Electrical can do Mechanical but not vice versa.

BTW, you probably should have posted this in your other thread. I feel your pain. My supercharged engine is virtually back to stock & problem still not solved.