ISC Duty Cycle question - RPM target too low?

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ISC Duty Cycle question - RPM target too low?

Postby blk91gt » Thu Jun 28, 2007 1:49 am

I have a few minor issues with my otherwise decent idle - with teh car idling steady if I tap the gas and release the RPM will drop below target, then surge and after a few occillations it will finally settle back at target rpm.

I've been watching my duty cycle carefully and I was hopeing for some feedback. With the ISC unplugged the car will just barely idle at around 650 rpm. I set the thorttle body airflow scalar based on my logs and my MAF curve is very close to ideal. I love a low idle and have tried 750-780 rpm. At 780 (the highest I wish to idle) logs an ISC duty cycle of ~28% at 770rpm which I prefer, I am in the low 20% range.

I have a feeling the ISC may be to blame for the surge on throttle release not being able to compesate due to the low duty cycle. Either than or my thorttle plate is too far shut. Now if I open the throttle up more physically the ISC DC of course will suffer as well.

Am I simply hoping for a lower idle than possible/practical?

Any thougts on the mater would be apreciated.
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Postby cgrey8 » Thu Jun 28, 2007 7:26 am

You could probably slow down the dashpot of the ISC so it doesn't repond quite so fast and thus doesn't overshoot and hunt to find idle. You can also decrease the width of spark control the idle spark vs RPM error function has. This way, spark isn't bouncing up and down and part to blame for you overshooting and undershooting.

With my engine, I can hit the clutch while the engine is coasting in gear, the RPMs drop below setpt, but bounce right back to about where they are supposed to be, and gradually drop into set idle. At first, I was concerned about letting it undershoot the RPMs, but it's never caused a problem, it catches itself gracefully. The interesting thing is my girlfriend's (completely stock) 2005 Scion tC does the EXACT same thing. So I leave it alone and call it good.
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Postby 86GT » Thu Jun 28, 2007 9:40 am

blk91gt

Take a look at the ISC neutral idle air flow function. Chances are it is not set right. The idle strategy will look up the RPM and then apply the correct DC to the ISC. If the look up values for the ISC DC is to small then theidle RPM will drop to much and the secondary idle control will kick in. The secondary idle control is the spark control.

Chris
The dip that you are seeing is probably caused by the dashpot decay function. Look at the bottom of the decay function. It dips down and then comes back up.. There is also a minimum dashpot that can be set in the scalars.
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Postby cgrey8 » Thu Jun 28, 2007 11:29 am

86GT wrote:...Chris
The dip that you are seeing is probably caused by the dashpot decay function. Look at the bottom of the decay function. It dips down and then comes back up.. There is also a minimum dashpot that can be set in the scalars.

My base tune is the X3Z. Is there something inherent to that tune that would make me need to change it to stop the overshoot? If so, I'll gladly tweak it especially since I haven't messed with it yet. Can you confirm with PID you are refering to? I think it's FN879, but there is the decay and the preposition.
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Re: ISC Duty Cycle question - RPM target too low?

Postby do'h » Thu Jun 28, 2007 12:26 pm

I had a similar issue that drove me nuts for a long time until I started messing with the lower end of the maf curve I Leaned it out a bit up to 1 v problem solved. Idled like stock. I am not suggesting that this is your problem however just something you might want to try out just for kicks.

blk91gt wrote:I have a few minor issues with my otherwise decent idle - with teh car idling steady if I tap the gas and release the RPM will drop below target, then surge and after a few occillations it will finally settle back at target rpm.

I've been watching my duty cycle carefully and I was hopeing for some feedback. With the ISC unplugged the car will just barely idle at around 650 rpm. I set the thorttle body airflow scalar based on my logs and my MAF curve is very close to ideal. I love a low idle and have tried 750-780 rpm. At 780 (the highest I wish to idle) logs an ISC duty cycle of ~28% at 770rpm which I prefer, I am in the low 20% range.

I have a feeling the ISC may be to blame for the surge on throttle release not being able to compesate due to the low duty cycle. Either than or my thorttle plate is too far shut. Now if I open the throttle up more physically the ISC DC of course will suffer as well.

Am I simply hoping for a lower idle than possible/practical?

