I just finished going through a similar problem on a 86 240 DL B230F. Of all the things I did the following seemed most important.

Flame trap cleaned and sealed with a clamp. I also drilled a 1/8" hole in the center of the flame trap. (I've never had a backfire with this car and started to remove the trap all together but opted for the drill.)

Replaced small vac line from flame trap to downstream of throttle body (it was so soft the vac pull from the downstream side of the throttle body was closing the flow of air).

Cleaned the throttle body while still on (had to use a small mirror to check for build up areas, remember to hold the throttle open and clean the opposite side of throttle butterfly as best as you can). Cleaning the throttle caused the idle to increase a couple hundred RPM. I'm sure some lazier mechanics had previously increased the idle setting to coverup the need to clean the valve.

The other thing that was creating havoc was the air box. There is a mechanic switch in it that directs air from either of two origins, 1) warm air from the exhaust manifold or 2) from he snorkle next to the right headlight. I removed the spring loaded position manipulator (it was stuck in the 'from the exhaust manifold position') to ensure only air from the cold air snorkle could enter (had to tape up the warm air entrance.

Once I did that I set the idle screw to between one quarter and a half turns past (into)just touching the stop. After that I set the distributor where the engine ran the best.

Minor things I did included cleaning all graound connections, new plug wires, rotor button and cap, battery connections cleaned, checked alternator, and taped and exposed wires (look around, there are probably at least two or three - if you clean everything you will run into them).

Hope this helps. The feeling you get when the damn thing runs great is, for me, worth the effort.

I've had this happen on my '86 also. Are you sure it's not just due to the current position of the idle turnscrew?

Try this. Keeping count of turns, turn the turnscrew in (clockwise) 2 full turns, then ground the testpoint again. If nothing, you could continue turning in and testing, until you reach the endstop (keep track of total turns in)! If you still get no response, turn it back to its original position, then do the same testing turning it out (counter-clockwise), keeping track of turns, then grounding the testpoint.

You may get no result doing this, but I know in my case, I wasn't getting a change when grounding the testpoint, then I did after adjusting the turnscrew. It may well be that responses like this indicate a throttle body cleaning is in order. I've done mine on the car, but intend to remove it for a thorough cleaning this spring.

Just a hunch, but it may be due to your throttle switch setting. By that I mean the idle contact is not making. Because the Blue White wire you ground comes into the ECU like the other (Wide Open) switch contact.

I think when the ECU "sees" both IDLE and WOT at the same time, it knows it's time to shut the IAC for adjustment purposes.

But even with that hunch, I couldn't make an '86 shut down with that wire once.
--
Bruce Young,
'93 940-NA (current), '80 GLE V8 (Sold), '86 240dl, '85 244 ti, '83 245t
'76 244 (R.I.P at 255k), 73 142 (98K), '71 144 (ex circle track car)
Used '70 144 from '78 to '92, New 144 from '67 to '78, Used '62 122 from '63 to '67