andrew m. boardman wrote:
> 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,

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...

> 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.

sure it would!

> More to the point, though,
> it does this even with the coolant temp sensor disconnected.

ok.

> (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.)

ok, but that's output - you need to find the input that's triggering
this response from the ecu.

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. and i'd test the ecu output with a potentiometer wired in place of
the sender. it's supposed to range between about 20k and 100 ohms, with
lower resistance being the higher temp, so you can read the effect it
has on ecu output accordingly. that area of the head tends to get badly
fouled with oil leaks from the distributor, so check the quality of the
connection as well.

if still no dice, i'd check the electric load sensor.

>
>
>>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.

that just corresponds with open eacv.

> 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.)

right, but the system also tolerates a degree of noise, so a full
disconnect is different to something fuzzy or high resistance.