jim beam <nospam@example.net> wrote:
>hmm, the #1 cause on this group is insufficient coolant level. check
>inside the radiator - if there's the smallest leak, air sucks back
>rather than coolant, so the expansion bottle level stays the same.

Yeah, I spent some time reading through the archives and was impressed by
all of the ways that low coolant level can make the engine misbehave.
Unfortunately, the coolant level is good, overflow isn't dropping after
use, and it's been bled by the book.

It's also jumping around very sharply; the EACV signal will jump from
3.5v or so up to 9v, sit for a bit, then back down to 3.5v; I don't think
a temperature sensor could react that fast. More to the point, though,
it does this even with the coolant temp sensor disconnected. (Voltages
quoted are the difference between the two EACV wires; it gets a constant
+12 plus a control line from the ECU which varies between +12 and ground
depending on how far the ECU thinks the EACV should be open.)

>insufficient coolant level in the block means the coolant foams and
>gives spurious temp readings to the ecu, so it hunts back and forth
>between "cold" and "normal" idle behavior.

What might also be important here is that the engine is being revved up
way way beyond what might be called for even with a very cold engine;
seeing it pop up to 4k RPM is not unusual. I suppose there's some chance
that flaky coolant temperature sensor wiring is making the engine look
really really really cold, but not far enough out of spec to flag the
sensor as failed. (It does flag it as failed when unplugged, though.)