Please explain DFCO (Deceleration Fuel Cut Off)

[mwebb]

newer GM products use DFCO as the time to test EGR of course on many of those systems the MAP sensor is just a tag along klingon for the purposes of the EGR test and has little to zero fuel trim authority
i do not see any thing related to that EGR test
in your stuff ? maybe that system is too primitive ?

Originally Posted by Olympiadis

Here is a screen-shot of some of the (stock) DFCO calibrations on my 1991 Chevy Truck.
link for larger photo:

Note that the factory calibrations shown are not very aggressive at all, and during normal driving conditions it is rare for DFCO to engage.

Also, DFCO and DECEL-enleanment are two completely different things.
DECEL-enleanment commands leaner than stoich AFRs during most deceleration conditions, but only functions when in open-loop mode.

Though it is very wasteful of fuel, closed-loop mode maintains a 14.6:1 AFR during deceleration conditions unless DFCO is engaged.

[Olympiadis]

Originally Posted by mwebb

newer GM products use DFCO as the time to test EGR of course on many of those systems the MAP sensor is just a tag along klingon for the purposes of the EGR test and has little to zero fuel trim authority
i do not see any thing related to that EGR test
in your stuff ? maybe that system is too primitive ?

Can you post this EGR test logic? I'd like to take a look at it, as I've never seen this particular strategy for the EGR diagnostic.

DFCO would seem to me to be a very poor time to test for EGR operation due to the relatively low exhaust pressure, that is if the MAP you're talking about is actually taking pressure measurement only from inside the intake manifold.
I have seen systems with a pressure transducer that measures the pressure in the EGR delivery tube as compared to the pressure inside the intake manifold. Basically the algorithm looks for an approximate offset in pressure that coincides at each point tested. If the manifold pressure changes by 10% (and EGR is in operation), then it expects the pressure in the EGR delivery tube to change by approximately 10%, give or take a window for slop.
Are you saying a test like this (above) happens during DFCO?
It's hard to imagine since DFCO is often very brief, and as soon as DFCO is initiated, the IAC starts reacting by opening up to reduce manifold vacuum.
I'd just like to take a look at that logic.

The code in my truck is indeed "primitive", though the EGR diag is pretty much the same as what GM used in the larger codes all the way up to the 1998 model year. It basically tests the O2 integrator offset during a steady load cruise condition once above 45 MPH.
Here are the calibration qualifiers for the EGR test on my truck:

;---------------------------------------------
; ERR 32 >> EGR <<
;---------------------------------------------
LD52B FCB 30 ; 30 SEC DIAG CYCLE TIME ENABLED IF IN MAP WINDOW
LD52C FCB 80 ; 40 Kpa LO LOAD DISABLE
LD52D FCB 208; 87 Kpa HI LOAD DISABLE
LD52E FCB 17 ; 6.6% LO TPS LIMIT (ENABLED IF IN TPS WINDOW)
LD52F FCB 64 ; 25% HI TPS LIMIT (ENABLED IF IN TPS WINDOW)
LD530 FCB 250; 98% EGR ON THRESHOLD
LD531 FCB 25 ; 2.5 SECs EGR ERR DELAY TIMER
LD532 FCB 3 ; 3 COUNTS FAIL COUNT THRESHOLD
LD533 FCB 10 ; 3.9% TPS, CHANGE TO DISABLE TEST
LD534 FCB 28 ; 2.8 Secs TEST DURATION
LD535 FCB 4 ; 4 COUNTS, INTEGRATOR DIFF FOR EGR OK
LD536 FCB 45 ; 45 MPH REQUEST TO ENABLE TEST
LD537 FCB 255; 99.6% EGR D.C, DECREMENT FOR TEST IN WORK (454cid TYPICALLY 50%)
;---------------------------------------------

[Nevyn]

There are known issues w/the new yet-to-be-released Prius PHEV-13 where it shows 0.02 GPH on SGII when in DFCO as well. I think it's a Toyota thing?

[04_Sentra]

Originally Posted by Nevyn

There are known issues w/the new yet-to-be-released Prius PHEV-13 where it shows 0.02 GPH on SGII when in DFCO as well. I think it's a Toyota thing?

Don't you mean SG2 showing 0.02 GPH while EOC? Anyway, I found this thread while trying to figure out if my car (Nissan Sentra) had DFCO. I have notice that once in a while the MPG on the SG2 goes to 9999 but had not really paid attention to when it was happening. I watched it carefully and it is only while decelerating in gear. GPH goes to 0 at the same time but while EOC GPH goes to 0.02 and MPG goes to ~2800 (varies with speed.) I do know that while the engine is off and the key is on the fuel pump is running so maybe that makes the SG2 think that the fuel consumption is >0, but in DFCO the fuel pump is shut off. That is just an idea and I'm not sure how to test that, not that it really matters since I am sure that while EOC and DFCO the fuel consumption is actually 0.

[SentraSE-R]

My '02 SE-R had DFCO, so your '04 does, too. You can't rely on the SG2 showing you when it occurs, since the SG2 relies on the TPS setting you set for cutoff, in combo w/open loop. If your cutoff is set wrong, the SG display will be wrong.

My Sentra's specs put it into DFCO with rpm >2500. In its top (6th) gear, that only occurred >65 mph. My xB goes into DFCO w/rpm >1500. In its top (5th) gear, DFCO kicks in at any speed >35 mph. DFCO is much more useful in the Scion. Plus, when the Nissan goes into DFCO, it wants to stay in open loop for the next 4 minutes, burning fuel inefficiently the whole time. When the Scion goes into DFCO, it returns to closed loop instantly, so it's burning stoichiometric (efficiently) right away.

[04_Sentra]

Originally Posted by SentraSE-R

My Sentra's specs put it into DFCO with rpm >2500.

Did you test this or is it documented somewhere?

My 04 stays in closed loop when coming out of DFCO, it does go into open loop whenever I go to neutral which is a little annoying. I did some digging to try and find a way to force it to stay in closed loop but most of the Sentra guys want to stay in open loop for racing purposes.

[Ryland]

early on in this thread someone asked about what can kick an engine out of DFCO, At least on Honda Civic's the computer keeps track of oil pressure, engine speed, engine coolant temp, intake manifold vacuum, the clutch has a switch that clicks as soon as you start to press it (before the even clutch disengages) along with vehicle speed.

[SentraSE-R]

Originally Posted by 04_Sentra

Did you test this or is it documented somewhere?

It's described in my service/repair manual. Nice to hear Nissan improved its ECU logic to return to closed loop after DFCO. My 2002 implementation was pretty primitive.