Inconsistent AFR near idle

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dpstark2
Helpful MS/Extra'er
Posts: 41
Joined: Tue Nov 20, 2012 10:22 am

Inconsistent AFR near idle

Post by dpstark2 »

Hi all,

I have tried tuning this issue myself in a number of ways, but I always end up in the same situation. Note that the car doesn't have a great tune in it, but it's generally good. Starting up, warm-up enrichment keeps my AFR in a reasonable range for the motor (225 /6 in my 72 Dart). By the time I get down the road and the motor gets to temperature, my AFR swings way lean (15-16:1). At this point, underhood temps are still pretty low. By the time I get to work some 10 minutes later, the AFR is more rich than I'd like it (12:1). At this point, underhood temps usually approach 125F or more on hot days.

I have tried the MAT/CLT blend, limited tuning to nice low underhood temps, etc. All end up in the same situation, so I am wondering at this point if I'm missing something. The current configuration has the blend to almost 100% near idle, rapidly coming off that by the time I'm cruising at a low speed. AFRs here are good.

In the attached log, you can see what I am describing. At 77s, I have reached the end of my main street and sat at idle for a bit. at about 213s, I'm down the road at a stop light and the motor is up to temp but I get about 16:1. By the end (884s) I'm parked at work and back down to 12.8:1.

Any ideas what I'm missing here? My other car does similar stuff but rarely sees this big of a swing and I haven't used the blend table at all with it. I hesitate to mess with the air density table because that affects everything and this seems limited to idle/low RPMs. I could raise the "warm" temperature, but the car sometimes doesn't stabilize above ~170F in the winter. I just feel like I'm missing something someone with more experience might know.

Thanks!
Attachments
2018-01-17_16.29.20.msq
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2018-01-17_15.31.36.zip
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SwedCharger-67
Experienced MS/Extra'er
Posts: 264
Joined: Sun Dec 23, 2012 2:20 pm
Location: Nordmaling, Sweden

Re: Inconsistent AFR near idle

Post by SwedCharger-67 »

When your engine is up to temp the EGO correction switches off below 1300 rpms, and engine goes a bit rich.
Might be a good idea to disable "include AFR target", disable EGO correction and fine tune the VE table a bit closer to ideal values. :)
Martin, Sweden
Mopar 512 cui, 8.4 liters, high compression, fuel E85, full sequential control by MS3X, burning rubber just blipping the throttle... :D
DaveEFI
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Location: SW London, UK

Re: Inconsistent AFR near idle

Post by DaveEFI »

Can be worth setting all the VE table cells round about idle to the same value, group them, and trim for the best idle.
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dpstark2
Helpful MS/Extra'er
Posts: 41
Joined: Tue Nov 20, 2012 10:22 am

Re: Inconsistent AFR near idle

Post by dpstark2 »

I've done both of these things, but that doesn't mean I did them correctly. So the opinion is that my VE tables are the culprit?

My main question is why the same conditions result in 16:1 AFR with the motor at 170F and MAT near 70F, but 12 or 13:1 when the motor is 175F and the MAT is more like 120F. I know nothing is perfect, but I would expect that the AFR wouldn't vary this much. I've gone the route of tuning the VE table with the car hot, but that results in a too-lean-to-run condition when the warm-up enrichment turns off. To even get it to run that way, the warm-up table had to be turned way up. Going the other of tuning as soon as the motor is up to temp but MAT is low, I get too-rich-to-run conditions after driving a while. And higher RPMs seem unaffected. This is why I focused on the MAT/CLT blend for a while, but nothing I've done there has fixed the issue. It got me closer- the swing is now 12 to 16:1, rather than 10 to 17:1.

I've also checked fuel pressure, which seems consistent (hard to do with a gauge under the hood).
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