bl85c

iTrader: (2)

Is there a point when DFCO can do more harm than good? The only situation I can think of is the short period after fuel cuts out and residual fuel is sucked up and combusted- possibly causing an extreme lean burn, like after an extended WOT period. In other words, how high a load is too high to enable DFCO?

DSmith

I could be wrong but,
if DFCO is activated the throttle will be closed. If the throttle is closed then the engine is not getting air. The idea behind DFCO is the drop the RPMs as soon as the throttle is closed but turn the fuel on in time to keep the engine from going dead. I don't see this causing a lean problem.

bl85c

iTrader: (2)

The throttle's closed (or very close to it, like 3%) when DFCO is enabled, but the engine still moves air. Otherwise your car couldn't idle.

RednGold86Z

iTrader: (1)

Too many people are "worried" about "lean" thinking it's some magical engine destroyer. At full throttle, yeah, you don't want to be lean for long on a hot engine or one that's timed at the knock limit, because it burns slower (can even be burning when the intake opens again, which is bad) which heats the piston tops, and has a reduced knock timing threshold. A fuel cut rev limit may cause a problem on a powerful engine.

At part throttle, idle, decel, practically anywhere, the only immediate side effect is reduced power. Slowly the engine will get warmer, typically, from reduced combustion speed.

DFCO, is decel aka overrun fuel cutoff. That means negative load, typically = less than free rev torque. No fuel is not considered lean - it's no fuel at all, and is cool.

The residual fuel that sucks through DOES cause a problem for the cat(s). Especially after a long WOT. It can spike the cat temps, especially if the high load was performed at 14.7 and closed loop (which many new cars do until the cat temp model is high enough). The residual fuel is usually quite rich at first, due to the buildup of fuel in the manifold during >60kPa operation, then quickly leans out. That quick lean out goes below the minimum fuel required for combustion, and passes HCs (fuel vapors) into the cat/exhaust - sometimes will make a pop, or a flame out the tailpipe.

DFCO dries out the port walls, which causes lean operation for the first dozen or so cycles after fuel returns. Most EFI systems can compensate for this with extra fuel momentarily to prevent a lean stumble. Many OBD emissions compliant systems will even go extra rich for an extended period to de-saturate the cat's oxygen. This is to allow down O2 sensors to begin influencing the main fuel loop sooner, with confidence.

DFCO is not dangerous to normal use though. Keeping fuel injecting can be worse, if they can't be controlled well enough, or if the engine has too much EGR at that point, and can't maintain combustion. That'll cook the cats quickly.

If it's stalling on you, watch the IAC to make sure it's not closing when you don't want it to. Make sure it's rich enough (i.e. correct) VE at the point it's turning fuel on. If you have any enrichment for DFCO exit, use it. Also make sure the spark timing that is used is what you think it is, and is enough (like is it using the closed throttle table, but you didn't realize it?).

bl85c

iTrader: (2)

Exactly what I was looking for.
Don't have a cat so no probs there!