PCM Diagnostics & Tuning - Idle Operation Part II

gameover has posted additional info to the original "Idle Operation Part I" Thread. Here is the New Info and a link back to the Original thread:


okay so here's some extra info re ETC. I'm just adding all this stuff to our editor at the moment so its a good time to write this down...

Let's pickup at the "Desired Idle airflow part, which is the final "airflow" value the idle control routines deliver to set either the IAC motor position or the ETC position.

After the "Desired Idle Airflow" is calculted , it is then translated into an "Effective area" value in square millimeters (mm2). This is the cross sectional area required to deliver the airflow desired (taking into account air density and pressure ratio across the throttle/IAC). Now at this point the calculation branches to either IAC or ETC.

If IAC is installed the "Effective Area" is translated into a number of "steps" that delivers this area (a table of IAC Steps vs. Effective Area).

If ETC is installed then there is a single value that translates "Effective Area" into "Desired throttle area percent" units of % area per mm2. This number is then handed over to the ETC routines that control the ETC TPS %.

The ETC logic is quite simple in that it has two main inputs the Accelerator Pedal Position (APP%) and the "Desired throttle area percent" (IAC%). In the ETC code there is a maximum value that the IAC% is clipped at as a safety check (mostly on the throttle cracker), but the Throttle Cracker, Follower, Adaptive and various compensation routines are all the same tables (ie. the ETC code has nothing extra).

Now, the ETC looks at the APP% and checks if it is 0. If it is not 0 then the commanded ETC position is a direct function of the APP%+IAC% (accounting for throttle cracker). If it is zero then the ETC position is controlled via the IAC% (as you would expect).

Once this ETCDesiredThrottleArea% has been passed thru the numerous limiter functions (ETC RPMlimiters, MPH limiter, TorqueMangement etc.) the final ETC Rotation% is calculated via a simple transfer function of ETC Rotation% vs DesiredThrottleArea%.

There are maximum slew rates, minimum position checks and a few other bits n peices here as well, but in terms of idle nothing else comes into it.

Unless you have modified your ETC in anyway, there whould be no reason to change the ETC% vs EffectiveArea scaler or anything else there as far as i can see.

Hope that helps,

Chris...
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Here's the Link-> http://www.ls1tech.com/forums/showthread.php?p=2044503#post2044503

Unless you have modified your ETC in anyway, there whould be no reason to change the ETC% vs EffectiveArea scaler or anything else there as far as i can see.


For the mechanical-throttle cars, a common mod is a throttle body
"porting". This often includes an epoxy filling of the PCV and IAC
air ports which are then drilled back open, generally at a smaller
orifice size.

This being the case, the effective area may be "clipped" by that
orifice / passage. People who underdrill the hole sometimes report
problems with idle stability.

Of course the "right answer" is probably in the drill bit and not in
the PCM.

here's a pic from a stock TB.

http://www.hptuners.com/pub/chris/IAC.jpg

Good point jimmy. Makes me wonder what affect the other mod we all did has? Namely diconnecting the coolant line to the TB. What would that do to the Base Running Airflow table that is based on ECT? Since the PCM would asume the TB is at that temp.

here's a pic from a stock TB.

http://www.hptuners.com/pub/chris/IAC.jpg
Well, that explains why I can't scale the "IAC Park" table by a percent.

Good point jimmy. Makes me wonder what affect the other mod we all did has? Namely diconnecting the coolant line to the TB. What would that do to the Base Running Airflow table that is based on ECT? Since the PCM would asume the TB is at that temp.
Good question. I've noticed with recent cold weather that there is a slight difference in idle transition during cold mornings until the engine heats up for a while. It takes more than just ECTemps being reached. I suspect that TB's expand and contract and the gap between the TBody and TBlade changes. Could the coolant line heat up the TBody faster and minimize this affect?

Good question. I've noticed with recent cold weather that there is a slight difference in idle transition during cold mornings until the engine heats up for a while. It takes more than just ECTemps being reached. I suspect that TB's expand and contract and the gap between the TBody and TBlade changes. Could the coolant line heat up the TBody faster and minimize this affect?
That would kind of make sense, especially since aluminum and steel expand and contract at differently. That's one reason I haven't decided whether or not to do the TB bypass on my S3...when I get it.