The short version: What, on an '89 DPFI 1.5l engine, does the ECU use to
determine target EACV opening and idle speed? What would cause the ECU
to suddenly decide to command a full-open EACV without also indicating an
error?

The long version:

I've got an '89 Civic Wagon, 1.5l DPFI, 2wd, 5sp, and its idle speed
bounces randomly up and down between normal and 1-3k RPM over normal.

The "check engine" light is off, and the ECU displays no trouble codes.
The EACV has been cleaned and then replaced (twice!) and the ECU once
with used and known-good parts, and the symptoms are identical in all cases.

We've spent some time driving around with a portable oscilloscope wired
up to the EACV and various sensors. When the idle shoots up, it's
because the ECU is actually commanding the EACV open. Readings from the
MAP sensor, TPS, ELD, and O2 sensor look perfect compared to the graphs
in the Helms manual while it's doing this. We've made briefer checks of
the intake air and coolant temp sensors and they look OK; more
importantly I've driven with them disconnected and the periodic idle
jumps still happen.

Running the car with either the EACV disconnected or with its bypass air
hose plugged results in the expected low idle and no other problems
except for possibly a 15% fuel economy hit. (What with rapildly changing
gas formulations, though, it's hard to be sure the fuel economy issues
really stem from the engine control problems.)

My best guess is that there's some intermittently failing sensor that's
important enough to the ECU at idle to make it want to whack the throttle
open. The thing is, outputs from what seem like the obvious culprits
(MAP sensor and TPS) check out fine at both the sensor and the ECU
harness. Wiring and vacuum hoses have been thoroughly checked. The
RPM/crank position sensor hasn't yet been checked during operation, but
my understanding is that they usually either work perfectly or fail badly
enough for the ECU to throw an error, but it's on the agenda for the next
time we have a few free hours that aren't rainy.

Any educated guesses would be deeply appreciated. Anyone who can provide
a fix gets both my undying gratitude and (if you want it) a free bonus
used-known-good EACV!

thanks,
andrew