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Open Loop --> Closed Loop engine operation


JimD

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Anyone know what the variables are? When does it change from open to closed? When would it change back to open? I would appreciate having a handle on the specific parameters and values for a circa 1998 model year engine.

For some time, I expected coolant temperature would at least be one of the factors. Based on some recent observations with an OBD monitor, it appears that I was wrong; very wrong.

Jim

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Historically, it was a factor of a few things. Coolant temperature was likely one of them because oxygen sensors were not heated for a long time. In essence, the oxygen sensors need to be awake and talking before the system can close the loop. So the computer would wait until a certain coolant temperature threashold was reached (say, 165*F), and then try for closed loop operation.

Likely, it's simply a matter of when those heated oxygen sensors (of today's cars) warm up and start talking. I'd imagine that the car might go closed loop within a matter of minutes of a cold start, or even less.

Jim, what are you finding with an OBD monitor? Closed loop much quicker than you anticipated? To answer your question of when it might go back open after being closed, I can't think of a situation...I would imagine that once those HO2 sensors are up and running, it wouldn't go back out (of closed loop).

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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Using the OBD monitor I am getting conflicting indications from a cold start. One indicator shows going into closed loop at a coolant temp. of 153 degrees. Another indicator starts toggling back and forth between open and closed at 46 degrees.

I need to run a dozen or so more samples to determine what these two indicators are really telling me. Sure could use a multi-channel recording o'scope for a day or two to get a record of O2 sensor voltage swings that I could then plot against coolant temperature.

I do not think coolant temp. is anything more than one way to know when to expect the engine "should be" in closed loop. The state of health of the O2 sensors has to be a critical factor; maybe the only factor.

Therefore, you should not see a SERVICE ENGINE SOON light that is caused by a faulty O2 sensor before the PCM says it's time to toggle to closed loop. Or maybe it will not even toggle to closed loop if one of the O2 sensors is not performing as expected.

There is so much I don't know about the details and I am eat up with curiosity. Being retired can be so time consuming.

Jim

Drive your car.

Use your cell phone.

CHOOSE ONE !

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I do not think coolant temp. is anything more than one way to know when to expect the engine "should be" in closed loop. The state of health of the O2 sensors has to be a critical factor; maybe the only factor.

Conventionally, yes, I think that coolant temperature was the one measurable metric that told the engineers when the engine SHOULD be warm enough that the O2 sensors SHOULD be outputting a signal. I'm sure there was a range there somewhere...and they picked a warm enough value (typically in the 155-165*F range) to ensure enough time for the O2 sensors to heat up (though you wouldn't think it'd take long).

But nowadays, with heated O2 sensors, I think they're really to roll in a matter of a minute or two (or less). So I'm in agreement that in a modern car with heated O2 sensors, they are possibly the only factor in determining closed loop (aside from the WOT switch). I don't know of an engine that stays closed loop when WOT is true.

Therefore, you should not see a SERVICE ENGINE SOON light that is caused by a faulty O2 sensor before the PCM says it's time to toggle to closed loop. Or maybe it will not even toggle to closed loop if one of the O2 sensors is not performing as expected.

I would not expect the SES light because of an O2 sensor while in open loop, no. That said, it could be that the computer expects O2 sensor data within xx seconds after a cold start (90 seconds perhaps, just to assign an arbitrary value), and maybe if it's not seeing that, it might light the SES, without the computer being in closed loop. I don't know -- that's just a total guess.

There is so much I don't know about the details and I am eat up with curiosity. Being retired can be so time consuming.

It's frustrating to know just enough to be curious, ain't it!? I know about retirement -- my folks are retired and full-time RVers. They're busier now than when dad was working...busy spending my inheritance! ;) I jest, of course.

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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Jim, I found this, not sure if this helps,

The vehicle is in closed loop fuel control, based on HO2S bank 1 sensor 1 and bank 2 sensor 1 voltages switching from below 350mV to above 500mV

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Is that Northstar specific data?

