I have a couple of ideas, but nothing conclusive at this point since I'm still trying to track down this exact set of symptoms in a friend of mine's '89 740 Turbo (which is also LH-2.2).

First, swap the Idle Air Control Valve (IAC) with a known good working unit that you removed from your buddies car. -or test your current valve in someone elses car. It's important to rule out the valve first before moving on. The 501 and 520 IAC valves were used in the LH-2.2 vehicles and are completely interchangeable. However, the 516 is unique to the LH-2.4 cars and is not compatible with the earlier LH-2.2 system.

Second, with a good IAC in the car, and with the engine running high, try to pinch closed the air tube that goes into the IAC valve. If your fingers are strong enough to pinch this shut (it can be really tough on older tubes), you may notice an RPM change. Use pliers wrapped in a rag if needed to close the tube.
---If the RPMs do not change, then the problem is probably in the Throttle Body, it's linkage, or the black thumbwheel is open wider than it should be.
---If the RPMs drop when the tube is pinched closed, then the Fuel Injection Computer is telling the Idle Air Control Valve to be wide open. (continue below)

If the Fuel Injection computer is causing the problem, it may be getting faulty data from somewhere that's causing it to think it needs to be running this fast. Check the resistance of your temperature sensor and make sure it is within spec. Swap your Air Mass Meter with a known functional unit with the same part number (007 in your case). If these have not correct the problem, try swapping the Fuel Injection Computers. I have had limited success with this final solution of swapping Fuel Injection Computers with a known good one. The car seemed fixed for a few days, but then the high idle speed returned. I haven't yet had the opportunity to get my ECU (FI computer) back from my friend with the problematic car to test it to see if the ECU has been damaged permenantly.

My next plan is to start backprobing the ECU harness connector pins and look for anything suspicious (like a shorted or opened wire connection in the harness, or power on some line where it shouldn't be). I'm planning to probe the TPS pins (throttle position sensor), the AMM pins, the coolant temp sensor pins, and anything else that comes to mind while looking through the wiring diagrams.

This has been quite frustrating, as I ran into this same problem before on an '87 240 wagon two years ago. Since the engine needed to be replaced anyways (had sustained severe damaged due to running the oil dry), I didn't bother to track down the problem. While I was replacing the engine, I also replaced the IAC valve, the engine wiring harness, and the Fuel Injection Computer all at once and the problem was gone. Hopefully, we won't need to go to this kind of an extreme for the cars that we're working on currently.

God bless, and please let me know if you find a solution.
Fitz Fitzgerald.
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'87 Blue 240 Wagon, 268k miles.
'88 Black 780, PRV-6, 149k miles.