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AFR closed-loop behavior

Old Mar 31, 2016, 01:40 PM
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AFR closed-loop behavior

I want to start by saying that I'm relatively new to the tuning scene. But thanks to the helpful individuals and posts on this site, I've been able to pick it up quickly. That being said, I have a couple questions.

My first question is in regard to the vehicle's AFR closed-loop behavior. Specifically, does the vehicle use the narrow-band AFR sensors to attempt to maintain stoich? Or does it actually try to hit the commanded AFR?

Most of the literature I've read on the subject (like Merlin's excellent tuning guide) state things like "in theory fuel economy during closed loop should be the same for any trim, seeing that the ECU is trying to keep a stoich 14.7:1." In an attempt to improve fuel economy and learn more about the vehicle, I've set my AFR to run leaner than stoich (~16:1) at idle and under low-load, cruise situation. In my situation, is the ECU still trying to maintain stoich or is it trying to maintain my commanded AFR? I have a wideband system set up and althought the readings trend updward when I lean it out, I'm still not hitting the commanded AFR while in closed-loop.

Of note: I've managed to dial in my idle and cruise fuel trims to within 1% (excellent, I know). But, I've noticed that my target idle (742RPM) is now very difficult to hit; it's constantly ~100-150 RPM above the target RPM. Also, although my LTFT at 60mph cruise hovers around 0%, when I increase my cruise to about 85mph my LTFT jumps to ~4%. I know I can fix this by adjusting my MAF scaling but before I did, I thought I'd check to see whether the issue could be related to the ECU's closed-loop AFR behavior.

Cheers and thanks in advance!
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Old Mar 31, 2016, 04:20 PM
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closed loop it goes off narrowband volts to try and maintain Stoich.

If you command the car to go into open load at those loads you can run leaner or richer (based on the main fuel map) but the narrowband loses resolution further from Stoich.
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Old Mar 31, 2016, 07:59 PM
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Razorlab,

Thank you for the response. I very much appreciate it.

Just to clarify: when the ECU is operating in closed-loop mode, it is ALWAYS attempting to maintain stoich (14.7:1, for gas), irrespective of the actual, commanded AFR?
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Old Apr 1, 2016, 08:42 AM
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Originally Posted by Dr. Jekyll View Post
Razorlab,

Thank you for the response. I very much appreciate it.

Just to clarify: when the ECU is operating in closed-loop mode, it is ALWAYS attempting to maintain stoich (14.7:1, for gas), irrespective of the actual, commanded AFR?
That is correct.
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