Hi everyone!
Newly rebuilt 350, aluminum heads, decent sized cam. I don't have the cam card anymore but it has a good lope to it. 1227165 $6E TPI.
I'm having problems when shifting from park or neutral to drive or reverse where I have to feather the throttle to prevent the engine from stalling until the IAC catches up. For some reason I have more instances of stalling in reverse.
Base timing is 6 degrees. I have the idle set to 700 rpm in the idle speed vs coolant temp table when at operating temperature. I have about 20 degrees of timing at idle.
I've replaced the IAC due to age, cleaned the throttle body, and reset minimum idle.
IAC steps at idle is around 20. Once I shift to drive the idle is stable at around 60 steps.
I was finally able to datalog with TunerPro and observed some behavior I wasn't expecting:

- Although the idle table specifies 700 rpm, the target idle speed is 775 rpm. I found the scalar that adds 75 rpm when in Park or Neutral.
- When shifting from park to drive the target rpm matches the table at 700 rpm.
- At first It seems counter intuitive to me to drop the target rpm when shifting in gear, but as load increases and rpm drops it's reasonable to target a lower rpm, but I need more IAC steps.
- I found scalars to add steps for fan anticipate and A/C, but nothing for just shifting out of park / neutral

Is there a way to tell the ECM to add 40 steps when shifting out of park / neutral? Is a park idle of 20 steps and drive idle of 60 steps too large of a gap?
Any other tips or tricks for how to combat this?
Is the scalar that adds 75 rpm in park / neutral compounding this situation? I don't know the logic at play but thinking simply about it:
- Engine is idling at 775 rpm at 20 steps
- I shift to drive
- EMC says oh, you're idling at 775 rpm and now that you've shifted the target is 700 so better start closing the IAC to meet the new target
- IAC starts to close
- Engine rpm drops rapidly due to shift to drive
- Stall saver kicks in adds IAC and timing to try and get back to 700

Thanks for the help!

 

On MAP cars like mine, I was finding that going from park to drive could cause a large enough change in manifold pressure to trigger a delta-MAP AE (accelerator enrichment) event, dumping excess fuel into the mixture and causing a near stall.

On a MAF car, with your "decent size cam" (not sure how big it is)... it could be that getting enough of a LV8 change that you're triggering a AE delta-LV8 event, which is making the mixture momentarily too rich. Maybe raise the threshold for minimum delta LV8 to trigger an AE event and see if that helps.

What converter are you running? Looser converters can also lessen the effect of wanting to stall (particularly with larger cams).

 

I think the cam is the 270H grind which is 224/224.
The converter is rated for 2,200 stall. I had a 2,800 stall converter in it and I had the same issue.
Interesting thought on AE being a factor here. I think AE on MAF setups require a minimum TPS input to trigger.

 

20 degrees advance at idle sound low for a car with a cam. Try a little more. I was surpried to find my motor ended up at 28 degrees @750 rpm before it was really happy.

 

I'd say a 30-40 (IAC) count delta is pretty reasonable going between a stable idle in park/neutral and in gear and vice-versa with a typical p/n offset.

Here's a stockish L98 going from drive to park/neutral with warm coolant: https://datazap.me/u/elijah/morning-...3458-3397-3375

You can see about a 30 (IAC) count delta in this example with a 50 rpm p/n offset. Interesting to note is that the LV8 delta is about 20 counts.

I will look for some logs going from park/neutral -> D.

 

Larry - Would you share your spark map with me please? Or your bin and mask?

Thanks everyone for your replies so far. Greatly appreciated!

 

[QUOTE=BCCHowdy;6483972]Larry - Would you share your spark map with me please? Or your bin and mask?

I am using, GM fastburn heads and hotcam, 10.25cr