ECT question
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Joined: Feb 2022
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From: New Dominion, PE
ECT question
Hi:
Arrived back in upper Canuckistan from winter hibernation in FL....
Truck fired right up with no problem. Gas mileage is still horrible. doing some tests on the ECT sensor I put in and the thermostst to kick the summer
season off.
When the truck is cold, I removed the ECT sensor wire and probed it with key in the ON position. Get 5V so the ECU is working.
I then put the meter on Ohm's and put the probes on the 2 connectors on the ECT sensor. got 35K which is in spec for temp of around 40 degrees out.
I then fired up the truck and brought it to operating temp (guage on dash in center) and the upper rad warm indicating the thermostat had opened.
Shut truck off and re-measured the ohm value of the ECT sensor. Got a bit over 3K which for 176 degrees is in spec I believe. Note I used a
laser temp light on the neck that the ECT screws into to confirm the reading in Ohms against and they jive.
Question, should not operating temp be around 185 for the truck to be considered at operating temp? I'm wondering if my thermostat is opening
up too soon and this is messing up the ECU.
Can anyone tell me what the ECU logic uses to switch over to closed loop mode?
thx
Wendell
Arrived back in upper Canuckistan from winter hibernation in FL....
Truck fired right up with no problem. Gas mileage is still horrible. doing some tests on the ECT sensor I put in and the thermostst to kick the summer
season off.
When the truck is cold, I removed the ECT sensor wire and probed it with key in the ON position. Get 5V so the ECU is working.
I then put the meter on Ohm's and put the probes on the 2 connectors on the ECT sensor. got 35K which is in spec for temp of around 40 degrees out.
I then fired up the truck and brought it to operating temp (guage on dash in center) and the upper rad warm indicating the thermostat had opened.
Shut truck off and re-measured the ohm value of the ECT sensor. Got a bit over 3K which for 176 degrees is in spec I believe. Note I used a
laser temp light on the neck that the ECT screws into to confirm the reading in Ohms against and they jive.
Question, should not operating temp be around 185 for the truck to be considered at operating temp? I'm wondering if my thermostat is opening
up too soon and this is messing up the ECU.
Can anyone tell me what the ECU logic uses to switch over to closed loop mode?
thx
Wendell
Last edited by mackendw; Apr 25, 2026 at 10:38 AM.
Thread Starter
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Joined: Feb 2022
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From: New Dominion, PE
F....the americans converted me 
when I did the thermostat, I used a new housing from Rock Auto (Motorad) which came with what the kit said 192 deg thermostat. From what I'm observing, it is opening at
a little over 170 degrees and keeping the engine at no higher than 176 degrees. I wonder if the unit is another CSFC.
when I did the thermostat, I used a new housing from Rock Auto (Motorad) which came with what the kit said 192 deg thermostat. From what I'm observing, it is opening at
a little over 170 degrees and keeping the engine at no higher than 176 degrees. I wonder if the unit is another CSFC.
Last edited by mackendw; Apr 25, 2026 at 11:46 AM.
Thread Starter
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Joined: Feb 2022
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From: New Dominion, PE
AI seems to indicate that the temperature of 132 deg F is where the ECU will switch over to closed loop operation. This *MAY* be based on the calibration code.
which I don't have....
which I don't have....
