165 ecm what does close loop give up in open loop?
09-18-2009, 04:00 PM
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Car: 1989 Chevy Corvette
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165 ecm what does close loop give up in open loop?
to paraphrase, there are 2 distinct modes with the ecm's open and close loop,
when the car is in open loop, what functions are still managed by the ecm?
and when it goes into closed loop, what functions are taken over by the ecm where it were not before entering closed loop?
be as specific as possible please
this would really help me understand if i would like to tune wideband open loop and just leave close loop off permanently....
when the car is in open loop, what functions are still managed by the ecm?
and when it goes into closed loop, what functions are taken over by the ecm where it were not before entering closed loop?
be as specific as possible please
this would really help me understand if i would like to tune wideband open loop and just leave close loop off permanently....
09-18-2009, 04:54 PM
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Re: 165 ecm what does close loop give up in open loop?
Pretty much everything thats done in closed loop is done in open loop. The only difference being that instead of the stoich AFR being used by default with O2 feedback, the ECM uses the commanded open loop AFR (or inverse lambda, or lamba multiplier, or whatever the ECM in question uses). There is also no closed loop feedback from the O2 using the INTegrator and Block Learn Memory cells. They are left at the neutral value, and whatever pulsewidth is calculated is outputted directly to the fuel injectors without any corrections.
Theres nothing wrong with running open loop, especially if you have a cam. At idle, my O2 volts are < 50 mV, and my long term (BLM) and short term (INT) fuel trims peg at max and drown the motor in fuel if I allow closed loop. So, I just unchecked the option for closed loop and just set the open loop AFR to what works best for economy and power. As long as your MAF sensor is clean and in good working order, your fueling will be pretty close in open loop.
Basically all you need to do is set the closed loop enable temp to max, set your desired open loop AFR, unplug the ECM to clear out any stored closed loop corrections, and thats pretty much it.
Theres nothing wrong with running open loop, especially if you have a cam. At idle, my O2 volts are < 50 mV, and my long term (BLM) and short term (INT) fuel trims peg at max and drown the motor in fuel if I allow closed loop. So, I just unchecked the option for closed loop and just set the open loop AFR to what works best for economy and power. As long as your MAF sensor is clean and in good working order, your fueling will be pretty close in open loop.
Basically all you need to do is set the closed loop enable temp to max, set your desired open loop AFR, unplug the ECM to clear out any stored closed loop corrections, and thats pretty much it.
09-18-2009, 05:22 PM
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Re: 165 ecm what does close loop give up in open loop?
Pretty much everything thats done in closed loop is done in open loop. The only difference being that instead of the stoich AFR being used by default with O2 feedback, the ECM uses the commanded open loop AFR (or inverse lambda, or lamba multiplier, or whatever the ECM in question uses). There is also no closed loop feedback from the O2 using the INTegrator and Block Learn Memory cells. They are left at the neutral value, and whatever pulsewidth is calculated is outputted directly to the fuel injectors without any corrections.
Theres nothing wrong with running open loop, especially if you have a cam. At idle, my O2 volts are < 50 mV, and my long term (BLM) and short term (INT) fuel trims peg at max and drown the motor in fuel if I allow closed loop. So, I just unchecked the option for closed loop and just set the open loop AFR to what works best for economy and power. As long as your MAF sensor is clean and in good working order, your fueling will be pretty close in open loop.
Basically all you need to do is set the closed loop enable temp to max, set your desired open loop AFR, unplug the ECM to clear out any stored closed loop corrections, and thats pretty much it.
Theres nothing wrong with running open loop, especially if you have a cam. At idle, my O2 volts are < 50 mV, and my long term (BLM) and short term (INT) fuel trims peg at max and drown the motor in fuel if I allow closed loop. So, I just unchecked the option for closed loop and just set the open loop AFR to what works best for economy and power. As long as your MAF sensor is clean and in good working order, your fueling will be pretty close in open loop.
