[W126 Coupe] 1985 500SEC stalling under load

[Andrew Seidel]

Group,
Down to business:
My Lorinser had a complete engine and transmission
rebuild about 10k ago where all plastic and rubber
bits were replaced, spark plugs, distributor, etc.

It was working fine for months (so rebuild not
suspect), until it came back from the body shop (SUV
spun out in front of me in a Colorado blizzard).  They
told me it would not start while at the body shop and
their indy replaced the ignition control unit.  It
now starts cold/hot perfectly but takes a good minute
of turning over to start warm.  Further, it
idles perfectly, fuel pump making usual sounds, and
does not stall on decel (a la classic idle
controller).

However, it dies whenever I get on the gas hard and
grey smoke comes out the tail.  I can get going any
speed I want, so long as I keep light on the gas pedal
-- dangerous starting out at an intersection though!

With the Mighty Vac, I've tested: warm-up compensator
limits, idle control valve (even swapped in another
one), EGR valve limits.

Help!  Any idea the cause and other tests to try? 
What subsystem should I suspect -- too lean or too
rich?  Sincere appreciation for the help, Andrew Seidel.

Andrew Seidel
1986 560SEC
1985 500SEC
1983 300D Turbo
1975 240D

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