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Hotshot
Joined: Jul 2020
Posts: 10,519
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From: Near Salt Lake City
Read up about what brings on closed loop operation
The O2 sensor switching is what determines when the processor goes into closed loop operation
Without the O2 sensor "switching" there is NO closed loop
You just about need a breakout box and a DVOM to watch the O2 sensor switch on your 1987
or
You can backprobe the sensor and watch the voltage closely
Processor sees a voltage signal coming back from that ECT
Hot operating temperature.60v is 194 degrees @ 2.8k ohms, (.64 volts will turn on the cooling fans in a Taurus for instance)
3.51 volts is 50 degrees F @58.75k ohms
1987 Ford should have a 192-195 thermostat in it
I'd buy a Motorcraft thermostat for it on Ebay NOS needs a RT 1152 IIRR
The O2 sensor switching is what determines when the processor goes into closed loop operation
Without the O2 sensor "switching" there is NO closed loop
You just about need a breakout box and a DVOM to watch the O2 sensor switch on your 1987
or
You can backprobe the sensor and watch the voltage closely
Processor sees a voltage signal coming back from that ECT
Hot operating temperature.60v is 194 degrees @ 2.8k ohms, (.64 volts will turn on the cooling fans in a Taurus for instance)
3.51 volts is 50 degrees F @58.75k ohms
1987 Ford should have a 192-195 thermostat in it
I'd buy a Motorcraft thermostat for it on Ebay NOS needs a RT 1152 IIRR
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Thread Starter
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More Turbo
Joined: Feb 2022
Posts: 657
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From: New Dominion, PE
Read up about what brings on closed loop operation
The O2 sensor switching is what determines when the processor goes into closed loop operation
Without the O2 sensor "switching" there is NO closed loop
You just about need a breakout box and a DVOM to watch the O2 sensor switch on your 1987
or
You can backprobe the sensor and watch the voltage closely
Processor sees a voltage signal coming back from that ECT
Hot operating temperature.60v is 194 degrees @ 2.8k ohms, (.64 volts will turn on the cooling fans in a Taurus for instance)
3.51 volts is 50 degrees F @58.75k ohms
1987 Ford should have a 192-195 thermostat in it
I'd buy a Motorcraft thermostat for it on Ebay NOS needs a RT 1152 IIRR
The O2 sensor switching is what determines when the processor goes into closed loop operation
Without the O2 sensor "switching" there is NO closed loop
You just about need a breakout box and a DVOM to watch the O2 sensor switch on your 1987
or
You can backprobe the sensor and watch the voltage closely
Processor sees a voltage signal coming back from that ECT
Hot operating temperature.60v is 194 degrees @ 2.8k ohms, (.64 volts will turn on the cooling fans in a Taurus for instance)
3.51 volts is 50 degrees F @58.75k ohms
1987 Ford should have a 192-195 thermostat in it
I'd buy a Motorcraft thermostat for it on Ebay NOS needs a RT 1152 IIRR
>>. where can a fella read up on the logic the PCM goes thru in transitioning from Open loop -> Closed Loop..is there an online publication that describes
>> this in some detail? I'm sure the engineers who wrote the code flashed onto the CMOS chips on the PCM are long retired or dead.
>>. the truck is not getting to 194 degrees for sure. I ran it for 15 minutes and it only reached 176 degrees. It was cold out mind you (~ 39 degrees F).
>>. I do have the Motorcraft RT 1152 ready to install in it. Waiting for a new felpro gasket as I'm sure it'll be destroyed when I take the one currently in
>>. it off.
>>
Hotshot
Joined: Jun 2015
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Engine running codes are irrelevant until you get code 11 from the engine off self test.
If it was mine, I would clear the codes, drive it and then check in a couple weeks to see what, if any, codes come back.
If it was mine, I would clear the codes, drive it and then check in a couple weeks to see what, if any, codes come back.
Thread Starter
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More Turbo
Joined: Feb 2022
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From: New Dominion, PE
found the section in the innvoa 3145 on how to clear the PCM codes...and it does in fact work. no longer seeing the 51 in KOEO test...just the EGR one.
and get the 11 code after it finishes.
and get the 11 code after it finishes.
Hotshot
Joined: Jul 2020
Posts: 10,519
Likes: 2,654
From: Near Salt Lake City
The PCED has system operation in it for each sensor and the system as a whole
It talks about the O2 sensor heating up and then switching to closed loop
That is why they added heaters to O2 sensors and now they are called HEGOs
That is so they can switch to closed loop operation quicker and save fuel
You can also simply Google open vs closed loop operation for 1990 era Ford vehicles
Cadillacs, BMW, Mercedes all work the same way with a lambda sensor
It talks about the O2 sensor heating up and then switching to closed loop
That is why they added heaters to O2 sensors and now they are called HEGOs
That is so they can switch to closed loop operation quicker and save fuel
You can also simply Google open vs closed loop operation for 1990 era Ford vehicles
Cadillacs, BMW, Mercedes all work the same way with a lambda sensor