Basically all you need to do is set the closed loop enable temp to max, set your desired open loop AFR, unplug the ECM to clear out any stored closed loop corrections, and thats pretty much it.
do i sacrifice what closed loop is good at, if i disengage it?
suppose i want to lean out on cruise? and close loop accomplishes this, with it taken out, it would be whatever AFR I program based on load, correct?
how does this affect state emissions compliance, do they inspect the functionality of O2 sensors?
isn't it the ideal that the ecm (close loop) is able to compensate and manipulate AFR's real time? especially at high load, acceleration and cranking and not necessarily idle which i find the most problematic with my cam, although I think after today's drive I have found a sweet spot.
so is it not a compromise to run open loop just to address idle, and forgo close loop where it is a walk in the park for the other engine operating modes?
isn't close loop some assurance that the ecm will compensate for moments not anticipated by running strictly open loop?
when you say unplug the ecm, this is not to say take it out completely this really means disconnect the battery to reset, correct?
Last edited by slickfx3; 09-18-2009 at 05:29 PM.
09-19-2009, 04:28 AM
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Re: 165 ecm what does close loop give up in open loop?
i have read elsewhere that it is proper procedure to tune in open loop, to eliminate the noise close loop introduces during tuning, inferring that open loop tuning will create a baseline of sorts whereby close loop can reference a center point, is this off base?
do i sacrifice what closed loop is good at, if i disengage it?
Basically all you sacrifice is closed loop fueling control during idle and cruise. With MAF, its not as much of an issue as the MAF explicitly measures the air coming into the motor. My PCM calculates the fueling to within +/- 2% during steady state conditions using the MAF, so closed loop isnt really a necessity. CL does help if you have a cat, though. It ensures that the AFR stays around 14.7:1 for proper operation. With a 165, the fueling is only 8 bits, so the fueling will drift a bit at very low and very high loads due to the limits of the hardware. Because of this, you may have issues with the cat not working at peak efficiency or possible damage to the cat. But, with a cam, the AFR is going to be off, anyway, so its basically perfect imperfection when your using closed loop during low loads.
suppose i want to lean out on cruise? and close loop accomplishes this, with it taken out, it would be whatever AFR I program based on load, correct?
Yes, you can use whatever AFR you wish, provided you do not have a cat. If you have emmissions, stick to 14.7:1 when not in PE and verify proper operation with a WB.
how does this affect state emissions compliance, do they inspect the functionality of O2 sensors?
With your ECM, open loop may effect the emmissions when your on the dyno/sniffer. But, since your OBD-1, they will not scan your car, and will have no way of knowing what the computer is doing. Only the exhaust emmissions matter from an inspection standpoint.
isn't it the ideal that the ecm (close loop) is able to compensate and manipulate AFR's real time? especially at high load, acceleration and cranking and not necessarily idle which i find the most problematic with my cam, although I think after today's drive I have found a sweet spot. so is it not a compromise to run open loop just to address idle, and forgo close loop where it is a walk in the park for the other engine operating modes?
The '165 does not calculate the fueling in true real time. The main loops within the ECM run at 80 Hz, and the injectors fire either once per rev or once per two revs (cant recall for sure). The O2 is also only used during part throttle and idle. At WOT, your essentially in open loop. The fueling logic is also rather rudamentry. The ECM just calculates the required duty cycle of the injectors based on the ratio of air coming into the motor vs. the injector flowrate. This is likely one of the reasons your having issues getting good drivability. I essentially had a '165 with MAF at one point, and I also had issues with getting the proper fueling due to the low resolution.
In the later PCMs, the fueling IS calculated in real time for each individual cylinder, and can be outputted to that cylinder as soon as it reaches its intake cycle. On top of that, the calculations are done in 32 bits, so the resolution is only limited by the sensors, hardware, and injectors. They get much closer, and also suffer from less drift in fueling during transient conditions.
Now, ideally what you would want to do is only use closed loop in the areas where the cam overlap is not an issue. This is typically at cruise with the RPMs high enough that the engine pulls good vacuum so the exhaust cant scavange air from the intake. But, very few computers had this capability to selectively enable and disable closed loop at will. Probably the only one that had it was the TBI/CPI PCMs. Those you could do open loop idle, but still have closed loop during cruising. As far as closed loop goes, those computers had the best routines IMO. The later PCM that I have now is not as good from a closed loop standpoint. For some reason, GM chose to change it, even though the logic worked very well.
when you say unplug the ecm, this is not to say take it out completely
this really means disconnect the battery to reset, correct?
