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Open Loop After Start Up. #5128642 06/08/19 02:58 PM
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George7941 Offline OP
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After starting and driving away on a cold engine, the O2 sensors start switching in about 15 secs but the loop status does not change for over 3 minutes.

The photos show the display on my Tech2. The frames are at about 5 frames a second. The first photo shows the sensors start switching. The solid thick line in the second photo shows the loop status change.

The O2 sensors will only switch when the ECM adjusts fuel delivery based on feedback from the O2 sensors, pretty much the definition of closed loop. So it is surprising the ECM does not declare closed loop for over 3 minutes.

100_1273.jpg100_1271.jpg

2006 GMC Sierra 4.3l, NV3500,G80 , Mobil 1 5W30, Dexcool.
1987 BMW R80 - sold
2015 Honda CBR300R

Staying out of the right lane a lot.
Re: Open Loop After Start Up. [Re: George7941] #5128654 06/08/19 03:27 PM
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bullwinkle Online Confused
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Doesn't closed loop begin when an engine achieves operating temperature (or a number close to it)? My xB has had weird O2 sensor faults that show up on a cold start, when still in open loop, but upon checking sensor voltages all is normal, and the faults go away when they are cleared.


06 Ram 3500 CTD 4X4(FG Venturi), 93 GMC C3500 6.2, 89 F-450 7.3, 98 XJ 4.0(XG8A), 05 xB(XG3600), 18 Transit 3.7, 03 Merc Grand Marquis 4.6 2V(XG2)
Re: Open Loop After Start Up. [Re: George7941] #5128661 06/08/19 03:38 PM
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Originally Posted by George7941
The O2 sensors will only switch when the ECM adjusts fuel delivery based on feedback from the O2 sensors, pretty much the definition of closed loop.

Yes. Apparently the definition of closed loop has slipped. Perhaps it means fully closed, where long term trims are being adjusted, etc.


2018 Hyundai Kona 1.6T DCT
2006 Pontiac (Holden) GTO AT
2001 Pontiac Grand Prix GT 3.8 NA (beater)
2005 Toyota Matrix FWD (ultra beater)
"Fob" and "Noack" are not acronyms.
Re: Open Loop After Start Up. [Re: bullwinkle] #5128662 06/08/19 03:38 PM
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George7941 Offline OP
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My truck engine is nowhere near operating temperature at three minutes after cold startup. It is at about 40C / 100F when the ECM changes the Loop Status to Closed.


2006 GMC Sierra 4.3l, NV3500,G80 , Mobil 1 5W30, Dexcool.
1987 BMW R80 - sold
2015 Honda CBR300R

Staying out of the right lane a lot.
Re: Open Loop After Start Up. [Re: George7941] #5128675 06/08/19 04:08 PM
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sloinker Offline
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Need to get to around 190F on the coolant I believe.


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Re: Open Loop After Start Up. [Re: George7941] #5128729 06/08/19 05:50 PM
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shanneba Offline
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My BMW will run the Secondary air pump for up to about 90 seconds after startup, maybe that needs to complete before the ECU goes to "closed Loop".

Does the engine adjust the Air Fuel ratio on a cold start. I know some vehicles will do that and will not get to closed loop until the richer mixture decays out over time.

Another vehicle I have has a temperature limit before the ECU actually uses the O2 sensor output. The O2 sensors on that vehicle are not heated.
( the closed loop limit is set to a head temp of 149 F, the engine normally operates at around 350 F head temp. It is an air cooled engine smile )


2003 BMW 330Ci
2013 HD Sportster XL1200C
Re: Open Loop After Start Up. [Re: George7941] #5128782 06/08/19 07:22 PM
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George7941 Offline OP
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The only way the O2 sensor will switch is if the ECM adjusts fuel delivery based on feedback from the O2 sensors and the A/F ratio is maintained close to stoichometric, and it is doing that about fifteen seconds after startup. The O2 sensors are heated, which is why they start working so quickly. No air pump on this engine.

