I have a 408 with a NW 102mm dbc throttle body and I'm feeling porting the IAC would be the only fix without replacing the throttle body. Warm idle iac counts are at 0 and on a cold startup at 310 and its barely enough air to start it in 90 degree weather. If I tun the ide set screw slightly to close the blade (enough to drop tps voltage by .02v) then it jacks the counts from 0 at warm idle to around 150. I've been pulling my hair out on this one, just wanted a second opinion.
Fueling has a 100% effect on IAC
Yes I agree, fueling is on point stft?s were 0 to -2% at idle.Quote:
Originally Posted by Ben Charles
I pulled my hair out on this with my buddies Frankenstein DBC car with a 102 mm BBK TB last month. I eventually fixed it but I took a bunch of unnecessary steps before I figured out the root problem we were having. I?ve tuned several cars but this was my first LS tuning experience and I couldn?t find this info elsewhere so take it for what its worth. It must have been a throttle body specific issue otherwise I don?t know why it?s not covered in the available idle tuning guides.
What I eventually found was that the IAC steps did not align with the IACs physical position. I took measurements of the IAC pintle extended vs retracted when installed in his throttle body and when removed. I found that from a fully retraced at position 310 to fully extended at position 0, it had about .5? of travel outside of the car. When installed though, from position 310 when I commanded position 0, unplugged then removed the IAC, I found only about .25? of travel. So the IAC only had about ? the range of travel that the counts allow for. I?d love to hear some other peoples measurements of their IACs actual range to see if this was unique to the 102mm throttle body.
As I recall, when the key is turned off, it homes the IAC to the fully retracted position then sends it to the fully extended position. You can see this with the scanner and also her it happening with a stethoscope. With his car on startup, an amount of airflow was being requested by the BRAF table that equaled a high value when looked up on his steps vs effective area table, thus sending the IAC back to its fully retracted position with 310 counts. From here, as the car warmed up, the IAC gradually moved towards the extended position. But because his IAC only has ?? of motion, fully extended happened at around 155 steps when it started from 310 steps. If an airflow quantity is requested that is less that that delivered by the sqmm at 155 steps, the IAC would bottom out on its seat but the step position would continue to increment as the stepper motor continued to run. I could hear this with the stethoscope. The sound changes when the pintle bottoms out at the seat around 155 then the stepper continues to ?skip steps? while the step counts continue to increment.
Before you go porting and drilling stuff, check your IACs range like I did and tell me if what I just typed sounds familiar.
I definitely feel like that's exactly what's going on, feels just like the IAC only has a fraction of the range it should. Question is, what's the fix & why haven't we heard more about this issue? Thank you for sharing your findings btw.Quote:
Originally Posted by Dan_W
That setup should not be ran at stoich at idle.. that?s most of your problem
It must be way off then because what I didn't mention before is that it's a 9.5.1 CR motor running E85. That being said, on a gas scale where would you put the Idle AFR, I already changed stoich idle to 14.1? Something tells me I probably should have it idling in OL.Quote:
Originally Posted by Ben Charles
If it?s e85 and pure... stoich should be at 9.8...
If changed in tune you can leave WB on gas scale and read it like regular gasoline..
Can?t tell ya what exact, Bc every combo is different, but leaner then stoich, I?ll give that info
OK thanks I'll, experiment and report back my findings. Maybe this will help someone in the future.Quote:
Originally Posted by Ben Charles
Ok, sounds like were in the same boat. I'd prefer you actually measure though before spending time on an assumption. I'd also like the data point.
Since the car already starts and idles, I assume that your spark and fuel are in a workable range. Set your base throttle opening > command your IAC to position zero with the scanner, then disconnect it. Disable the adaptive spark. Run the car (fully warm) and adjust the set screw until the car wants to stall, then open the throttle body just a hair more open from that point. For his 6.0 car it was able to low idle around 7-750 rpm... his normal idle is set at 950 for now. Watch your TPS volts and %. Reset your TPS when you're done so its reading 0% with the throttle blade closed.
This is now your base throttle opening. It's the foundation of the rest of your idle tuning. It doesn't change unless you need to go back and change your idle spark advance or fueling. Everything builds of each other so best to pause and tune in your spark and fueling at idle now if you need to.
On top of that foundation, the sqmm vs effective area table and BRAF tables are the pilliars of the idle tune. The sqmm vs effective area table is a lookup table similar to a VE table. It describes the opening of the iac relative to the iac openings cross sectional area and is used to determine what iac opening is needed for a requested airflow given the current temp and pressure differential (manifold vacuum vs atmosphere). If you changed to an aftermarket throttle body, odds are that this table is no longer accurate. The car can still be idle tuned without addressing this table but in my opinion, you are asking for trouble and may end up having to put extra effort into desensitizing the IACs PID controls since demanded flow and actual flow are not aligned.
I needed a way to limit the IAC, to keep it below approximately 155 steps. To tune it the way I wanted, I cant allow it to skip steps... I need to know the IAC's actual position. I chose a value of 128 to be conservative to start with, and partly because that was the upper limit in the bi-directional controls. I tried doing this with the desired IAC area max table but its apparently not a hard limit. You might be able to limit it with the desired airflow max table, I don't know, I didn't try. I spent a bunch of time modeling and scaling a stock airflow table to make it fit my projection but that's not needed since its just a rough first hack before you can start gathering data. I suggest drawing a straight line on your steps vs effective area table. Start at about 20sqmm with 1 step , and end at 120 with 128 steps. This is your starting point to tune this table.
Next, do some cold startups and log data like you are going to tune BRAF from Krambos guide... you'll need to capture warmup cycles from both P/N and in gear to capture the airflow needs of the engine through its full range. Since the data is noisy, you will want to repeat this to make the model accurate. Note that on the stock table, its not a straight line.
To be continued...
Thanks, I don't have the vehicle still, its a (off road) rock bouncer buggy, the customer was in a rush, and good enough was ok with him despite me asking for more time. Fuel injectors were at 82% dc at 7.5#of boost so we already have plans of getting it back in so we can hang some more boost to it, I will post up on this thread when I do.Quote:
Originally Posted by Dan_W
Ok, if you would like my idle config, pm me your e-mail... I don't see where I can upload the file here.
PM sent, thanksQuote:
Originally Posted by Dan_W
It's worth noting that we can log actual vs. commanded position of the IAC, but, there is no feedback from the IAC (positional) to the ECM that is used.... the "actual" isn't actual at all. It's some software value that "looks" real when you see it trail commanded, but it's definitely a fictitious number. So I tend to now ignore IAC actual position. If the unit ran into no resistance during it's extension, that number maybe right. If, as noted in this thread, it stops in the bore during extension (early), the ECM won't actually recognize this behavior and the "actual counts" will still be, incorrectly, increasing.