1988 244 DL; B230F; LH-2.2; Manual 5-speed

This is a continuation from my post on a Stumbling Problem that has me now working on a high idle.
see
(http://www.brickboard.com/RWD/volvo/1560041/220/240/260/280/fyi_stumble_resolved_running_problem_unsteady_idle_rich_mix_blowing_black_smoke.html)

The upshot being ...
What will cause and engine to idle high when the throttle body air control knob is closed?

Other posts would say this is a vacuum line / intake air leak ... but based on below and previous line checks and tests of the intake manifold I am not finding any place for air to get in.

I'm afraid I may still be working with the same O2 sensor, IAC, AMM loop from my "stumble resolved" post.

Below is the work done so far.
--------------------------------------
Pour water over the throttle body gaskets and intake air manifold gaskets:
- no change in rpm.

Check Throttle Plate:
- fully closed

Replace:
Oil Filler Cap gasket. New gasket is more flexible than old gasket.

Check IAC/Vacuum leaks:
Block air path to IAC. Car will start and idle though obviously at a low and unsteady rate.
- Tighten hose clamp on intake side of IAC.

Check major air paths by blocking at IAC (coming and going), Charcoal Canister (EVAP), Oil Vapor Trap Inlet with sandwich bags.
*All blocked - Car will start but not hold idle.
*Open IAC outlet - will start but not hold idle.
*Open Charcoal Canister - will not start.
*Open IAC inlet - will start and hold idle.
*Open Oil Vapor Trap inlet - still running.
- Idle does not sound lower.

20.26 mpg

Check/Adjust Idle:
Very little reduction in RPM when AIC is grounded. Was able to close Idle Air Control Knob a little more (from where I thought it was previously closed) and lower RPM a bit but it still is 750+ not 700.

water injection clean engine:
Run 1 cup distilled water through engine.
Drive car. Repeat w/ another cup of distilled water and drive till tail pipe is dry.

replace:
Swap original (first) Oxygen Sensor installed while working on stumble with (second) O2 sensor installed during emissions work in 2011.

Engine may start more readily and sure seems quieter and smoother.

21.16 mpg

Mileage was 22 +/- mpg.
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1988 244 DL; B230F; LH-2.2; Manual 5-speed