Last edited by George7941; 06/08/19 07:23 PM.

2006 GMC Sierra 4.3l, NV3500,G80 , Mobil 1 5W30, Dexcool.
1987 BMW R80 - sold
2015 Honda CBR300R

Staying out of the right lane a lot.
Re: Open Loop After Start Up. [Re: George7941] #5128792 06/08/19 07:34 PM
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Popsy Offline
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Engine management (albeit very primitive) in my Barchetta doesn't consider the O2 sensor information till coolant temp reaches (I think) 40°C. Also the variable intake system is deactivated till coolant reaches a certain temp.

I guess the O2 sensors are "ready" way before that, perhaps the same thing is happening in your car, as others have said.


96 Barchetta, 300V 5W40 | Forfour 95, M1 0W40
Mazda 6 2.3, Castrol 0W30 | Colt Ralliart, 300V 5W40
Re: Open Loop After Start Up. [Re: George7941] #5128847 06/08/19 08:45 PM
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I believe it starts in open to heat everything up using a set fuel map. Then I think in most ECU's the ECU goes from open to closed loop based on several parameters like catalyst temp, RPM fluctuations (settling down) and throttle position. Plus I don't think it'll go into closed loop until the cat is fully heated. Until then it stays in open loop. How long it takes for everything to get heated up to where the ECU goes into closed loop can be effected by outside air temp, altitude, humidty. Your O2 sensors can be bouncing around but the ECU is ignoring them until the catalyst heats up, at which point it goes into closed loop using feedback from the O2 sensors and not set fuel map.

Again, that's just my understanding and I could be off/wrong so it's worth double checking.

Last edited by Mad_Hatter; 06/08/19 08:53 PM.
Re: Open Loop After Start Up. [Re: Mad_Hatter] #5128981 06/09/19 01:16 AM
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George7941 Offline OP
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The graph of the O2 sensor shows that it is switching exactly the way it does when the engine is at operating temperature. It is not bouncing around, it is reacting to changes in the mixture.

Can the ECM control fuel delivery enough to accomplish switching without feedback from the O2 sensors? It uses fuel maps and the learned STFT and LTFT values to produce a lean mixture one second and richen it the next second without feedback? Maybe or maybe not and the way to find out is to artificially enrichen the mixture with external propane or lean the mixture by disconnecting a PCV hose. That will be my next experiment. I will post the results if I can get it done soon enough before this thread becomes stale.


2006 GMC Sierra 4.3l, NV3500,G80 , Mobil 1 5W30, Dexcool.
1987 BMW R80 - sold
2015 Honda CBR300R

Staying out of the right lane a lot.
Re: Open Loop After Start Up. [Re: George7941] #5129092 06/09/19 08:16 AM
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Originally Posted by George7941
The graph of the O2 sensor shows that it is switching exactly the way it does when the engine is at operating temperature. It is not bouncing around, it is reacting to changes in the mixture.

Can the ECM control fuel delivery enough to accomplish switching without feedback from the O2 sensors? It uses fuel maps and the learned STFT and LTFT values to produce a lean mixture one second and richen it the next second without feedback? Maybe or maybe not and the way to find out is to artificially enrichen the mixture with external propane or lean the mixture by disconnecting a PCV hose. That will be my next experiment. I will post the results if I can get it done soon enough before this thread becomes stale.


Yes, unplug your O2 sensors and watch it do just that. You wouldn't want your engine to just shut down in the event one of your O2 sensors fails. That's because the ECU will go into open loop and pick up where it left off and begin to manage the fuel trims using the fuel map and stored adaption table (self learning). The ECU's goal is to operate in closed loop but the ECU will go into open loop during certain situations like heavy load.(like towing or acceleration) because in open loop it doesn't have to "wait" around for the O2 voltage (lambda) readings. This is the compromise between emissions standards and mfgs performance requirements. The ECU generally operates in closed loop when things are steady like cruising on the hwy (steady rpms) and/or the throttle position is less than a certain %.