Any thougts on the mater would be apreciated.
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Postby blk91gt » Thu Jun 28, 2007 12:45 pm

CGrey: I adjusted DASMIN scalar per cougar5.0s advice and it did improve the hanging before the idle settled when I clutched in to a stop. However this error on a slight rev still causes an uncomfortable dip and surge. It may also be the blame for slow speed parking lot manouvering and the occasional stall. No ammount of messing with dashpot decay and the dasmin scalar helpped unfortunatly. Al the did was leave me with excessive rpm and a long hanging time. I feel the hang time is fine now as is the slow drop to idle, but this issue apears to be caused by somthing else. Perhaps it is the timing rpm error control. I took your advice and set it to the same value as I'm using in the spark tables for idle and timing is now much more stable at idle. Perhaps I tool away too much timing control though?

86GT: Ive set up the neutral idle airflow ith EA, but I probably didn't do it correctly. The stock A9L only has 3 rpm points: 4080 1952 and 672. I tried adding a few more which I imported from the EA calculations. ranging from 700, 900, 1100 1500 and 4000. It did seem the stabilize things a bit, but This persistant drop in rpm continues. What do you recomend as far as the export of ISC airflow calculations from EA. Shoul I just change the 672 RPM box to my actual idle rpm and use the calculated value for that rpm and leave the rest alone?

This stil leaves the ISC duty cycle which acording to what i've read here is too low. Should I be content with high 20% range or should I increase my idle until I see 35% (which will have me idliing around 850 rpm (higher than I want). Does the TB airflow scalar effect the duty cycle - ie. does reducing it cause a change in duty cycle directly?
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Postby cgrey8 » Thu Jun 28, 2007 12:55 pm

The only negative I've noticed to narrowing the amount of timing the idle timing vs RPM error function has to work with is that when I turn the blower motor on or my headlight on, the RPMs drop by maybe 20RPMs. This is miniscule and easy for me to ignore. However my concern is how the engine will behave once I get my AC charged and ready to use. How is the engine going to handle the AC kicking in or is any AC compressor load issues easily handled by the AC scalars?
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Postby Cougar5.0 » Thu Jun 28, 2007 1:16 pm

If your ISC DC is low, aren't you supposed to close up the throttle plate a bit so that the ISC circuit will have to pass more air (ie, higher DC)?
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Postby cgrey8 » Thu Jun 28, 2007 1:52 pm

Was that directed at me??? I have no clue. I've never messed with the throttle plate tweaks in my tune except for the scalar that indicates how much air (in lb/min) the TB flows at closed throttle. I pulled the ISC, saw avg what the MAF was saying, used EA to calculate that kg/hr into lb/min, and typed that in for a scalar. I think the default was .55 and mine was .53??? It's been a while since I messed with that.
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Postby blk91gt » Thu Jun 28, 2007 5:34 pm

Cougar5.0 wrote:If your ISC DC is low, aren't you supposed to close up the throttle plate a bit so that the ISC circuit will have to pass more air (ie, higher DC)?


Yep, and it does, but if I close it any futher it wont idle when the ISC is unplugged - hence I wonder if I am hoping for to low an idle.
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Postby Cougar5.0 » Thu Jun 28, 2007 8:38 pm

blk91gt wrote:
Cougar5.0 wrote:If your ISC DC is low, aren't you supposed to close up the throttle plate a bit so that the ISC circuit will have to pass more air (ie, higher DC)?


Yep, and it does, but if I close it any futher it wont idle when the ISC is unplugged - hence I wonder if I am hoping for to low an idle.


Oh, c'mon - who cares if it idles when the ISC is unplugged? I never unplug it, I just let EA calculate it based on the normal idle speed. I dial in the ISC DC while logging live. Kinda difficult for the idle circuit to work properly if the ISC is so close to the min level (17%) that it cannot properly control the idle?
Last edited by Cougar5.0 on Thu Jun 28, 2007 9:06 pm, edited 1 time in total.
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Postby Cougar5.0 » Thu Jun 28, 2007 9:05 pm

cgrey8 wrote:Was that directed at me???


No, you were talking about the idle speed adjustment using timing. I will comment that the A9L has different idle speed timing adjustment depending on whether it is in gear or not (FN841N/D). I use the neutral switch to lockout starting in gear (track legality cause I have no clutch switch) but noticed that the timing swung more when I slid it in gear because the ECU thinks I am putting it in drive. I made them the same (the smaller neutral adjustment is all that should be needed). The neutral and drive air are also different (FN875N/D) so I made these the same as well (and dialed in for the big TB). Kinda odd for a processor that is supposed to be for a manual transmission huh?
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Postby blk91gt » Thu Jun 28, 2007 10:27 pm

Cougar5.0 wrote:
blk91gt wrote:
Cougar5.0 wrote:If your ISC DC is low, aren't you supposed to close up the throttle plate a bit so that the ISC circuit will have to pass more air (ie, higher DC)?