Jim, that came out of the SI data for a 2000 Seville, do you want me to look for a specific year or is that adequate. There are also details about a DTC P0125 Engine Coolant Temperature (ECT) Insufficient for Closed Loop Fuel Control if you want me to I will send you the details in an email send me your email address in a PM if you want it...

here is a little more:

The vehicle is in closed loop fuel control, based on HO2S bank 1 sensor 1 and bank 2 sensor 1 voltages switching from below 350mV to above 500mV.

The PCM monitors the HO2S bank 2 sensor 1 voltage for 120 seconds (2 minutes).

During the 120 second monitoring period, the HO2S bank 2 sensor 1 voltage remains within 350 and 500 mV for approximately 16 seconds

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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The PCM monitors the HO2S bank 2 sensor 1 voltage for 120 seconds (2 minutes).

During the 120 second monitoring period, the HO2S bank 2 sensor 1 voltage remains within 350 and 500 mV for approximately 16 seconds

Good info! Do you read that to say that if the HO2S bank 2 sensor 1 voltage does NOT remain within 350 and 500 mV for approximately 16 seconds, after 120 seconds after a cold start, that the system will NOT go into closed loop and a MIL might be lit?

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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Is that Northstar specific data?

Jim, that came out of the SI data for a 2000 Seville, do you want me to look for a specific year or is that adequate. There are also details about a DTC P0125 Engine Coolant Temperature (ECT) Insufficient for Closed Loop Fuel Control if you want me to I will send you the details in an email send me your email address in a PM if you want it...

here is a little more:

The vehicle is in closed loop fuel control, based on HO2S bank 1 sensor 1 and bank 2 sensor 1 voltages switching from below 350mV to above 500mV.

The PCM monitors the HO2S bank 2 sensor 1 voltage for 120 seconds (2 minutes).

During the 120 second monitoring period, the HO2S bank 2 sensor 1 voltage remains within 350 and 500 mV for approximately 16 seconds

Let me clarify sorry:

Conditions for Setting the DTC

The vehicle is in closed loop fuel control, based on HO2S bank 1 sensor 1 and bank 2 sensor 1 voltages switching from below 350mV to above 500mV.

The PCM monitors the HO2S bank 2 sensor 1 voltage for 120 seconds (2 minutes).

During the 120 second monitoring period, the HO2S bank 2 sensor 1 voltage remains within 350 and 500 mV for approximately 16 seconds. (it appears that for the sensor to be between 350 and 500mv is a fault, its either below 350 (open loop) or above 500 (closed loop)

Action Taken When the DTC Sets

The PCM illuminates the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test runs and fails.

The PCM stores the conditions present when the DTC set as Freeze Frame/Failure Records data.

Conditions for Clearing the MIL/DTC

The PCM will turn the MIL OFF after the third consecutive trip in which the diagnostic runs and passes.

The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.

The DTC can be cleared by using the scan tool Clear DTC Information function.

Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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For some time, I expected coolant temperature would at least be one of the factors. Based on some recent observations with an OBD monitor, it appears that I was wrong; very wrong.

Allow me to correct that coolant temp. expectation.

The key to closed loop operation is O2 sensor performance. Period. But there is no practical way for the driver to know, or predict, or even care, if the engine is in open or closed mode.

In the big picture, the O2 sensors form the feedback portion of a standard old closed loop servo system. When the PCM does this to fuel flow, the O2 content of the exhaust gases (should) cause the sensor to report that in fact happened. The this and that depend on many many factors including engine coolant temp., engine run time, vehicle speed, intake air flow, intake air temp., throttle position and on and on.

Different expectations under different operating situations. When the O2 sensors are operating properly, the system is happily controlling fuel delivery to maintain the desired 14.7:1 air / fuel ratio under conditions that a carburetor could not handle near as well.

Jim

Drive your car.

Use your cell phone.

CHOOSE ONE !

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