I was digging around in the ASDZ '747 hack when I noticed a couple of variables (also listed in the 747-mine.ecu for GM ECM, at LD2E3 to LD2E8) entitled:

DFCO Lower RPM Threshold (1400 RPM)
DFCO Upper RPM Threshold (1000 RPM)
DFCO Upper MAP Threshold (34 kPa)
DFCO Lower MAP Threshold (13.3 kPa)
DFCO Upper Speed Threshold (18 MPH)
DFCO Lower Speed Threshold (15 MPH)

Am I interpreting these correctly as DFCO is only engaged *between* these values? Looking at the disassembly (of which I can at this point only read the notes), I can't tell if these values are always applied, or only during certain conditions. If they are worth playing with, would there be any reason not to set them to cover wider ranges? Other than possible driveability issues, are there mechanical (high temps / fouling / something else) reasons not to engage DFCO?

 

For the RPM & MPH the vehicle/engine needs to be above these points. How they work is that if the engine is at 1200 RPM (using above values) it will not go into DFCO. If above 1400 RPM it will go into DFCO. As the RPM drops (vehicle slowing down)once below 1000 RPM the ECM exits DFCO. The MPH parameters operate in the same manner.

The MAP parameters are similar but opposite. The MAP needs to go below the lower threshold before entering DFCO and will exit DFCO if it goes above the upper threshold.

Myself, I use DFCO in all of my current vehicles.

RBob.

 

Thanks RBob. It seems obvious, I know, but it seems like every time I "figure out" how something works, it doesn't work that way. I have a couple of follow-up question, if you don't mind.

1. I'm tempted to crank up the values to include much higher speeds and MAP values, but I'm concerned: why would the factory values be set so conservatively? They seem to engage only at the last few moments of deceleration

2. At LD2ED, there's a parameter "2 Sec REQ FOR DECEL FUEL Cut Off". I'm wondering what "REQ" is: I'm reading it as "request" or "requirement", which would lead me to believe I have to be within the three operational windows for 2 seconds before DFCO engages. Or not?

3. At LD2EF, there's a value with the note "Min Time Between Decel Fuel Cut Off", w/ "val = msec x 0.80". Between what? The 160 value (decimal?) seems too long to be a pulsewidth modifier (and is there any pulsewidth associated with DFCO? Aren't the injectors "off"?) The address is referenced at D97C in the disassembly in a section titled "Check Decel Fuel C/O Timer":

D8FC: LDAB LD2EF ; 3 Sec's MIN BETWEEN DECEL FUEL C/O

I'm reading this as a timer for disengage/re-engage of DFCO; I'm slowing, I touch the throttle a bit, it waits some time period before re-engaging.

4. I'm posting a separate thread about tuning for differing air temps.

Thanks for your help.

 

You're still not reading the hysteresis curve right. According to the MPH values you supplied, anytime you are above 18mph, DFCO will be allowed to engage, pending proper conditions. As you are slowing, as soon as your MPH falls below 15, DFCO turns off.

HTH

 

1) What kevm14 posted.

2) 'required'. Yes, needs to qual for this time before entering DFCO.

3) The comment is incorrect. It is an additional delay based only on the TPS qualifier before entering DFCO.

4) OK.

RBob.

 

Does it have to meet ALL parameters before entering DFCO, or just one? ie, are they AND or OR??

 

DFCO engage is like an AND gate...or you could think of it as disengage is an OR gate. MAP and RPM tresholds in the hysteresis must be simultaneously satisfied for DFCO to work. There is also a min coolant temp parameter.