Closed Loop vs Open Loop
08-19-2005, 10:55 PM
#1
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Closed Loop vs Open Loop
So I see this in several threads, but in simple terms...what's the difference b/w closed loop and open loop. Is closed loop with the PCM starts taking signal from sensors, where open loop the PCM runs on factory settings? What sends the car into closed loop...time elapsed, speed reached, RPM reached. What is it that actually makes a car run in closed loop. Also, what resets the car back to open loop...turning off the car and re-starting, or could you be driving and then from closed loop, it goes back to open loop.
Educate me, please
Educate me, please
08-20-2005, 12:11 AM
#2
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Re: Closed Loop vs Open Loop
Closed Loop = Computer is reading all the sensors and adjusting fuel mixture, timing.. what not.
Open loop = computer relying on pre-set specs (runs rich)
When the oxygen sensor reaches like 600degrees, the car enters closed loop... It will not enter closed loop if certain sensors are not found.. I'm not sure if it's the same for newer cars, or all new cars, or what. Example is my 1987 Camaro will not enter closed loop if the MAF (mass airflow sensor) is unplugged. Not sure if newer camaros require the MAF or not, i can only assume it does.
If a sensor's data is CRAZY (really high or really low), or non-existant, it can cause an open loop condition. The oxygen sensor senses how lean or rich your car is running according to how it reads the unburnt gasses from the exhaust pipe. It adjusts the air/fuel mixture accordingly. If you turn the car off and on, it will be in open loop only for a VERY short time, until it can read all the sensors fine again, and the oxygen sensor is heated up.
Open loop = computer relying on pre-set specs (runs rich)
When the oxygen sensor reaches like 600degrees, the car enters closed loop... It will not enter closed loop if certain sensors are not found.. I'm not sure if it's the same for newer cars, or all new cars, or what. Example is my 1987 Camaro will not enter closed loop if the MAF (mass airflow sensor) is unplugged. Not sure if newer camaros require the MAF or not, i can only assume it does.
If a sensor's data is CRAZY (really high or really low), or non-existant, it can cause an open loop condition. The oxygen sensor senses how lean or rich your car is running according to how it reads the unburnt gasses from the exhaust pipe. It adjusts the air/fuel mixture accordingly. If you turn the car off and on, it will be in open loop only for a VERY short time, until it can read all the sensors fine again, and the oxygen sensor is heated up.
08-20-2005, 05:58 AM
#3
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Re: Closed Loop vs Open Loop
In the LT1's, the only sensors that are not used in open loop are the O2 sensors. For all practical purposes, everything is identical in open loop, except the air is proportioned to the mass-air measured fuel by using a standard calculation, ,modified by the learned long-term fuel corrections in Cells 16 (idle), 18 (load) and 17 (coast down). The A/F ratio will be variable, as a function of coolant temperature (cold start enrichment). The PCM will not "learn" as far as feedback from the O2 sensors, and will not use the short term fuel correction capability, or change the stored long term corrections.
Three things need to happen.... 1) O2 sensors reach minimum operating temperature (~600degF as indicated in post above), 2) coolant temp must reach a minimum value (variable) that is in the range of 120-140degF, and 3) a timer (about 200 seconds) from start must time out.
Once it enters closed loop, the A/F ratio is fixed at the stoichiometric value of 14.7:1 and uses the O2 sensors feedback (in essence, accepting feedback from the O2 sensors is "closing the loop") to adjust the A/F ratio and keep it oscillating slightly above and below 14.7:1 to store O2 in the catalytic convertor, releasing it from the NOx in a reduction reaction, and then using the released O2 to use in an oxidation reaction to convert CO to CO2 and unburned HC's to CO2 and H2O. If the short term corrections can not control the A/F ratio, the PCM "learns" and adjusts the stored long term corrections until it get the A/F ratio back to 14.7:1.
When you go WOT (actually a variable throttle postion vs. RPM) the PCM once again drops the feedback from the O2 sensors, disables the short term corrections, and sets a richer target A/F ratio (unfortunately an overly rich 11.7:1). While it would appear to be another case of "open loop", GM calls it "power enrichment mode".
For more info on the quirks of the system, read the "A/F Management" section of my online Scanner writeup:
http://members.aol.com/InjuneerZZ/ScanMast.htm
Three things need to happen.... 1) O2 sensors reach minimum operating temperature (~600degF as indicated in post above), 2) coolant temp must reach a minimum value (variable) that is in the range of 120-140degF, and 3) a timer (about 200 seconds) from start must time out.
