andrew m. boardman

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

jim beam

andrew m. boardman wrote:
> 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

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.
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.

jim beam

andrew m. boardman wrote:
> 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

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.
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.

jim beam

andrew m. boardman wrote:
> 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

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.
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.

jim beam

andrew m. boardman wrote:
> 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

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.
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.

jim beam

jim beam wrote:
> andrew m. boardman wrote:
>
>> 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
>
>
> 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.
> 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.

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. the eacv provides supplemental
air in addition to the "hard" idle setting - it's not a substitute.

jim beam

jim beam wrote:
> andrew m. boardman wrote:
>
>> 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
>
>
> 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.
> 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.

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. the eacv provides supplemental
air in addition to the "hard" idle setting - it's not a substitute.

jim beam

jim beam wrote:
> andrew m. boardman wrote:
>
>> 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
>
>
> 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.
> 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.

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. the eacv provides supplemental
air in addition to the "hard" idle setting - it's not a substitute.

jim beam

jim beam wrote:
> andrew m. boardman wrote:
>
>> 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
>
>
> 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.
> 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.

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. the eacv provides supplemental
air in addition to the "hard" idle setting - it's not a substitute.

andrew m. boardman

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.)

andrew m. boardman

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.)

andrew m. boardman

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.)

andrew m. boardman

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.)

jim beam

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.

jim beam

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.