Return-to-idle tuning for a 2002 Corvette

[antus]

In the earlier aldl delco pcms stall saver only kicks in at very low rpm, during a stall event. so it wont make a difference in normal running or idle. I think it adds a lot of spark, maybe fuel too to try and up the torque dramatically and get the engine above stall rpm before it stalls. hence 'stall saver'. I assume the logic is same or similar on these pcms.

[AngelMarc]

Reason I ask was a wandering idle in my bastardized 12587603 tune for the CB450 with most of the sensors gone. The one that shouldn't be speed density based on descriptions, but behaved like it was. Most idle stuff was gone/zeroed out, no IAC, no spark correction... as far as I know anyway.

[NSFW]

I would expect idle to wander around if you turn off all features that are there to stop it from wandering around.

With that stuff disabled, mine oscillates so much I'm sure people assumed I was blipping the throttle on purpose.

[AngelMarc]

It was more of a slow creep and hold, then eventually creep the other way.
And I had all the things disabled. No IAC, closed throttle drive spark table only and no modifiers affected it, as far as I was able to tell with a scanner recording data. Open loop fueling.

[antus]

So the question becomes, what feature can the pcm still use to correct it? IAC and idle spark correction would normally do this.

[AngelMarc]

The question is, why would it wonder in the first place? Data in, data out.
Eh, never did much tuning with it. Maybe spark was over-retarded at low end in a way that once it went high it'd stay there, or something.

[antus]

Its an analog world, with analog hardware, on a digital control system. in a perfect simulation it would not, but in real life everything isn't 100% stable and has minor variations.

[AngelMarc]

Eh, all the filtering, a good tune should sort it without the extra fluff.

[NSFW]

Spark timing seems to do most of the work... The factory Corvette tune doesn't make airflow corrections until the engine speed is more than 80 RPM too high or too low, but within that range it will vary the spark +/- 5 degrees. In the logger I can watch it make brief small airflow adjustments every second or two, but that's with an overmarket cam. I never looked when it was stock but I have a hunch that airflow correction wouldn't be needed at all for steady-state idle. Probably still useful for transitioning to idle from cruise or stall though.

With a "big" cam (lots of overlap) you get exhaust contaminating the intake charge, kinda randomly, so combustion isn't predictable and the air/spark feedback loops stay pretty busy trying to keep RPM where it should be.

With a fixed throttle blade angle and no spark corrections, it seems like there's a cycle between between high-ish RPM, high vacuum, lots of exhaust reversion, which causes RPM to sag, which reduces vacuum and therefore reversion, so RPM climbs again, and the cycle repeats.