Yes, I usually just unplug mine so other things like the head unit dont lose power (and my preset stations, CD names, and so fourth). Disconnecting the battery will do the same thing. Some of these computers have capacitive back up, so you may need to leave it unhooked overnight. Although some computers dont store their closed loop data, this will help ensure that you clear everything.
do i sacrifice what closed loop is good at, if i disengage it?
Basically all you sacrifice is closed loop fueling control during idle and cruise. With MAF, its not as much of an issue as the MAF explicitly measures the air coming into the motor. My PCM calculates the fueling to within +/- 2% during steady state conditions using the MAF, so closed loop isnt really a necessity. CL does help if you have a cat, though. It ensures that the AFR stays around 14.7:1 for proper operation. With a 165, the fueling is only 8 bits, so the fueling will drift a bit at very low and very high loads due to the limits of the hardware. Because of this, you may have issues with the cat not working at peak efficiency or possible damage to the cat. But, with a cam, the AFR is going to be off, anyway, so its basically perfect imperfection when your using closed loop during low loads.
suppose i want to lean out on cruise? and close loop accomplishes this, with it taken out, it would be whatever AFR I program based on load, correct?
Yes, you can use whatever AFR you wish, provided you do not have a cat. If you have emmissions, stick to 14.7:1 when not in PE and verify proper operation with a WB.
how does this affect state emissions compliance, do they inspect the functionality of O2 sensors?
With your ECM, open loop may effect the emmissions when your on the dyno/sniffer. But, since your OBD-1, they will not scan your car, and will have no way of knowing what the computer is doing. Only the exhaust emmissions matter from an inspection standpoint.
isn't it the ideal that the ecm (close loop) is able to compensate and manipulate AFR's real time? especially at high load, acceleration and cranking and not necessarily idle which i find the most problematic with my cam, although I think after today's drive I have found a sweet spot. so is it not a compromise to run open loop just to address idle, and forgo close loop where it is a walk in the park for the other engine operating modes?
The '165 does not calculate the fueling in true real time. The main loops within the ECM run at 80 Hz, and the injectors fire either once per rev or once per two revs (cant recall for sure). The O2 is also only used during part throttle and idle. At WOT, your essentially in open loop. The fueling logic is also rather rudamentry. The ECM just calculates the required duty cycle of the injectors based on the ratio of air coming into the motor vs. the injector flowrate. This is likely one of the reasons your having issues getting good drivability. I essentially had a '165 with MAF at one point, and I also had issues with getting the proper fueling due to the low resolution.
In the later PCMs, the fueling IS calculated in real time for each individual cylinder, and can be outputted to that cylinder as soon as it reaches its intake cycle. On top of that, the calculations are done in 32 bits, so the resolution is only limited by the sensors, hardware, and injectors. They get much closer, and also suffer from less drift in fueling during transient conditions.
Now, ideally what you would want to do is only use closed loop in the areas where the cam overlap is not an issue. This is typically at cruise with the RPMs high enough that the engine pulls good vacuum so the exhaust cant scavange air from the intake. But, very few computers had this capability to selectively enable and disable closed loop at will. Probably the only one that had it was the TBI/CPI PCMs. Those you could do open loop idle, but still have closed loop during cruising. As far as closed loop goes, those computers had the best routines IMO. The later PCM that I have now is not as good from a closed loop standpoint. For some reason, GM chose to change it, even though the logic worked very well.
when you say unplug the ecm, this is not to say take it out completely
this really means disconnect the battery to reset, correct?
Yes, I usually just unplug mine so other things like the head unit dont lose power (and my preset stations, CD names, and so fourth). Disconnecting the battery will do the same thing. Some of these computers have capacitive back up, so you may need to leave it unhooked overnight. Although some computers dont store their closed loop data, this will help ensure that you clear everything.
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