Idle tuning help Vortech SC / Comp Cams XE270

[Bullitt5566]

I need some help with dialing in my idle tune. The setup is fairly common. Vortech supercharged with Comp Cams XE270 cams.

The engine will idle fine once idling, but the transition to idle is the problem.
When coming to a stop the idle will drop to 900-1000 and hang for a second or two, then it suddenly will drop and almost stall before catching itself and jumping backup up.
It will continue this a few times, then the idle will "lock in". It is like the idle catches and it is rock solid.
If I datalog it I can see the rpm oscillate in a perfect sine wave.

The IAC duty cycle is between .20 and .30.

The IAC Intergrator is nominal.
The timing clip is set to 0.
I've increased the dashpot predisposition and decresed decay at lower air flows. I've done about 10-15% each.

I'm curious if anyone else has this setup and what their predisposition and dashpot settings are?

Does anyone have any suggestions how to cure the hunting idle?

 

[decipha]

10-15% is nothing, make some 50-75% changes and see what you get

 

[Bullitt5566]

Don't know why I didn't try this yet. But I decreased the lowest dashpot decay setting by half and it seems to be a big step in the right direction. Thank you.

 

[decipha]

yeah no problem, I have all the details in my dashpot write up if you want to check it out

http://info.efidynotuning.com/dashpot.htm

 

[Bullitt5566]

Well, the car stalled out twice today so I think I'll have to start over due to the new cams. I'm going to just start at the beginning with the MAF Transfer function, then the idle air, then the dashpot. I'm using the SCT PRP software so, while the write-ups on efidynotuning are awesome, they don't always translate to SCT nomenclature.

 

[decipha]

yea SCT is stuoid for not putting ford PIDs, they get the freakin strategy doc handed to them by henry youd think the least they could do is put actual labels on things

 

[Bullitt5566]

Getting closer thanks to Decipha's write-ups, I thought I had it last night but this morning same crappy stalling idle.

I started on Saturday. I disconnected the IAC and TP plugs then set the throttle stop at the lowest RPM the car would steadily idle. About 800 rpm.
Then I datalogged the MAF counts, RPM, IAC Integrator (Air Flow Error), and IAC Duty Cycle.
I translated the MAF counts to Air Flow #/Min because I could not find another value to give the data directly.
Then I found the air flow at 800 rpm by multiplying the AF*(800/840 (actual rpm)). I multiplied that by the IAC duty cycle. And added the IAC Integrator Error #/min.
I set the Throttle Body Air Flow (100% - IAC Duty Cycle%)*AF
Set the dashpot decy and preposition.

That worked. Steady 800-850 rpm idle, IAC duty cycle 30%, and IAC Integrator 0 error.

Blip the throttle... and... Idle comes down, hangs for a second then drops to 400rpm almost stalls and recovers. FAIL!

Sunday back to datalogging. I noticed that the idle source kept hitting the MBT/MAX limit (Idle source 1). Datalogged the sprark, load, rpm.

Found that after a throttle blip the ECU was advancing the timing to catch the idle but clipping. I raised the MBT and MAX timing tables at low load and rpm.

Ended up at over 60 degrees. Then I ran into the Borderline Knock table limiting spark error, I raised that too.

The result. Seemed like it worked. Blip the throttle, rpm would drop, and stop at 800 rpm. NO DIP!

This morning, FAIL! The car kept stalling. So I flashed back to the tune from Saturday, before messing with the timing.

So I think raising the MBT/MAX timing to allow the ECU the room to correct the idle was OK, but raising the Borderline Spark was incorrect.

This afternoon, I'll set the borderline spark back to stock at low RPMs and see how that works.

Does anyone have any insight/recommendations into setting spark advance at idle?

Some key parameters
Spark Gain is 1.0999
Min Spark is 0
MBT/MAX is now 60 degrees in the lower 12 cells RPM below 2000/Load below .200

 

[decipha]

your issues are due to the DASCTL value being too low, that scalar is the maximum dashpot to allow control, kick it up to 0.400 lbs/min and all of your issues will vanish

 

[Bullitt5566]

Well I looked for an SCT equivalant to DASCTL. Couldn't find anything close. I get what DASCTL is doing and tried a few variables that were close but no change. HOWEVER, I unplugged the MAF so the ECU would read from the Load with failed MAF table for and the idle was nearly perfect. So TP sensor? The MAF is brand new.

 

[decipha]

nope, tp sensor has no affect on idle

it sounds like your maf isn't dialed in or its getting some turbulence

 

[Bullitt5566]

Yes, sounds like the problem is with the MAF. I swapped the mAF out for a different one of the same model. Same result.
I also removed the filter and tried different MAF orientations. Same result.
I tried loading the SCT provided MAF transfer function for the 90MM LMAF, car would not idle at all. Put the tuner provided MTF back.
I datalogged the actual load and compared it to the load with failed MAF. The actual load is about 50% higher than the Failed MAF load.
Increased the engine displacement by 10% to bring the load down. No change.
I think I'm going to start a new tune with the MAF transfer provided for the 90MM MAF and ditch the tuner provided MTF. It's a Ford 90mm Lightning MAF so the MAF tranfer function is well known. I don't know why the tuners MTF would be so different.

 

[RacerXHG]

Did the tuner scale it? Then if you changed just the mtf, it would be messed up. Start from scratch and load maf last.
Btw, this is a file I got a few years back from the SCT tuning forum. It gave me a good start on fixing my idle issues with bigger cams and overlap.
View attachment stage_3_cams_idle_fix.zip

 

[Bullitt5566]

Thank you for the reference. I don't think my MAF is scaled. It is draw though and I'm not (or didn't) maxing out the MAF. I scaled MAF would be pretty obvious right?

 

[Bullitt5566]

@RacerXHG. Thank you for the file. I looked over the settings and while I've made adjustment to all those values at one time or another, I don't think I hit the right combination. The settings file helped a lot.

The car idles much better, but I simply cannot get the idle below 800 rpm. If I pull the IAC plug the lowest steady idle is 800. I don't know if that is normal or if it points to a mechanical issue like a vacuum leak.

Anyway, I set the throttle stop, adjusted the IAC and TB air values. It's idling steady at 800 so I'm calling it GTG.

 

[RacerXHG]

You're welcome! Btw! What's your vacuum at that 800 rpm idle? How many degrees of overlap is there?

 

[Bullitt5566]

Sorry it took so long to get back. Vacuum is 18hg at 800 rpm. Camshaft overlap is 10 degrees @ .050. The car will idle fine at ~825rpm but not any lower. If It is like there is a drop off. I have also noticed that when the power steering pump is under load during turning or braking the idle will drop. For example it stalls when backing out of the driveway or backing into a parking spot. Both use the brakes and steering. The ECU does not compensate. RPM drops ~100 rpm I've adjusted the tables to add IAC with no affect. I also looked for a power steering pressure switch and I'm convinced it doesn't exist. Does anyone know how the ECU knows when the brakes or steering are applied