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I keep hearing/reading that the engine temp (coolant) must be at "operating temperature" before the switch occurs to closed loop.
What I cannot find definitively is what that magic temperature is that the ECU is using. Surely this is stated somewhere.
Not much of a mystery
A PCED stands for Powertrain Control Emissions Diagnosis manual
They have made one every year now since about the time EEC4 rolled out in one form or another
They added the words powertrain control in the late 80's IIRR
It's a big thick loose leaf type book
They used to cost about a hundred bucks used on Ebay they are a lot cheaper now
Here is a picture of my 1989 PCED that was given to me by our Asset student who had had enough
ASSET students were college trained entry level Ford service techs (trained as well as books could)
Then they sent them out to work in the field for a few months at Ford dealers, then they graduated the program and got hired
I removed the thermostat that was in the truck after draining the coolant. Man getting that right bolt off the thermostat housing was an ordeal.
Putting the Motorcraft RT1192 (192 degree) one in.....it was almost impossible to get the right bolt in and the threads started. I ended up
sacrificing the box end of a 1/2" wrench to allow it to fit in between the housing and the water pump boss.
Will post an update once I get it refilled with coolant and burped.
in the meantime....I removed the O2 sensor (bosch) and bench tested it using a propane torch while probing the wires in the harness. goes from .1V to .8V and goes back to almost zero
when heat ie removed from the bulb end. so I would gather this indicates the sensor is OK,
coolant added to radiator. brought engine up to temp. closely monitoring the engine temp at the boss where the ECT screws in. got to 180 before the thermostat opened...
quickly dropped down to around 150 and began slowly climbing back up. Normal behaviour for a 192 deg Motorcraft thermostat?
as these trucks (ODB1) don't have live data this is about as reliable / accurate I can be in obtaining the temp. perhaps there is some degree of heat
being lost from the fluid inside the boss and the outside metal. just guessing.
Last edited by mackendw; Apr 29, 2026 at 08:28 AM.
Yes, it won't hit 192 or higher until you hit Phoenix
It takes a while to get all that coolant hot, and meanwhile, the radiator is cooling it down
You can watch the temp cycle on the gauge about 3 times as the thermostat opens and closes as the engine warms up
Once at operating temp, measure what comes out of the vents for heat or panel vent while on max heat
It should blow within 3-4 degrees of the thermostat
See if it blows 188 out of the vemts
It should get better mileage now, running hotter like it should
neck of the boss where ECT screws in is now 185F. coolant in radiator where cap is ~103F. this with the truck just idling.
getting the air out of the system. lots of heat in the cab.
>>. 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.
Yes there is, its called "POWERTRAIN ELECTRONIC CONTROL STRATEGY BOOK", that is "FoMoCo, PROPRIETARY & CONFIDENTIAL".
The book is about 1400 pages long.
It is all controlled in the 'Fuel Control Strategy':
It can either be a
predetermined value that is calibration dependent and can vary with engine
operating conditions (Open Loop Control); or the EEC may ramp the value up
and down in a limit cycle to maintain an average stoichiometric mixture, as
determined by the EGO sensor (Closed Loop Control).
FUEL MODE DESCRIPTION:
The purpose of the FUEL_MODE module is to determine the fuel control mode
(Open Loop/Closed Loop) and the value of LAMBSE. The fuel control strategy
consists of 3 mutually exclusive modes:
OPEN LOOP (OLFLG = 1)
CLOSED LOOP (OLFLG = 0)
SELF TEST OPEN LOOP (OLFLG = 1)
OPEN LOOP MODE
During open loop operation, the computer calculates the injector fuel
pulsewidths required to provide a pre-determined A/F ratio or lambda
value. The desired lambda value (LAMBSE) can vary with engine operating
conditions and is calibration dependent.
CLOSED LOOP MODE
During closed loop operation, the computer ramps the desired lambda value
(LAMBSE) in a limit cycle manner about stoichiometry. Using the EGO
(Exhaust Gas Oxygen) sensor, the computer increases or decreases LAMBSE
at a calculated rate of change. The rate at which LAMBSE changes is
calibration dependent.
SELF TEST OPEN LOOP MODE
During Self Test, the computer calculates lambda values (LAMBSEL,
LAMBSER) that will exercise the fuel, EGO, and thermactor systems. These
calculations are done in Self Test, outside of "Base Fuel Strategy" (See
the SELF TEST SECTION).
Originally Posted by mackendw
I keep hearing/reading that the engine temp (coolant) must be at "operating temperature" before the switch occurs to closed loop.
What I cannot find definitively is what that magic temperature is that the ECU is using. Surely this is stated somewhere.
Basically, the EGO sensor must be warm, and ECT temperature must be stable.
There's alot more going on than just a 'Magic Temp'
The Warm EGO Logic determines if the EGO sensor is warm enough to enter Closed Loop control.
Time since start-up and coolant temperature at start-up are used to determine if the sensor is warm.
The output from the logic is the flag, 'WRMEGO'.
The setup of the bung that the O2 sensor is welded onto the exhaust is offset in a section around 3-4 " on a pipe perpendicular to the main exhaust pipe just before the CAT.
The downpipe from the heads that does the air injection feeds into this area. I'm wondering if the bung not being in the direct path of the exhaust is preventing the O2 from
getting hot enough.
I can take a picture of how it's setup...just questioning if my truck is STANDARD or has been jerry-rigged in past years before I owned it.
in my software years of doing development, these modules with state flags like this could be monitored by logging information to a log which you could
inspect during a test. does the Rotunda STAR test tool allow you report on this information in a live manner?
Last edited by mackendw; Apr 29, 2026 at 11:27 AM.
another possibility is the 12V that is to be supplied to the O2 sensor to turn on the heater may not be working. I never tested that part of the sensor.
I'm guessing I can just turn the key to the ON position (not start it) and take Voltage readings on the 2 white wires on the connector harness plug on the
truck to verify that this is being supplied?
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