Yep, and it does, but if I close it any futher it wont idle when the ISC is unplugged - hence I wonder if I am hoping for to low an idle.


Oh, c'mon - who cares if it idles when the ISC is unplugged? I never unplug it, I just let EA calculate it based on the normal idle speed. I dial in the ISC DC while logging live. Kinda difficult for the idle circuit to work properly if the ISC is so close to the min level (17%) that it cannot properly control the idle?


I hear ya, but i am concerned that if the car won't idle with teh isc unplugged, it is that much more likely to stall if the ISC dips closed (which its seems to be doing).
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Postby 86GT » Fri Jun 29, 2007 12:06 am

DASMIN is what I was referring to chris.

The ISC neutral function needs to be adjusted each time the MAF is adjusted. If you do not re adjust the ISC neutralk function then the idle can either hang or dip to far. It is pretty important that the ISC neutral function matches the MAF function.
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Postby blk91gt » Fri Jun 29, 2007 12:39 am

clint:

Can you give me a little guidance on how exactly to enter the Neutral Idle Airflow? As I asked above, The A9L only has 3 RPM points in which to enter data: 4080 1952 and 672 do you sugest adding more RPM data points or just modifying the 672 RPM box; should it be changed to my idle RPM target?

Thanks!
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Postby 86GT » Fri Jun 29, 2007 9:26 am

I use one point for the zero referrence and then next one is for the idle that you are targeting. In my case it is 760. I then use 1000 and then 1500. I will keep using points until i get upto 1.99 on the air flow.

Does that make sense.
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Postby blk91gt » Fri Jun 29, 2007 12:22 pm

It does thanks

Should the ISC Drive function mimic the Neutral Idle Airflow?
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Postby 86GT » Fri Jun 29, 2007 7:45 pm

Yes, it is a tad higher at the idle to compensate for the load caused by the trans.
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Postby blk91gt » Sat Jun 30, 2007 12:56 am

Ok, so I was outside playing with the idle tonight and I decided to set all the ISC and timing functions relating to idle back to stock and then do the Idle reset procedure one more time.

I was able to get the car to idle at 690-710 rpm with the ISC unplugged I logged TB airflow and entered it into the scalar. Now if 690 is as low as it will idle does that mean I *MUST* set my desired idle rpm to 122 rpm higher (IE 812 RPM)? I like it to idle around 780 RPM and when I set it as such the ISC Duty cycle drops down to 17.5% (the minimum). When I turn my A/C on though the rpm is kicked up 25 rpm and the ISCDC is about perfect at 33-34%.

i'd rather not idle over 800 RPM, but If I have to to keep things working properly I'll do it I guess.

The side effect to all of this is that the dip and suge is now gone!

Man EEC IV idle is frustrating when you are as picky as me! :)
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Postby blk91gt » Sat Jun 30, 2007 3:42 am

Ok here is somthing interesting from over at Stang Net:

Another method of setting idle that Clint hints around at and that I've seen in Don's SCT training videos is to lock the injector duty cycle at 35% in the tune and then adjust the TB set screw to achieve the idle that you want then set the ISC D/C back to stock.

This involves setting the ISC DC multiplier to 0 and adder to 0.35 and resetting the KAM.


The multiplier part I found, but anyone know what adder he is talking about???

This would really be great if I could hang the ISC at 35% and then run the screw until I get desired idle RPM.
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Postby Cougar5.0 » Sat Jun 30, 2007 10:18 am

Perhaps the ISC Adder vs RPM (FN810) is what they are talking about. This is added to the calculation and if the multiplier for the first part of the equation is "0" then the output will be 0.35 if you set the ISC Adder vs RPM (FN810) to 0.35 for all RPMs.

What multiplier did you find? I'm not sure about this one.

I did notice that FN810 (ISC Adder vs. RPM) and FN824 (ISC Gain vs. RPM Error) are set-up based on the stock 672 idle speed. I offset these two functions for my 768 RPM which can only help - thanks for bringing this up.