Fwiw, I believe emissions standards require a ECU to get into closed loop upon start up, within a certain time. I don't know what that time limit is...

Last edited by Mad_Hatter; 06/09/19 08:42 AM.
Re: Open Loop After Start Up. [Re: George7941] #5130859 06/10/19 09:38 PM
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clinebarger Offline
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To give a specific answer to YOUR controller (PCM's) behavior which is a Delphi P59.....

It has what we call Open Loop STFT control. As soon as the O2 are driven rich (Producing their own voltage) which indicates they are hot enough. The PCM will use the last updated STFT base fueling tables/maps/cells to begin Proportional Fueling.
Proportional Fueling is the "Switch/Oscillation" function that drives the O2's Rich-Lean without Over/Under shooting it too much.

Your able to see this more so on engines that don't warm-up very fast, The 4.3L in particular as it's base architecture is so old, But the PCM running the show is relatively modern. The first year for the Delphi P59 Controller was 2003. The only reason it wasn't used past 2007 was because it didn't quite have memory for AFM control.

Minimum Closed Loop enable Coolant Temps range from 100°F to 131°F on GMT800 platform's (P01 & P59 Controllers)


2001 Chevy Camaro L92/4L80E
2006 Chevy 2500HD LBZ/Allison 1000
2010 Toyota Corolla 2ZR-FE/U341E
2000 Toyota Avalon 1MZ-FE/A541E
Re: Open Loop After Start Up. [Re: George7941] #5132795 06/12/19 11:57 PM
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I haven't started my Camaro in a while, So I decided to data log a cold start......I'm using a 2005 GMT800 Delphi P59 to control a "Cammed" 2008 L92 (6.2L) that's converted to 24X/1X Crank/Cam reluctors

Drops into closed loop at the 2:48 mark, The time it took ECT to reach 132°F.
79°F coolant start-up temp, Ambient in my garage was about 85°F which to IAT reflected.

*if you watch the O2-mV....They heat-up & drive full Rich within 8 SECONDS!!!!. Granted I had the key ON for @30 seconds loading the data logger. My O2 heaters are powered by ignition HOT & Case ground. Not by the PCM. A stock P59 equipped vehicle will control the O2 heaters via Duty Cycle/PWM.

*At 28 seconds....Open Loop STFT begins by driving the fueling lean. Then O2 voltage oscillation is logged....Followed by STFT activity .

*While not technically in closed loop.....It IS using O2 feedback to control fueling!!!

Turn your speaker volume down!, I was listening to music on my laptop while filming the datalog. While my speaker were at a low volume.....My phone was at max volume






2001 Chevy Camaro L92/4L80E
2006 Chevy 2500HD LBZ/Allison 1000
2010 Toyota Corolla 2ZR-FE/U341E
2000 Toyota Avalon 1MZ-FE/A541E
Re: Open Loop After Start Up. [Re: clinebarger] #5132917 06/13/19 07:07 AM
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George7941 Offline OP
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Very interesting! Thanks, clinebarger. 132F temp is reached at about 19.17.05 and the graphs at the bottom right show absolutely no change in behaviour at this point.


2006 GMC Sierra 4.3l, NV3500,G80 , Mobil 1 5W30, Dexcool.
1987 BMW R80 - sold
2015 Honda CBR300R

Staying out of the right lane a lot.
Re: Open Loop After Start Up. [Re: George7941] #5133517 06/13/19 07:22 PM
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clinebarger Offline
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The fuel trims won't change a lot unless something is wrong, The graph scale is +25% to -25%.....


2001 Chevy Camaro L92/4L80E
2006 Chevy 2500HD LBZ/Allison 1000
2010 Toyota Corolla 2ZR-FE/U341E
2000 Toyota Avalon 1MZ-FE/A541E
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