Please explain DFCO (Deceleration Fuel Cut Off)

[some_other_dave]

As to why your car drops out of DFCO after a while, there are almost always RPM limits for DFCO. After all, if they left the injectors off all the way down to 0 RPM, you'd have to use the key to start again and that wouldn't be so great.

It is also possible that Ford engineers decided that running in DFCO for more than X amount of time ran some risk they didn't want to run. I'm not sure what sort of risk that could be, though. You should still be pulling a vacuum so the brakes should keep working; with no fuel and no spark you shouldn't be putting a thermal load on the engine (just compression heating which isn't nearly as much as combustion heating); the field coil on the alternator should still be energized so the charging system should still be charging... No, no idea why they'd do that.

-soD

[cfg83]

some_other_dave -

Originally Posted by some_other_dave

As to why your car drops out of DFCO after a while, there are almost always RPM limits for DFCO. After all, if they left the injectors off all the way down to 0 RPM, you'd have to use the key to start again and that wouldn't be so great.

It is also possible that Ford engineers decided that running in DFCO for more than X amount of time ran some risk they didn't want to run. I'm not sure what sort of risk that could be, though. You should still be pulling a vacuum so the brakes should keep working; with no fuel and no spark you shouldn't be putting a thermal load on the engine (just compression heating which isn't nearly as much as combustion heating); the field coil on the alternator should still be energized so the charging system should still be charging... No, no idea why they'd do that.

-soD

My car usually exits DFCO around 2500 RPM in 3rd or 4th gear. I wish it was around 1000 RPM, but maybe that's too close for comfort because idle RPM is about 850.

If the Ford has unheated oxygen sensors, then a long period of time would cool off the sensor and it wouldn't read correctly. A cold oxygen sensor would encourage the ECU/PCM to switch from closed-loop to open-loop mode, which is not good for MPG.

CarloSW2

[euromodder]

Originally Posted by cfg83

Yes, but I think DFCO has a strategic purpose for us.
However, if I can get engine-braking + DFCO on the downhill, I can avoid using my brakes and risking dangerous speeds.

Going down a rather steep incline, you obviously have no choice but to remain in gear and use engine braking.

If gravity is propelling your car, DFCO is better than coasting with the engine on as there is no fuel use at all using DFCO.

But on the near-flat terrain where I live, engine-on coasting is definitely better for FE than DFCO and trying to brake as little as possible.

[comptiger5000]

Exactly as Euromodder said. DFCO is best used for when you have no choice but to bleed off momentum. At least you can keep the engine from using fuel while doing so, plus it saves wear on your brakes.

[round.boater]

The beauty of many of the roads I drive, I suppose, is lots of time to watch the guages.
My DFCO cuts off at ~2000 rpm regardless of time.
Coming down a hill, the water temp will drop to ~192 with engine-on coasting, temps hold ~198 with RPM's in the mid 2000's and no fuel, temps climb to ~206 with RPM's in the mid 3000's during prolonged no fuel engine braking. An 8 mile down grade of consistent 6 or 7% (I-70 tunnel to Silverthorne) will have me cycle between DFCO and engine-on coasting several times. Pulling a trailer I can't really coast out of gear though or I'll melt my brakes.
I'll bet the exhaust gas temps are the reason for cutting off the cut off. Too bad, most of those descents transition into climbs, and temps will jump right back up with hardly any wait at all.

[trikkonceptz]

Understanding its purpose and use how come DFCO reads 9999 on the scangauge while EOC reads somewhere in the ball park of lets say 2800 mpg's, why not 9999 as well?

[320touring]

In my daily commute, I've started adding a DFCO phase to the endo of my EO(n)C sections..

this presents the ideal opportunity to slow the car for the ROundabout/junction

Using no fuel and being in the right gear for picking away again!

The OBC shows its a positive step..

fingers crossed it works on this tank!

[euromodder]

Originally Posted by 320touring

In my daily commute, I've started adding a DFCO phase to the end of my EO(n)C sections..

If you can't start coasting from further out, DFCO is the way to go for the last part of the glide.

I've done the opposite, adding E-On-Coasting to a long habit of DFCO

[euromodder]

Originally Posted by trikkonceptz

Understanding its purpose and use how come DFCO reads 9999 on the scangauge while EOC reads somewhere in the ball park of lets say 2800 mpg's, why not 9999 as well?

As you're not using any fuel anymore, both should see "infinite" mileage ...
It's got to be a software thing.


Driving my sister's Skoda Fabia 1.4 Diesel, I've noticed its fuel consumption indicator switches to the hourly fuel consumption when you're stopped.
(I got 3,6 L/100 km or 65.3 mpg BTW, without even trying hard)

[NachtRitter]

Originally Posted by trikkonceptz

Understanding its purpose and use how come DFCO reads 9999 on the scangauge while EOC reads somewhere in the ball park of lets say 2800 mpg's, why not 9999 as well?

When you say EOC, do you mean Engine On Coast or Engine Off Coast?