I'm struggling with high idle on a 200k mile 1986 240 with LH 2.2 and AW70. The idle is high in cold and hot conditions, probably close to 2500 RPM.

I started by smoke testing the intake side but didn't find any obvious vacuum leaks. I disconnected the charcoal canister circuit anyway, just to be safe. Then by pinching the intake side of the IAC plumbing I was able to get the idle down to something like 500 RPM based on ear (no tach) when the IAC was removed from the air intake circuit. Similarly, grounding one of the pins on the diagnostic port on the driver's side (which disables the IAC) would also drop the idle to very low levels.

So, thinking I found the culprit (a stuck open IAC), I measured resistances between pins and found thousands of ohms between the pins. A normal IAC is around 20 ohms between 2 of the 3 pin combinations, and 40 ohms on the 3rd combination. So I disassembled the IAC as the LH 2.2 units are rebuildable. The internals were actually very clean and not gummed up, however, I found giant gouges in the stator surfaces and extremely worn and oxidized armatures from arcing. This seemed consistent with the high resistance values. I then cleaned away the oxidation and reassembled the unit, which now measured the correct 20 and 40 ohm values. The unit sort of worked after that, though I would need to knock it with a screwdriver occasionally to get the idle to move into the correct speed. I attributed this to the armatures getting "stuck" in the deep grooves they wore over 200k miles.

At this point, it seemed the best option was to replace the IAC. Bosch units are NLA, except for used unknown units on eBay, so I bought one of the fairly common aftermarket IAC's from IPD. Upon installing the new unit, the idle remains high in almost the same way. Unplugging the battery to force the ECU to relearn has not helped.

If I blip the throttle enough to disengage the TPS, the idle will drop to a normal speed for about 1 second and then slowly build back to up around 2500 RPM over the course of about 5 seconds, almost like the IAC is slowly opening all the way up. I can get the idle to drop to normal levels if I ground the diagnostic pin or pinch the hose, same as the first IAC.

I'm looking for ideas on what my next steps should be. It seems like the IAC circuit is working, but something is driving the ECU to open the IAC all the way open. The car drives normally otherwise, and gets what I consider decent mileage (around 16 mpg around town). No other indicators of issues that I can notice. I've previously done most of the stage zero items (plugs, wires, air filter, distributor, PVC clean, fluid changes, etc).

Any thoughts as to what I should tackle next?