Your desire to set idle with the ISC unplugged is causing you a lot of problems - I can't comprehend why you are causing yourself these problems when there is no need to unplug the ISC - oh, well.
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Postby blk91gt » Sat Jun 30, 2007 3:50 pm

Cougar5.0 wrote:Perhaps the ISC Adder vs RPM (FN810) is what they are talking about. This is added to the calculation and if the multiplier for the first part of the equation is "0" then the output will be 0.35 if you set the ISC Adder vs RPM (FN810) to 0.35 for all RPMs.

What multiplier did you find? I'm not sure about this one.


Pretty sure the multiplier is FN820B - ISC Duty Cycle Mult vs. Load. I can't belive I didnt see the adder, thanks for clarifying that! I'm going to go try this now.

I did notice that FN810 (ISC Adder vs. RPM) and FN824 (ISC Gain vs. RPM Error) are set-up based on the stock 672 idle speed. I offset these two functions for my 768 RPM which can only help - thanks for bringing this up.


No prob. how did you calcu;ate offset?

Your desire to set idle with the ISC unplugged is causing you a lot of problems - I can't comprehend why you are causing yourself these problems when there is no need to unplug the ISC - oh, well.


Well the throttle plate being too far cosed seems to be the cause of the dip and surge I was experiancing wheb the acellerator was released. Logic dicatated that if the plate was slamming shut (too far) and the isc was then opening fully to compensatethis must be the cause. Increasing base idle does seem to have cured it but the result is the ISC being now too far closed at regular idle. I'm going to have to find a happy medium.
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Postby blk91gt » Sat Jun 30, 2007 7:06 pm

Well I tried the posted method - didn't work changing the adder all to .35 and the multiplier to 0 yeilded a datalog that shoewed no ISC duty cycle at all. So I changed the multiplier to 1 and it brought the ISCDC to 30.7 flipping between it and my normal tune that shows the ISC pegged at 17.5.

The car idles a bit high with the ISC pegged at 17.5% but concidering the surge is gone i can live with it. Reducing the throttle screw slighly raised DC but the surge came back. I think I must be asking for too low an idle given the size of my cam and the low 10in of vacume it produces.
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Postby Cougar5.0 » Sat Jun 30, 2007 7:58 pm

Ahhh, I didn't see the Stage2 cam & didn't know about the 10in vacuum. I settled at 768 RPM based on 14" - 14.5" of vacuum. There was NO way to keep it stable much below this. Knowing this, I would be surprised if you could get a really good idle below about 850 RPM. Did you realize that there are like 10 - 15 different items that need changing when you up your idle like this? It's a real pain and hardly anybody understands ISC & the associated scalars & functions completely. Add to that a low vacuum which will be extremely difficult to make stable at a low idle speed and you are looking at surging.

Here is a best estimate of what I changed to up idle to 768 RPM which also included a larger TB (A9L in parentheses):

Scalars

Idle Speed Drive/Neutral - 768 (672)

- these two up limits so you don't hit them with a higher idle RPM
ISC Max Idle Closed Throttle (IDLRPM) - 975 (850)
ISC Max Drive RPM (ISCLPD) - 952 (824)

- this helps when declutching a scortching S/C engine @ 100 MPH in 3rd
ISC Min DASPOT for Declutch (DASMIN) - 0.25 (.0938)

- these helped ensure nice stable cold/warm startups
ISC Startup Kickdown Time (TKDTM) - 50 (25)
ISC Startup RPM Adder (BZZRPM) - 104 (64)

- this is for the 85mm TB
Throttle Body Airflow (ITHBMA) - 1.00 (0.55) (EA calc'd)

Functions

- due to hot S/C air causing high idle!
ISC Adder Versus ACT ISC Adder vs RPM (FN825B) - max of 8 RPM added versus 72

- this adds air when idle dips low - added 100 RPM due to ~100 RPM higher idle
ISC Adder vs. RPM (FN810) - changed RPM scale 600 to 700, 450 to 550

ISC DASPOT Decay Rate (FN879) - extended the decay time to minimize dips on declutch

ISC Neutral/Drive Air (FN875N/D) - adjusted using EA & datalogs

ISC Startup RPM vs. ECT (FN826A) - added RPM @ temp - 200 @ 100F (48 )

- this one is centered on idle set speed.
ISC Gain vs. RPM Error (FN824) - centered "1" at 768 RPM (672 stock) - see below:

4080 3.00 4080 3.00
992 3.00 896 3.00
784 1.00 688 1.00
752 1.00 656 1.00
592 3.98 496 3.98
0 3.98 0 3.98
0 3.98 0 3.98

Clint - note that FN824 should be part of the IDLE level in BE.
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Postby blk91gt » Sat Jun 30, 2007 9:48 pm

Thanks for the look at your settings Cougar. I'll try some of them tomorrow and see if they help my cause - thanks for posting them! i'm particularly curious as to the ISC adder vs RPM function might help my dip/surge issue. You as so right about people not really having a lot of info on how the ISC functions work and react to changes.