Once it enters closed loop, the A/F ratio is fixed at the stoichiometric value of 14.7:1 and uses the O2 sensors feedback (in essence, accepting feedback from the O2 sensors is "closing the loop") to adjust the A/F ratio and keep it oscillating slightly above and below 14.7:1 to store O2 in the catalytic convertor, releasing it from the NOx in a reduction reaction, and then using the released O2 to use in an oxidation reaction to convert CO to CO2 and unburned HC's to CO2 and H2O. If the short term corrections can not control the A/F ratio, the PCM "learns" and adjusts the stored long term corrections until it get the A/F ratio back to 14.7:1.
When you go WOT (actually a variable throttle postion vs. RPM) the PCM once again drops the feedback from the O2 sensors, disables the short term corrections, and sets a richer target A/F ratio (unfortunately an overly rich 11.7:1). While it would appear to be another case of "open loop", GM calls it "power enrichment mode".
For more info on the quirks of the system, read the "A/F Management" section of my online Scanner writeup:
http://members.aol.com/InjuneerZZ/ScanMast.htm
08-22-2005, 04:55 PM
#4
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Re: Closed Loop vs Open Loop
Thanks Fred for the detailed explanation. Also, my compliments to your ScanMaster document....well done and it's answered a lot of questions about terms I see here and there on the Board.
So, this is very interesting about the long-term fuel correction. Let me explain:
I had a stumbling problem a while back when I first install the Random Tech Y-pipe with hi-flow cats...already had SLP shorties installed. I had new O2 and new plugs already, so decided to install new wires. After the wires, still had the stumbling and misfire, so installed new opti. That's where I stopped. I didn't mind installing new wires or opti, since I've had the original parts for 94K miles so good time to change it anways.
But even after that I had some stumbling. Changed fuel filter..probably been 20K miles or so. With wires, opti, fuel filter, stumbling definitely improved but still there.
Here's where it gets interested. I've noticed that my stumbling's been getting less and less over time...kinda like in stages:
1. First stumbled randomly at speeds below 40mph, all day long.
2. Then stumbled at low speeds, but after cruising on the highway, the stumbling went away. Even going back into town idling at stop lights and low speed driving..no stumbling. Now, if I shut off my car, and came back later and re-started my car, stumbling was back. But if I get up to crusing speeds, stumbling went away again. This is where I started to question the open loop vs. closed loop.
3. Lately, the stumbling's there at low speeds, but not as bad. Now as long as I can get the speeds around 50mph..not necessarily highway at 90mph...I mean 65mph
, the stumbling would go away the rest of the time, unless I turn off the car and re-started.
My car is running the rich the last time I checked.
So, where am I going....I'm really interested on how this long term correction works. Is my car "learning" from the increased air flow now with the y-pipe? If so, is there a way to make it learn quicker...say longer trips...driving throughout the rpm band? The stumbling situation is seriously noticeably getting better the more I drive.
So here's my thought and correct me where I go off the tracks...car starts in the morning in open loop and stumbles b/c not reading the O2 and using the "learned" long term fuel corrections, which are based on airflow prior to new headers, CAI, catback, and y-pipe. As car moves to closed loop...probably cat heating up is the last step, it's starting to get feedback from O2 and correct A/F ratio, so not running rich and stumbling. The more I drive, the more it learns and adjusts the long term corrections.
Now when I park and turn off the engine, all learning has stopped. When I start again, stumbling's back, but hopefully using "more accurate" long term corrections. That's why it seems that stumbling's getting better.
Is this even close to something plausible or am I out in left field. Also, is there a way to learn quicker?
I plan to dyno tune my car in a few months...installing new converter and new 1.6RRs first. I assume in the dyno tune, they'll be adjusting the fuel corrections based on estimated new air flow.
Any suggestions would help.
So, this is very interesting about the long-term fuel correction. Let me explain:
I had a stumbling problem a while back when I first install the Random Tech Y-pipe with hi-flow cats...already had SLP shorties installed. I had new O2 and new plugs already, so decided to install new wires. After the wires, still had the stumbling and misfire, so installed new opti. That's where I stopped. I didn't mind installing new wires or opti, since I've had the original parts for 94K miles so good time to change it anways.
But even after that I had some stumbling. Changed fuel filter..probably been 20K miles or so. With wires, opti, fuel filter, stumbling definitely improved but still there.
Here's where it gets interested. I've noticed that my stumbling's been getting less and less over time...kinda like in stages:
1. First stumbled randomly at speeds below 40mph, all day long.
2. Then stumbled at low speeds, but after cruising on the highway, the stumbling went away. Even going back into town idling at stop lights and low speed driving..no stumbling. Now, if I shut off my car, and came back later and re-started my car, stumbling was back. But if I get up to crusing speeds, stumbling went away again. This is where I started to question the open loop vs. closed loop.
