iac position during driving

[Seight-v8]

hello all,

driven my car for the first time today with a new engine & vems fitted (rover v8)

just been wondering what position the stepper motor should be at while i'm driving.

I was under the impression that it should be closed (ie position zero), but on idle would be at position 22.

When i looked at my log when at 2500rpm its at position 39.

Its the standard 4wire bosch/rover v8 stepper motor, with vems V1.1.64.

cheers

scott

[rob@vems.co.uk]

isnt there a IAC follows TPS setting in the idle settings?
I think this means the IAC opens with the throttle - which I'd imagine would be a bad thing on a turbo'ed motor if your IAC goes to atmosphere.

[Seight-v8]

yes mine was enabled when i drove the car, i wondered what this settings meant.

Engine seemed to dive when throttle was closed, but this might be fuel cutoff, looking at log, as it was set for 20kpa.

Not sure if i should be using fuel cutoff either....

Lowest map reading i got was 8kpa...

Idle speed still seems a little erratic .

Not bad for first drive, and it did feel good, after 2 years hard work.

scott

[Sprocket]

With the 'follow tps when not idle' disabled, the stepper should resume the refference position set in the refference table. With 'follow tps when not idle ' enabled, the stepper motor will move its refference position as the throttle is opened, so when the throttle is released, the stepper will return to the refference position gently smoothing the return to idle.

On the Rover Mini, I tune to Rover's specification of - idle advance 15 deg+-5, idle air stepper refference position warm 20 to 40 steps, warm start refference position 80 steps.

Ideally you need to fix the stepper position to 30, and fix the idle advance to 15 degrees, by disabling ignition idle control, then tune the throttle stops to give you your idle speed. Then enable and set the ignition idle control thresholds for 10 degrees advance, 10 degrees retard and IAC degrees to 5.

Stepper motor should not ever be zero, if it is, it needs tuning.

Hope that helps

[tweak]

It looks like the IAC will return to it's reference position when the throttle is released in disregard of this setting. The only advantage i can see is that more air can be feed to the engine thus giving the pedal a feel of better response. But it does not help for the real issue, which is the IAC control algorithm building up a huge integer when in overrun.
Which again will lead to the engine stalling when clutch is released. I spend hours and hours trying to overcome this and i found that i had to use both lambda enrichment and ignition advance at idle level, in addition to the main throttle screw physical fine-tuning, plus of course the PID settings. I found that the giving the D value very high weight and the I value almost no weight (1) worked best for my car. The Proportional value has to be high as well, but not too high to get oscillation. In a fast moving system like an engine the D value is very helpful in my opinion, but unfortunately it seems that the D value only starts to give in when the P value is exceeded, which means that if RPM still falling rapidly when at the setpoint RPM, it will not start to counteract it until it is starting to overshoot. Having the D value very high helps though. Ideally it would start weighting in the D value way before the setpoint is reached so that the set RPM  can be reached without overshoot. Or the integer value has to remember it's previous setting from normal idling and take control at a level where 'real' idling is present.

In my opinion the easiest way to deal with this at SW level is that this control algorithm needs to be activated only when the RPM is under a certain level, or the MAP is above a certain level. Otherwise the PID idle will learn the system under false circumstances. Giving the Derivative value more to play with also will help, but tuners will have to use it with care of course.

[GintsK]

isnt there a IAC follows TPS setting in the idle settings?
I think this means the IAC opens with the throttle - which I'd imagine would be a bad thing on a turbo'ed motor if your IAC goes to atmosphere.

What relates to turbo engine with IAC to atmosphere, closing rate seems too slow for me. It is understandable at small throttle openings, where fast closing can hurt driveability. But when boost hits fast, 1 second or more prolonged closing means boost leak

Gints

[mattias]

No turbo engine has the IAC routed to atmosphere.

Personally I have no trouble at all with the way I tune the IAC. I set the reference position to be 200-300 rpm above the target idle rpm at any particular coolant temp, and let the integral decrease have enough room to close the valve enough to meet the target. I don't use "follow TPS" and I set the "close delay" to 10+ secs to avoid driveability problems.

[GintsK]

No turbo engine has the IAC routed to atmosphere.

First generation of Imprezas  IMO...

[Denmark]

close delay of 10+sec,

Is the time in sec, in vemstune?,

as i got 77, in mine, and have had that all the time.


And yes impreza´s up to 99´have iac to atmo


/Skassa

[mattias]

And yes impreza´s up to 99´have iac to atmo

My mistake, but I bet they are one of few manufacturers who do it that way. If you write the IAC algorithm and use the valve accordingly it's not a problem, but a DIY person that replaces the stock ECU must know better or be aware.

I assume Subaru only use MAP sensor + intake air temperatur (aka "Speed-Density") in the stock ECU.  Do they route the air through the air filter?

[rob@vems.co.uk]

Nope, Subaru have only a MAF sensor, and an IAT in the airbox - dnb tells me that the only function of the IAT is to retard the ignition by a few degrees once the temp exceeds a certain threshold.
There are a load of small differences in the implementation of european and japanese management systems, the most obvious being the triggers they choose to use.