IAC logic "Topic of the week 2/19/12"

[EagleMark]

I would love to see some definations like those in XDF parameter comments! And the info in there thats in between the parameters, could you add a read me in between the diffrent parameters for that info?

That can be done if someone was willing to spend the time...

If all XDF's were set up like that, life would be great.

Or we could have one of these?

[RobertISaar]

my A1/nAst1 XDFs are in the range of 700-800KB...

though A1 does things at least a bit differently than described for 42.

[JeepsAndGuns]

Dont we all need a easy button!

If I had a list of super awesome discriptions like those, and a matching list of what parameters they go to, and where they are in the XDF, and knew HOW to add them to the parameter comments, I would do it.

Though, at first, I would probably be selfish and the first one I would do would be the one I use the most ($0E)....lol

[93V8S10]

The Advanced $0E is 1.15 MB!

So JeepsAnd Guns is taking over expanding the $0E xdf, Cool! :)

[JeepsAndGuns]

So JeepsAnd Guns is taking over expanding the $0E xdf, Cool! :)

HAhaha, not quite. If I was to try it right now, if I didnt mess something up, most all parameters would end up saying "I have no idea what this does"

[PJG1173]

"I have no idea what this does"

LOL gee thats how I feel even when there is a description.

[daleulan]

I haven't played with the $0d/$0e idle control algorithms (I mostly tune my own algorithms these days). But a few things to think about. The later algorithms have PID control and usually a deadband right around idle where the spark control takes over. This prevents the IAC from moving back-and-forth and wearing itself out quickly. There is usually a time delay to enter PID mode. One thing to check is to log IAC stepper position, ignition timing, O2, MAP, pulse width, and RPM and compare a 'good' return to a 'bad' return. Getting more detail out would require logging separately the IAC terms for dashpot, proportional, integral, derivative, and stall-saver. I don't know if you can do that with a stock EPROM but I would be a bit surprised. However, compare (carefully) the various engine inputs and you should be able to see the biggest difference.

One thing that can be an issue is if the 'learned idle' ends up being off - ie. if the basic position is not right - that can cause wierd return-to-idle. Normally the table should be set up so very little PID is required to maintain the correct idle speed - the basic idle position table should be equal to the number of idle steps required. If you see a table for 'dashpot' or 'throttle cracker', you can try reducing those. I'd set one table at a time to zero and carefully log the response. If there was no difference, put the table back. Once you've got the engine not hanging up and maybe even stalling you can put that table back and slowly tweak it down. Some of the later GM ECMs have a VSS dependant throttle cracker (dashpot) which would make the engine behave differently whether the vehicle was moving or not.


You probably want a bit of a hangup - maybe by a hundred or so RPM. There is a risk of taking out all the dashpots and it'll return to idle perfectly 99% of the time. The problem is that 0.8% of the time it will stall, and the other 0.2% of the time it will stall then flood - I've done that too many times to count.

The later GM IAC algorithms think in a '% airflow' instead of IAC position steps; there is a lookup table that converts % airflow to idle steps. This allows someone to change the IAC after the rest of the idle has been tuned in; it also linearizes the function which allows for more stable PID gains to be used. Anything that is nonlinear can get unstable if the effective system gain increases. This physically-based model system is now the norm in automotive ECM calibrations and software. It allows mid-year equipment changes without having to redo the entire engine calibration. It also makes the calibration, well, just more.

Getting the return-to-idle perfect, every time, is a very large challenge - it sometimes can take me weeks of eight-hour calibration days to get it right. Usually there is a bit more hangup than I'd like, but I do need to worry about emissions in all of my calibrations. Getting the idle return to feel right and never undershoot or stall is probably more challenging than meeting the emissions at that point, though. One of my acid tests is to apply power steering full-lock, air conditioning off-to-on, headlights on, and move from neutral to drive, all at the same time, during a recovery from a couple of pedal stabs.

[PJG1173]

wow might have to re-read that one a couple times to absorb. What is a PID?

[RobertISaar]

proportional integral derivative.

math term from calculus IIRC.

[93V8S10]

Would "throttle kicker" be another name for "throttle cracker"?

[RobertISaar]

i've never understood what either meant.

maybe it's my lack of experience with TBI stuff though, since that's mainly the only place i've seen either term.

[JeepsAndGuns]

It seems I have seen the term "throttle cracker" in $0E before. However, at this moment, I cant remember if it was a actual parameter, or if it was just mentioned in a parameter comment.
(I dont have my laptop on at the moment to pull up TP and look)

Also, what is "dashpot"?

[PJG1173]

the only mention of throttle cracker in $0D is a flag "Use Extended Throttle Cracker in DFCO". there is a whole bunch of throttle kicker stuff but no explaination of what it does.

[EagleMark]

Thanks daleulan for a great post of IAC information!

Dashpot is a mechanical deveice used on carburated vehicles to slow return of throttle linkage to idle. I remember these being introduced to cure an emmission when slowing the throttle to idle instead of full snap because of my mechanics training in 1974 to 1980 (before ASE certification) they were being phased in even on auto cars, manaul were even more so and same vehicle with auto and manual had differant part number, manual was slower return to idle. Although they may have been used before emmissins to prevent stalling.






Some were called dashpots but were actually soliniods to kick idle when AC was on or how ever they were wired to increase idle. Little for AC and a lot for say a hydralic PTO pump on equiptment.

To tranalate that to EFI, IAC is idle control and all the IAC parameters are some form of daspot.

Throttle kicker in $OD, $OE is known to be part of some TBI units used for equiptment as we have seen pictures of them to raise idle so if it is worded correctly in XDF would be a solinoid and no use to us if we did not have one of those TBI units with some kind of stepper motor built in to raise idle.

Throttle cracker would be a stall saver and some of what is commented in XDF as Throttle Kicker could be same?

IAC delay would be a dashpot to slow throttle closing (by adding air through IAC) for emmisions or idle stability.

One point made by daleulan that I thought interesting is mention of Idle, VSS control of IAC to prevent them from wearing out. I haven't done enough data logging with masks with Idle and Off Idle tables to notice but he's saying when switched to OFF idle IAC would stop moving. This would not only help wear of IAC but also force more air over throttle blades to further mix/atomise fuel and air in TBI units instead of air coming through IAC which would do less, IMO. My 1227747 $42 has one table and IAC moves constantly even at highway speeds. My LT1 $EE has 2 tables like the $OD, $OE stuff but I never paid attention to IAC counts other then idle. On MPFI injected air through IAC or throttle plates would be of no gain for fuel atomization...

[PJG1173]

doesn't the IAC add to overall CFM that the TB can flow? if you stop it from moving when off idle i would imagine that it would cut off some of that flow.