I was datalogging today and started looking at the history tables for spark timing. I'm noticing that the spark timing is (seemingly randomly) dropping like 20 deg or more at low-throttle/low-MAPs. Sometimes the spark timing is normal and sometimes it drops; and I'm not sure what's causing it.

I'm getting a bit of knock retard here and there, but mostly nothing in the suspect region and not enough to warrant ~10-20 deg of retard.

I'm posting the running min and running max on the spark history and my spark tables. I'm also posting my datalog .csv file in case someone is able to take a look at it.

Here's a screenshot of spark timing history showing the timing drop outs... The areas in the low MAP regions where the timing is down to around 10-20 deg (where as in the spark table it should be up into the 30-40 range... I pasted my spark map further down for comparison of what it should be vs what I'm getting).



Screen shot of the spark history showing normal spark timing...




Spark retard history




Spark map from my bin...


Closed Throttle spark table



My max allowable spark retard doesn't even allow for any retard in some of these MAP regions..

 

Fairly certain you are looking at DFCO spark retard. Basically when you lift on the throttle it cuts timing with the fuel cut.

 

Hmmm... I was thinking it would be something like that. But I wasn't finding anything like it in $8D. Unless I missed it?

But then the inconsistent nature of it is also confusing....

 

Just had another chance to scan through Tunerpro and I'm not seeing anything spark related on the DFCO subroutine...

 

Need to look through an $8D hack file. It may not be defined in the XDF you are looking at. Some of the TBIs have a simple 1 cell value. During DFCO, set spark to X value. TBIs I am more familiar with are usually commanded to -5° during DFCO.

 

I have the 8D hack... I'll check it out.

 

Found this in the ANHT hac... the only thing I saw relating DFCO to spark timing.

LDAA $85F2 ; SA BLEND VAL IF IN DECEL FUEL C/O
STAA L013B ; SPK ADV BLEND MULT

That prompted me to go back to Tunerpro where I found this in the SAUJP V5 XDF.

DFCO, SA Blend Value

which says...

X * (90/256) or 0.351563 * X + 0.000000

This term at $85F2 is used whenever the ECM is in DFCO.
DFCO uses the term at $85F3 to blend the SA back in.This will reduce and hold the SA until DFCO exits.

The value currently in this scalar is .70.

So the multiplier for the SA DFCO would be .70 * .35 = ~.25

For example, if I go to my spark map, 2800 rpm at 20 kPA, I see that I'm commanding 35 deg.

35 x .25 = 8.75 deg

The data shows the ECM is commanding 10.195 at that cell. Its not an exact match, but it seems to be really close to explaining what's happening?

In the SAUJP V6 XDF, it has this for the same parameter...

DFCO, Spark Blend Multiplier Sets SA-REF =0 Degrees

If in DFCO, this Spark Blend multiplier causes SA-REF to be set =0. SA-REF remains =0 while in DFCO. After DFCO exit, SA-REF blending begins to return SA-REF from 0 to the code-calculated value.
Factory Default = 2

I'm not sure I understand the V6 version though...

But also in V6, there's this other parameter...

DFCO, If NOT in DFCO, Counts added to SA-REF Blend per DRP

When in DFCO, SA-REF is set =0. Spark Blending is used to gradually return SA-REF to its calculated value by adding these Counts each time a DRP is detected.
Factory Default = 2 Counts per DRP

I guess the ECM starts ramping up the timing back to normal when out of DFCO...

So the question is how do I disable this? I don't see any particular reason to be pulling timing out like this... (maybe for emissions or something)?

 

Pulling the SA softens the driveline hit when fuel is turned back on. Can be real annoying on stick shift cars. The purpose of DFCO is to protect the cat-con on long decels. Just a little misfire with the low density, hard to light-off mix will melt a cat.

It looks like the reversion to no EST is done in the code. No option flag or parameter to change it.

May want to just disable DFCO, I did that on a early no-cat car with a stick shift.

RBob.

 

Thanks Rbob. I'll try that. Mine is an automatic trans.

Thanks Fast also for your input as well.

 

DFCO and retarded ignition timing also help prevent some of the exhaust after firing that happens when the air/fuel suddenly spikes rich when the vacuum jumps up on a throttle lift. Without the DFCO in operation the exhaust will pop and bang on a throttle lift at higher rpm.

DFCO disabled, the 383 pulls so much vacuum the stock 350 settings were cutting fuel over and over and over even at light sustained throttle. I eventually altered the DFCO to enable where it was actually needed.
https://youtu.be/1xXFPoS3Y2Y

 

Right now, ANHT has the following...

DFCO enable if RPM >= 1400

DFCO disable RPM <= 1100

DFCO enable MAP = 20 kPa

DFCO disable MAP = 28 kPa

So I guess I could move the enable to 6300 and disable to 6200 and see what happens. I typically don't like to rev past ~5500 anyway just to help engine longevity. Given the timing removal appears to be inconsistent in the data, it seems like I'm not noticing any backfiring or popping when DFCO is on or off. So it must not be buying me anything to have it.

 

DFCO helps fuel mileage slightly and cleans up unburned hydrocarbons when decelerating. It also helps slow the car to some extent when you suddenly lift off the throttle at speed.

 

Set the CTS enable high:

L85E8: FCB 0 ; DEG C FOR DISABLE OF C/O

RBob.

 

I’ve done this on my LT5 but I still see areas in logs where timing drops by 10*. Haven’t investigated it too rigorously but I think it has something to do w motor shutting down when in a slow crawl and the disengaging clutch while applying load of additional steering
lock.

 

Could be the stall saver kicking in. In $8D, $32/B, and $6E, along with likely the LT5 engines, when the stall saver is invoked the EST is disabled (ECM opens the BYPASS line) and the timing reverts to the base.

RBob.

 

RBob,

Several years ago we discussed Stall Saver on the $D0/D0A mask id’s which be pretty much identical to the other masks for the LT5.
I thought I eliminated the effect, but I will re-check since its been a few years since I visited that. Since getting a new short block installed, I’ve been playing mostly w VE and SA.

 

Oh cool. I'll try it that way first.

 

Found it... TP has it listed as DFCO, Min Coolant Temp. It's currently set to -40F. So I'll set it up to something ridiculous like 280F.

I'll be taking the Camaro out to the local car show tomorrow, so I'll see what this does.

 

It worked. No more timing drop outs. No negative effects that I can tell...

 

I took a look at my Closed Throttle SA constant and decided to drop it by 2*. Idle was much more stable. You can tell when the fans kick in. There’s a momentary drop to idle speed.

 

Should be able to adjust the IAC counts with the fan 1 and fan 2 turn on if they are ECM controlled. My 7730 controlled both fan speeds and the AC clutch. All had adjustable settings to stabilize the idle.

 

The IAC does do that. Its the moment Fan 1 kicks in that u see the idle drop and then comes back up w ECM intervention.