This is my last major issue, and this tune will be nearly perfect.

I know i'm probably targeting too low an idle given my cam and lack of vaccume but the car actually does idle very nicely without a hnt of surging with the only exception being this short blip of the thorttle. This sure makes staging the car and creeping into my carport late at night difficult.
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Postby 86GT » Sun Jul 01, 2007 12:44 pm

Cougar5.0 wrote:Clint - note that FN824 should be part of the IDLE level in BE.



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Postby blk91gt » Mon Jul 02, 2007 2:15 am

Cougar: did you touch Dashpot pre-position at all?

I'm curious also what are your decay rate settings?
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Postby blk91gt » Fri Jul 06, 2007 12:22 am

Well, after raising my target idle to 800 going thru the re-learn process with the ECU and trying to set the DC as clos to 34% as possible thing seem to be a bit better - No more surge, but the idle hangs higher than I want - as 1st I thought this was a dashpot issues untill I reduced those settings as discovered no differance exceppt occasional stalls on clutch in.

What I did discover is that I'm fighting a timing vs. ISC DC issue.

Per CGrey's sugestionions I had reduced timing control by the ECU so that it opperates in a range of 20-22 degrees and never falling below that. Unfortunaly this has led me to a high idle no mater what I do (~830-850 RPM). this also leads to an ISC DC that always drops to 0 (17.5%). No mater how far out I back the thorttle set screw. Eventally I close it completly and this leads to the surge/stall issue i discribed earlier.

If I relenquish timing control to the ECU the timimg drops very low at idle (~10 degrees) in order to maintain my targeted RPM with this method I can hold a steady 34% duty cycle but the low timing causes a lack of torque and a weak off idle. his can cause stalling when the power steering is strained in parking lot/garage manuvers and occasionally leads to the surge I discribed beause the timing drops so low when the rpm drops the ISC cannot recover no mater how agressivly I modify the ISC adder vs RPM/.

I can't seem to find a happy medium. For now i'm living with a slightly high and hanging idle with very low ISC duty cycle. Frustration has set in :)
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Postby cgrey8 » Fri Jul 06, 2007 7:13 am

20-22 degrees of timing is a little high. My engine runs best idling around 18-18.5. I'd slide that window down a tad more and see if that gets your idle RPMs a little lower.
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Postby Cougar5.0 » Fri Jul 06, 2007 9:06 am

Do you think he should use that tight a timing window with a cam lope? I am using stock timing offsets w/RPM deviation at idle and see about 20 +/- 2 degrees typically. Is this what you are talking about? I think the goal may be to limit timing deviations w/RPM offset through proper setting of other parameters. I judge how well I have set ISC parameters by how little the timing moves at idle. Is it possible that setting an artificially close "wall" for the timing window may force the ISC to have more trouble compensating? Not criticizing here, just thinking out loud.

blk91gt - yeah, idle hang is quite annoying. I had it for a long time before I realized that the RPM adder vs. ACT was the main culprit. I also had the "hit the 17% wall before the ISC control is stable" issue as well. I'm trying to remember how I fixed it. One thing I did was deliberately force just a little more DC from the ISC at idle while it was still learning (close TB). If it is getting enough air to idle too high, then it must follow that the TB plate is open too far or you have a vacuum leak or PCV issue - it's that simple. It's interesting watching the ISC learn - it may take a little while, so be patient. You can adjust the TB down (ISC DC up) while the car is running, but NEVER adjust up (ISC DC down) even the smallest amount while the engine is running. This will put you in part throttle mode until you cycle the key off then on again.

As far as DASPOT decay, I add more - I'd rather the engine never stall than worry about the RPM hanging a second longer on coast down. A smooth transition to idle can never be bad. If the ISC never goes closed loop, you will have a hanging idle, so you need to get it to go closed loop somehow. Patience is a virtue when adjusting ISC. :)
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