I have started tuning the idle. Not sure I am going about it the right way. I am following Chopperdoc's video tutorial and my IAC steps in the tune look miles off from where they should be. I am hesitant to make adjustments as it just barely idles now. I am attempting to get it derivable enough to track tune the VE tables. Though the previous owner has stated those are very good. It does run strong above 2000 rpms and the afr looks good when up there. I have attached the tune and an idle log. Any help with this would be greatly appreciated.

 

Start with your Base Running airflow table. I've been meaning to put together a guide on idle tuning, one of these days. It's a tough thing to tackle.

To adjust the RAF table do this:

Plot "Desired Air flow" on the ECT axis of the table, both in park and gear. So log ECT, and desired. Should already be logging it. You can use an old log, build the table and the numbers will populate. It will not work if the gas was touched, so throw in the TPS filter like in the IAC video. You can leave dynamic out.

Next, tune the VE in idle areas. It has to be REALLY close for everything to work right. GMVE is the calculation of MAP, injectors, dynamic air, IAT, RPM, AFR, and so forth. You get the idea, VE is pretty much the sum of everything. You're just adjusting the "final answer" by adjusting the table, which in turns changes other things in the tune, like dynamic airflow.

So you see that if the VE is off, so is dynamic air, because the "final answer" is wrong. I could post the formulas to show this, but I won't for now because it's pretty hard to understand the basics as is. Just trust me, you want the VE to be correct, at least in your idle cells.

Now, you can add the % of airflow change to your adaptives If you have 25% more air than before, add in 25% to the undershoot side your proportional, integral, and derivative airflow. Add about half that to overshoot, because you don't want the PCM to pull it too fast and cause a stall.

Next, set your idle spark tables to around 18-20* in idle cells. Many set the 400 column to something like 40* to act as a stall saver.

Adjust adaptive idle spark as well, push the numbers up to 10-15* depending on how the engine is behaving, again biasing the larger numbers on the undershoot side. You want the max value to be in the middle of the table, as it should NOT just suddenly pull 15* for a 50 RPM error. Start at like 2 and go up from there accordingly. For overshoot, the numbers have to be negative, but make them a bit less than undershoot.

I forgot to cover it in the video, but you also must readjust the RAF (Base running airflow) again each time you make big changes to the IAC effective area. It may or may not change much, but it affects "desired airflow" so rechecking your RAF table each time is a must.

Also, actual idle RPM may be too low for your cam, depending on what you have. Bigger cams need more RPM at idle. It's just the way of the beast. I wish I could get mine back down to 600, but there's no way that's gonna happen on my own car. So mine for example is set to 800.

Oh and to find the best spark for your cam, watch the MAP value as you adjust it and in the logs. The lower the MAP the better. More spark will typically lower the MAP and pull more vacuum, up to a certain point. Keep an eye on that too.

Hope that gives you some areas to focus on. Pretty much covers all of the idle related tables you need to mess with.

 

I will look at your tune and log later. I'm at work and can't right now.

 

Thanks, Chopperdoc, I have my idle pretty clean as of now. It just wants 1150 rpm. The cam is big, 243/259 651/651 112 LSA. 27 degrees overlap. With the super victor intake and FAST 4150 throttle body do I have a chance getting it down to 900? It will idle there but bounces around a bit and stumbles. I am travelling for work today and don't have the log to post with me. I have timing in the idle areas set to 18 advance. Any lower and it is real lazy. Any higher and the overspeed has to keep pulling it down. The best I can get vacuum wise is 5 lbs at idle and that is at 18 degrees.

Idle video

 

Subbed for later

 

Add the following inputs to your scanner:

1. Idle adapt STIT (Short Term Idle Trims)
2. Long Term Idle Trims (LTIT)
3. Desired RPM
4. IAC Desired position
5. Cylinder Airmass

Those should do for now, but will help in trying to get your idle lower. You should take you dynamic air values and plug them into you Base Running Airflow Table for now, as it appears your desired is pretty low in comparison. Desired is saying 9 g/s while Dynamic is at 19 g/s. Dynamic is where the real calculations are happening as that number comes from g/cyl, which is the ultimate fueling number for the PCM. So long as your VE is looking good that should help in getting it dialed in.

The Base Running Airflow Table (RAF) is a lookup table for the IAC, where it starts it's corrections. From there the adaptives take over and attempt to maintain the idle based on RPM error. Proportional is the main one (proportional RPM error), then there is Derivative (Fast acting changes and stall saver) and finally the Integral, which is basically the LTIT and fine tuning after the other 2 have made corrections.

Post another log with those channels/PIDs and lets see if we can get the idle down some for you.

Sounds good so far though.

 

Lots of good information in this thread, I am subscribing as I am about to jump into tuning my own project (4.8 Silverado with SS2 and 7875). Thanks for all the very helpful videos and responses so far.

 

Good info here, but I would like to remind everyone to check your mechanical parts. I was once SD tuning a setup where I only had access to it in extremely hot weather. I knew for fact that the idle tuning would be off in winter. When winter came around, sure enough the idle was problematic. I chased my tail on it for longer than I care to admit before realizing the IAC motor had gone bad at almost precisely the same time the weather changed. It was reporting different values, but they were not repeatable to actual IAC opening amounts. I changed it out and had everything fine in two warmup cycles. If you follow the tutorials here and fail to get results, you probably have a mechanical issue.

 

Absolutely.

I'll add in to this, as to mechanical things, that also includes the idle adjustment screw. TRUST ME you will end up messing with that more than a few times in very small increments until the idle is where you want it. If you want the idle to come down, you have to choke the air off. There's no other way. So once you have a round of adjustments in and looking good and want to adjust the RPM lower, you will find that you'll probably have to adjust the screw again, and again, and again in addition to making tune changes after each adjustment. It's better to control it with the blade itself than the IAC motor and tons of numbers and adjustments.

Note, as a sanity check, you can ditch the stupid spring on the adjustment screw and install a small jam nut on there instead. I spent a bit of time making this happen to make sure I wasn't chasing my tail. Just take the screw down to ACE or Lowes and get a small nut that fits it, thread the screw through and then put it into the TB just like that. Tighten it slightly, but you will find adjusting it doesn't take a wrench as it will still move with the nut tight, but it takes like 20" lbs to move it, at least. It's a sanity check lol.

 

Chopper, here is the latest tune and log with the PID's you asked for. Thanks again for sharing your knowledge. I know it was done warm, but the idle warm is my biggest concern. It fires up cold great and warms up pretty quick. When it cools I will get a log from cold to operating temp.