Thanks again for the replies; sorry about the long delay in answering the
latest. (Some unexpeted travel, then some unexpected getting-sick.) I'm
back under the hood of this thing, though, so here are replies to
suggestions that were offered while I was in la-la land:

>also, make sure your "hard" idle setting is correct - disconnect the
>eacv plug and set idle for 750 with the throttle body adjusting screw,
>just like it says in the honda manual.

It's within spec, though that spec is 625rpm (+/- 50) in my Honda manual
with the EACV unplugged and no load, 750 with the EACV plugged in.
(Well, sometimes it does 750 just fine, sometimes it does 1800+.)

>> Unfortunately, the coolant level is good, overflow isn't dropping after
>> use,
>
>but it should! level in the expansion [overflow] bottle should go down
>as it cools. and you /did/ check inside the radiator, not just look at
>the bottle? need to be sure...

Sorry, I meant it wasn't dropping between checks with the engine cool.
The overflow bottle fills up an inch or two when the engine is hot, then
goes back down when it cools. I did indeed check the radiator (full up)
and the bleed point (bleeds, no bubbles) after driving. I've also just
put another thousand miles on it, which I'd hope would chase any bubbles
out.

>i read before that you disconnected the temp sender, but there are three
>on this vehicle: one for the radiator fan, one for the gauge on the
>instrument panel and one for the ecu - just check you have the right
>one.

I do believe so. "TW sensor" per the book, 20k ohms cold and a few
hundred hot, just under the distributor, six blinks on the ECU when it's
disconnected. (But even disconnected and blinking, the ECU pops the EACV
mostly open from time to time.)

>I don't know if you have a Fast Idle Valve, but on multi-port cars this is
>the prime culprit.

Nope, no Fast Idle Valve. (Also not multi-port; throttle-body injection
with stacked main and and aux injectors.) I do have a dashpot diaphragm
that cracks the throttle during cranking, but that's not misbehaving
either.

New data after 1k+ miles: I'm really confident the coolant system is
bubble-free. Mileage is definitely down the toilet, though, so it
appears that the ECU is not only opening the EACV but increasing injector
opening as well, which makes me think one of:
- throttle angle sensor (ECU is seeing "full open" when it's not)
- crank position sensor (ECU is seeing "high RPM" when it's not)
....even though neither of them is far enough off for the ECU to indicate
an error.

Side question: the Honda manual says "replace throttle angle sensor" if
it's bad. It doesn't have instructions for doing so, though, and a quick
look at the sensor on the throttle body shows what looks like a pair of
rivet heads. Is this actually replacable without swapping the entire
throttle body?

I've got a friend coming by with a multi-channel digital recorder this
weekend; we'll wire up *all* the ECU inputs plus the EACV control and see
what corresponds to the EACV getting thrown open.