3. Lately, the stumbling's there at low speeds, but not as bad. Now as long as I can get the speeds around 50mph..not necessarily highway at 90mph...I mean 65mph
, the stumbling would go away the rest of the time, unless I turn off the car and re-started.My car is running the rich the last time I checked.
So, where am I going....I'm really interested on how this long term correction works. Is my car "learning" from the increased air flow now with the y-pipe? If so, is there a way to make it learn quicker...say longer trips...driving throughout the rpm band? The stumbling situation is seriously noticeably getting better the more I drive.
So here's my thought and correct me where I go off the tracks...car starts in the morning in open loop and stumbles b/c not reading the O2 and using the "learned" long term fuel corrections, which are based on airflow prior to new headers, CAI, catback, and y-pipe. As car moves to closed loop...probably cat heating up is the last step, it's starting to get feedback from O2 and correct A/F ratio, so not running rich and stumbling. The more I drive, the more it learns and adjusts the long term corrections.
Now when I park and turn off the engine, all learning has stopped. When I start again, stumbling's back, but hopefully using "more accurate" long term corrections. That's why it seems that stumbling's getting better.
Is this even close to something plausible or am I out in left field. Also, is there a way to learn quicker?
I plan to dyno tune my car in a few months...installing new converter and new 1.6RRs first. I assume in the dyno tune, they'll be adjusting the fuel corrections based on estimated new air flow.
Any suggestions would help.
08-22-2005, 11:06 PM
#6
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Re: Closed Loop vs Open Loop
Your thoughts about "learning increased air flow from the Y-pipe" don't work. If you add a part that increases the volumetric efficiency of the engine, such as in improved intake component or exhaust component, the PCM has nothing to learn. If more air is really flowing, the MAF sensor measures the air flow, the PCM adds the required amount of fuel to match that air flow, and the end result is the correct A/F ratio. Nothing to "learn".
The PCM would only have to "learn" if the MAF was not indicating the correct air flow. If the MAF was reporting less than actual air flow, the PCM would only be supplying fuel for that lwer than actual air flow. The O2 sensor would see a lean condition, and immediately adjust for the lean condition by adjusting the short term corrections. If that situation persisted for more than a few seconds, it would then start raising the long terms to make a more permanent adjustment.
The only thing that might take a while to "learn" in a situation like that would be the BLM's (long term corrections) for ALL of the 16 cells that the engine can operate in.... various combinations of RPM and MAP. If it isn't run more than a few seconds in say Cell 09, its going to take a while for that cell to adjust the BLM's fully. But its a fairly accurate response.
Stumbling could be related to a component that you changed, that wasn't "good" when you put it in.... e.g. cracked a spark plug, lost the gap setting, a damaged wire, etc. Could be related to a faulty/leaking EGR valve. Could be a component that is heat sensitive, like the IC module. Could be the sensors that are used in "transition".... a faulty TPS sensor would not provide the PCM with correct info regarding the need for A/F ratio enrichement when the throttle is opening.
If you know its running rich, you have to find the cause. Have you ever changed the O2 sensors? Is there any possibility you contaminated them with the Y-pipe swap? Any chance you used anti-sieze on the O2 sensors, or sprayed them with any sort of penetrant to remove them from the old exhaust? Is the Y-pipe cera-metallic coated? Any chance you have an exhuast leak before the O2 sensors? All of those things might cause the O2 sensors to read incorrectly, and cause it to run rich. Exactly how rich is it running, and how did you measure the rich condition?
"Stumbling" seems to be the most common problem, and the hardest one to track down. I don't know if you've tried a "search", but I know there are some threads that have gone on for 5 pages before the problem was finally tracked down. Reviewing some of those threads might give you additional info.
The PCM would only have to "learn" if the MAF was not indicating the correct air flow. If the MAF was reporting less than actual air flow, the PCM would only be supplying fuel for that lwer than actual air flow. The O2 sensor would see a lean condition, and immediately adjust for the lean condition by adjusting the short term corrections. If that situation persisted for more than a few seconds, it would then start raising the long terms to make a more permanent adjustment.
The only thing that might take a while to "learn" in a situation like that would be the BLM's (long term corrections) for ALL of the 16 cells that the engine can operate in.... various combinations of RPM and MAP. If it isn't run more than a few seconds in say Cell 09, its going to take a while for that cell to adjust the BLM's fully. But its a fairly accurate response.
Stumbling could be related to a component that you changed, that wasn't "good" when you put it in.... e.g. cracked a spark plug, lost the gap setting, a damaged wire, etc. Could be related to a faulty/leaking EGR valve. Could be a component that is heat sensitive, like the IC module. Could be the sensors that are used in "transition".... a faulty TPS sensor would not provide the PCM with correct info regarding the need for A/F ratio enrichement when the throttle is opening.
