just my opinion but I would want to preserve some utility from the biss, so when tuning if i had to adjust it in an extreme either way, almost fully open or closed etc. then I would probably adjust the tune. This of course assumes no mechanical issues. I'm only working from a sample size of myself so perhaps more experienced people will chime in with their approach.

 

Getting trims to zero is tricky to figure out on your own (I had to). You have to know when to pay attention to the trim numbers and when to look for your changes to take effect. Nobody has ever written a "how to" but they should. It took me half a day to figure it out LOL. The trim is a direct representation of what the IAC is doing and you use the BISS, which is directly responsible for influencing it, so I can't imagine tuning idle any other way. Just my opinion, though.

 

 

I have some free time today. Maybe I'll do a quick YouTube vid. It would be easier to show than explain.

 

 

American Idle

 

 

I'm no Doug DeMuro, but I made a quick vid that hopefully makes sense.

 

biss screw north of the ticking lifters youve replaced 3 times but still tick...lol that had me crying haha...the video is perfect man...clear as day...makes alot of sense...i always thought to mess with the biss screw you had to activate the speed adjustment screw mode in evoscan and make the adjustments then...didnt know you could just turn the screw without it...good to know...thx brother...now, lets see how this works with gsc s2 cams...if my car wasnt in pieces id go outside right now...soon!...

 

It's super easy, once you know to only pay attention to values after the fan goes off and they settle.... aaand that it takes a couple cycles for the ISCV to adjust. The first time I did this, I didn't know and chased my tail for three days before paying closer attention. I started to realize it took a couple cycles for adjustments to finish (I thought it was real time/instantaneous). Also, with the fan on, it usually reads +/-0.392 from where the actual value is. I was like, "son of a......". But now I know.
Anyway, good luck! You definitely need the adjustment with a set of S2s.

 

 

You need to log "ISCV demand trim AC off" and "ISCV demand trim AC on"

It works the same was as fuel trims. You want them closest to 0. Just like fuel trims, you don't want the ECU to have to be adding 15% or something.

You use the biss to zero the trim on the Ac OFF on. Then you use the ECU tables for the AC on demand to make the AC on trim perfect (if its off much).

Best way to do the idle/BISS

 

Great video. Giving away my secrets. (totally joking )

Just some info to add.

In evoscan, the value -1.17 is technically "zero" as this is where this ecu is after a reset.

I feel that anywhere from 0 to about +2 is fine. You don't have to rack your brain on getting it to be perfect 0 all the time. It will drift a lil (mine is always between 0 and +1.5 checking it for years)

Last, make your change, then wait. It takes some time for the trim to "settle' and if you keep making adjustments, you'll chase your tail forever, like Kaj mentioned. You may even wanna make a couple adjustments, get it close to zero, then drive the car for a day or two then check it again and if its between 0 and +2, you're ok.

 

yeah, using the Speed adjustment mode in evoscan would be the "service manual/forum way". In the service manual, it says to use that mode and target 850 (because thats whats the stock idle is) And Id say the forum way, is using that mode and then setting it to your desired idle ie 1000

But yeah, you will NOT use that mode to set your BISS using the trim way.

 

Oh its also best to have your Idle tables all done correct, by demand and not steps.