Wait... That doesn't make sense. There are only two possible results, the engine stalls or it doesn't stall. Even if everything was perfect, one of those two things would have to happen when you plug the IACV intake, so they can't both indicate a problem.

Yes, I'm aware of that. I have a full DPFI intake manifold sitting on my garage floor, and four more throttle bodies in a drawer. I'm saying, take the metal plate (the one with the PGMFI label on it) off the air chamber above the throttle body opening, and cover the port that the flexible tube draws air from, rather than trying to pinch off the tube or something.

How do those two clauses indicate anything else? My statement followed the pattern "If... If not..."

Yeah, I figured that out after leaving the site. Just didn't come back to edit things.

If the engine stalls, the IACV is bad... If the engine doesn't stall, then there's a vacuum leak.... What would the engine do if nothing is wrong? See what I'm saying. With only two options, an engine with nothing wrong will have to do at least one of them....

The engine stalled by the way (although in retrospect, I don't think I unplugged the IACV before I checked). Maybe the IACV is bad, but we bench tested it last week and the actuator motor responds to voltage perfectly, and the current draw is smooth and consistent on the oscilliscope as the plunger moves back and forth. Also, with the throttle plate completely closed, I can't see how blocking all the major intake air to the engine (via blocking the IACV tube) would cause anything but a stall in any circumstances. Maybe I'm missing something vital, but everything sure looks like it checks out to me.

Anyway, over the last week or so, I've replaced/readjusted the TPS, Added an additional return spring to be sure the throttle is closing that last few fractions, readjusted the idle, and reset the ecu several times. None of those things made any huge difference on their own, but the accumulated effect has been that the fast idle on startup is down to a consistent 1500 rpm, and it eventually drops to between 700 and 950 rpm. It still pulses a little occasionally, but not like it was. If it's not steady at 7-800, then it'll only bounce by about 150 rpm or so and not get up over 1000 rpm. Considering how it was, that's a major improvement. I'm going to drive it for a few days to see if it stays that way.

If everything were working properly, the engine would have stalled when you disconnected the IACV and backed off the idle set screw. If the engine keeps running under those conditions, air is leaking into the manifold somewhere it shouldn't be. If blocking off flow through the IACV makes the engine stall, that means the IACV isn't closing completely... Air is leaking through it under conditions that should cause it to fully block air flow.

Gotcha. So it's not sealing properly even though the actuator is functioning properly. I can buy that.

Anyway, it stalls when the IACV port is covered no matter wether the IACV is plugged in or unplugged. So I guess I'll check the local parts stores for an IACV (not likely that they'll have one). I'm assuming from looking at it that there's no way to service the IACV and replacing it is the only option.

Thanks for all the help. This was def. a PITA to track down. In summary, the problem was caused by an accumulation of 1) a bad IACV, 2) a bad tps and 3) slight binding of the throttle linkage in the last tiny bit of travel. I'm not sure why they didn't cause problems when the car had an auto trans, but did when I swapped in the manual, but I was definitely barking up the wrong tree for a while as a result. With luck the new IACV will resolve the last little fluctuation and idle adjustment issue and I'll be back to good again.