If you know its running rich, you have to find the cause. Have you ever changed the O2 sensors? Is there any possibility you contaminated them with the Y-pipe swap? Any chance you used anti-sieze on the O2 sensors, or sprayed them with any sort of penetrant to remove them from the old exhaust? Is the Y-pipe cera-metallic coated? Any chance you have an exhuast leak before the O2 sensors? All of those things might cause the O2 sensors to read incorrectly, and cause it to run rich. Exactly how rich is it running, and how did you measure the rich condition?
"Stumbling" seems to be the most common problem, and the hardest one to track down. I don't know if you've tried a "search", but I know there are some threads that have gone on for 5 pages before the problem was finally tracked down. Reviewing some of those threads might give you additional info.
Last edited by Injuneer; 08-22-2005 at 11:10 PM.
08-23-2005, 12:15 AM
#7
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Re: Closed Loop vs Open Loop
I had the same problem. Mine was running so rich that it would turn my back bumper black( my car is white ). I finally tracked it down to a bad connector or my ECT. Replaced that, and it went away. No problems since.
08-23-2005, 01:04 PM
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Re: Closed Loop vs Open Loop
Fred,
Yes, I did do the search first under "stumbling" and "hesitation", and you're right, this is a VERY common problem in our cars. I've read through threads and threads about stumbling and haven't found one that has the exact same symptoms as mine. There's over 200 of them, and that's only by searching with "stumbling" or "hesitation" in the title only. I thought I was frustrated...there's folks out there that I can really feel their pain. For me, I just bare with it until it gets up to highway speeds, and it's good until I turn off the car.
It doesn't seem heat related (cooling mod for ICM/coil), because it will start stumbling right in the morning. I doubt it's the wires/plugs/opti, because those are new.
O2s were replaced about 10K miles ago...85K miles on original O2s...not too bad. The O2s were new, so didn't use any WD-40 to pry them from an old y-pipe. My y-pipe is aluminized...externally, not sure about internally. Could that contaminate them.
I'm meeting with my local tuner next week to sit and do a detailed scan. I rather not just throw parts at the car to see if it fixes it.
69gto96z,
I read your post, and your problem's a little different. Yours runs good cold, but starts stumbling when warm. My stumbling is there whenever I re-start my car...cold or warm. It stumbles at first until it hits about 40mph, which is will run good. But if I get to a stop light, and start from idle again, it still stumbles. But if I get on the highway and drive for a while, the stumblings completely gone. Not just on the highway, as long as I don't turn off the car, it's fine. I can stop at lights, turn on the AC...no stumbling at all.
Now as soon as I turn off the car, and re-start it, stumbling's back. That's why I thought it was PCM related, because it comes back when the car is re-started, like it had amnesia or something.
How did you trouble shoot it to be a bad connector on your ECT?
Has anyone had this symptom on your stumbling?
Yes, I did do the search first under "stumbling" and "hesitation", and you're right, this is a VERY common problem in our cars. I've read through threads and threads about stumbling and haven't found one that has the exact same symptoms as mine. There's over 200 of them, and that's only by searching with "stumbling" or "hesitation" in the title only. I thought I was frustrated...there's folks out there that I can really feel their pain. For me, I just bare with it until it gets up to highway speeds, and it's good until I turn off the car.
It doesn't seem heat related (cooling mod for ICM/coil), because it will start stumbling right in the morning. I doubt it's the wires/plugs/opti, because those are new.
O2s were replaced about 10K miles ago...85K miles on original O2s...not too bad. The O2s were new, so didn't use any WD-40 to pry them from an old y-pipe. My y-pipe is aluminized...externally, not sure about internally. Could that contaminate them.
I'm meeting with my local tuner next week to sit and do a detailed scan. I rather not just throw parts at the car to see if it fixes it.
69gto96z,
I read your post, and your problem's a little different. Yours runs good cold, but starts stumbling when warm. My stumbling is there whenever I re-start my car...cold or warm. It stumbles at first until it hits about 40mph, which is will run good. But if I get to a stop light, and start from idle again, it still stumbles. But if I get on the highway and drive for a while, the stumblings completely gone. Not just on the highway, as long as I don't turn off the car, it's fine. I can stop at lights, turn on the AC...no stumbling at all.
Now as soon as I turn off the car, and re-start it, stumbling's back. That's why I thought it was PCM related, because it comes back when the car is re-started, like it had amnesia or something.
How did you trouble shoot it to be a bad connector on your ECT?
Has anyone had this symptom on your stumbling?
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