volkswagens-for-life

I understand what dfco is, but is that an item that's 'tunable'? What gains are to be had by messing with the dfco, if any?

Thanks for answering my random question.

Cliff Harris

The point of DFCO is to increase fuel economy. You could change the vehicle speeds that it cuts in and out, but I don't know what that would do for you.

volkswagens-for-life

Well I know some cars use it in strange ways... for instance the new McLaren has the dfco setup so that when you let off the throttle at high speeds it sends a fireball out of the exhaust. I'm sure they did that purely for the exotic "whoa!!" impact, but I'm a total noob at tuning and will never attempt to do so, so I figured I'd ask the pro's.

MrWillys

DFCO is designed to keep the engine from loading up upon deceleration. Some swappers think VSS is not needed, but they quickly find out when the vehicle dies when coming to a stop. I have never found a reason to make adjustment to these values in the constants though.

ANTI VENOM

I have no idea if it is a tuneable parameter but my understanding is that it was used to preserve the cats, reduce emmisions and maybe a small fuel savings. I'm not sure if there is anything to be gained by tweaking it. Interesting though.

mike100

Do the smallblock 350's pop and backfire under decel loudly when going lean due to dfco? It is almost insufferable on the LT5's with headers (and even just loud cat-backs), but if it doesn't backfire, I would leave it enabled. If anything due to the fact that you get more engine braking from pumping losses when defueled.

volkswagens-for-life

^Are you saying that intense popping through the exhaust during decel is due to a dfco strategy?

DanielRicany

Mine started doing this when I installed the Corsa full exhaust. Except it happens on decel when not in DFCO. When it goes into DFCO it stops doing it. I heard it something about a free flowing exhaust or something. Don't really know. Also heard it's a common thing with Corsa.

volkswagens-for-life

But dfco happens upon decel... or so I thought.

DanielRicany

It does. But it takes like 2 seconds to go into DFCO from the time you release the gas. Get an AFR gauge, you'll see.

mike100

On the LT5 it is a delayed backfire that apparently is ignited by the sudden lean condition on dfco. I know that "turning it off" made that go away. We are talking about exceptionally loud and embarrassing gunshot backfires- I don't have experience with L98's or LT1's, but I suspect they may be less annoying. What I did to turn it off was to have the minimum speed to engage dfco set to 200 mph or some similar setting that will make it never be able to activate.

Additionally, on the ZR-1's, a similar backfire can occur during the warm up sequence when the air injection is still running (I chose to retain air injection on my headers). It doesn't matter if you disabled dfco- the lean air is coming from the electric air pump. You can avoid the warm-up pops by shifting quickly and/or keeping the throttle cracked open a touch as soon as you get the next gear. This is a little less prevalent if you are running catalysts.

Tom400CFI

Not when it's active. Fuel is completely off. No fuel = no fuel to make a popping noise.

They will pop, however up to the point that fuel cut occurs.

MrWillys

Adding 2 to 4 degrees to the Closed TPS spark advance vs RPM table can help this.

Tom400CFI

Copy that. A light popping/burble doesn't bother me. It seems to me that Mike100 may have a problem though, if he's getting loud reports after going into dfco mode. At that point, there should not be enough fuel to explode.

mike100

It's a special LT5 thing. Remember it's also a speed density system too. Headers cause pops on the 32v engine. There is also a split blm fuel distribution anomaly on them that is pretty much normal for some reason. We just chalk it up as a lack of GM tuning refinement and deal with it by turning off the feature because these cars run so much better with the exhaust and intake mods.

volkswagens-for-life

Burbling and popping when a LSx or LTx lets off is just.... orgasm.

Tom400CFI

With regard to bubbling and popping (or gun shot sounds), there is nothing mystical/magical about the LT5. You need 3 things to make a back fire;
1. un burned fuel in the exhaust
2. air in the exhaust (usually from exhaust leaks)
3. ignition -from glowing exhaust parts or engine combustion
...doesn't matter the number of valves, MAP, MAF, # of cams etc. My LT1 is MAP, and no backfiring. If you have "gun shots" hen going into or when in DFCO, you've got a problem. First thing I'd ask is, "where is the fuel coming from?"

RWDsmoke

On my L98 decel enable temperature is 60C. It pops a little when cold but not after it is warmed up.

GREGGPENN

On L98's, only one bank is monitored (O2 sensor) by the ECM. If the injectors on that bank are leaner than the non-monitored bank, you can get popping during decel. That's because the non-monitored side is dumping more fuel (on decel) than the monitored side.

Checking injector resistance is a good check when you hear excessive popping. Often, you'll find lower resistances on the monitored side.

I tried playing with DFCO once, I wasn't happy with changes from the factory bin -- so I put it back to "stock".

Cliff Harris

DFCO (Deceleration Fuel Cut Off) is exactly that -- fuel is cut off. The ECM turns off the injectors during DFCO.

That was a big surprise to me. I had a misfire problem and hooked up LEDs to the injectors. When I took my foot off the gas at highway speeds the LEDs went out.