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ISC duty cycle max clip

Posted: 2023 Feb 04, 18:16
by Turbo4V
I've noticed recently that the ISCDTY seems to clip at 0.602 (60.2%) when at cold start it is trying to reach DSDRPM (say 1200 RPM) for the first 30 seconds. The ISCDTY stops rising at exactly 0.602 and does not increase even though I can see ISPIBR incrementing up until it reaches it's max clip. So I can't reach DSDRPM if the required air flow exceeds the ISCDTY of 60.2% which per FN8000 is 1.1 PPM. Also the ISCDTY will clip at 0.602 when driving and the DASPOT will continue to rise with RPM but the ISCDTY stops at 60.2% which doesn't provide the required air flow needed to match the DASPOT preposition. So, any thoughts where I should look for what is limiting the ISCDTY?

Re: ISC duty cycle max clip

Posted: 2023 Feb 04, 18:49
by decipha
post the tune file

Re: ISC duty cycle max clip

Posted: 2023 Feb 04, 19:07
by Turbo4V
Here you are, and thank you sir for taking a look.

Re: ISC duty cycle max clip

Posted: 2023 Feb 06, 18:51
by Turbo4V
In the meantime, possibly unrelated but I've always used an ISC valve for an 03/04 Cobra. Not sure where the FN8000 table in the FBGIO stock V8 auto binary came from, but it is quite different from the same table in an AMZ3 03/04 Cobra file. Its always acted OK without using the AMZ3 table so I've left it alone. Just throwing it out there in case it is of any potential relation to the issue.

Re: ISC duty cycle max clip

Posted: 2023 Mar 10, 10:31
by Turbo4V
I was hoping that maybe it was an issue with the older definition I was running, but the ISC clip persists even after downloading the latest 2023 version. Oh well, I will keep searching for the culprit. For now, I just reduced the cold start DRSRPM adders so that it didn't try to exceed 1100 RPM which seems to require just slightly less than 60% ISC on cold starts.

Re: ISC duty cycle max clip

Posted: 2023 Sep 02, 08:09
by Turbo4V
Bringing this one back as I haven't uncovered the root cause and continues to be problematic at start-up. The maximum ISC duty cycle it will command at cold start is always clipped at exactly 0.602 (60.2%) until required air mass drops enough for the ISC to begin to close. This is in park. However, when driving dashpot will rise and ISC will open accordingly but hesitates at 0.602 momentarily then shoots up seemingly as if to catch up. At which point the ISC duty will rise beyond 0.602 up to the 0.98 clip. So the ISC will exceed 0.602 at times, just never during cold start, which prevents hitting cold start desired RPM. Basically nothing greater than around 1100 RPM when engine is cold, so for now I just compromise and limit the cold start RPM adders to limit DSDRPM to nothing greater than 1100 RPM. Well anyway, just wanted to throw it back out there; open to ideas what the issue might be. Latest tune attached. Thanks.
TURBO4V_Working.bin
(256 KiB) Downloaded 235 times

Re: ISC duty cycle max clip

Posted: 2023 Sep 05, 12:27
by decipha
it appears you have 80lb siemens deka injectors, why do you have the injector data cut in half ?

sounds like your maxing out ipsibr. You have your idle airmass set to .38 lbs at 700 rpm idle and ithbma set to .32 that leaves .06 + psibrm clipped at .5 == .56 in fn800 that correlates to ~60% some odd

Re: ISC duty cycle max clip

Posted: 2023 Sep 07, 17:55
by Turbo4V
The injector data, MAF, and engine size are scaled 50%. Is there a correlation possibly?

IPSIBR only climbs if I set the start-up RPM adders such that the total DSDRPM exceeds around 1200 RPM for the first few seconds of cold start. That seems to be because the ISC won't open far enough to reach DSDRPM. IPSIBR is always very close to 0.000 at all times otherwise. I reduced the ECT based start-up RPM adders to no more than 1100 RPM (700 NUBASE + F825A + FN826A + BZZRPM) total which is all I can get out of the ISC during cold starts.

This is really only an issue for cold starts. It doesn't have any issues when DSDRPM is 700 for Park/Neutral when ECT is at normal temps, the ISC duty is 29%, IPSIBR is very close to 0.000. The idle airmass of 0.38 at 700rpm is correct based on logging at normal temps with no fans or A/C. The ITHBMA of 0.32 is also correct. Throttle Body air was determined by capping off and unplugging the ISC, then let it idle at normal temp (no fans or A/C) and 0.32 PPM is the measured air flow with no ISC. The air flow values are based on logging 'AM_UNSCALED_CALC' Is this correct?

Re: ISC duty cycle max clip

Posted: 2023 Sep 07, 21:22
by decipha
since you have under 256 lb injectors just plug the actual data in.

you need to double both fn875 and the ithb function
the scalar ithbma doesnt exist on fbgi0 nor any of the 99+ strategies

adjust the isc multiplier to get the iscdc kicked up where it needs to be

also make sure the crank iscdc is high enough that your not clipping yourself

Re: ISC duty cycle max clip

Posted: 2023 Sep 08, 07:48
by Turbo4V
Agree there isn't an ITHBMA, just meant the FN818 (ITHBMA) function.

Regarding your suggestion to increase the ISC multiplier, Initially I did add a bunch to those tables in an attempt to increase the ISC air flow to try and reach DSDRPM during start-up. But it eventually became apparent it didn't matter how much I added to the tables because the ISCDTY would hit 60.2% and not go higher and never reach DSDRPM, and of course the IPSIBR would just start to climb while the ISCDTY was stuck at 60.2% and never reach DSDRPM until the start-up adders expired and DSDRPM came down to something less than 1100rpm. So then I just reduced the ISC multiplier tables back down a little to something typical/reasonable because it was not helping, and pulled RPM out of the adder tables to reduce DSDRPM during start-up. That is when I started looking for whatever is limiting the ISCDTY to no more than 60.2% in idle control. Can't seem to find a root cause.