Please explain DFCO (Deceleration Fuel Cut Off)

[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?

Ya, I've got to guess that you mean Engine On Coast, since Engine Off Coast would mean no fuel going through the injectors and therefore infinite (9999 and then some) miles per gallon.

With Engine On Coast, you're still using some fuel to keep the engine idling while coasting. Otherwise (obviously) your engine will stop running. So the mpg's can be really really high, potentially even so high that the scangauge will show 9999 mpg, but never infinite.

[320touring]

Originally Posted by euromodder

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

i use the coast from circa 60mph to 35ish (over about 400m)then engage 3rd and dfco down to 15 selecting 2nd and ready to stop or accel if the give way is clear:-)

Good luck with the coasting!

[trikkonceptz]

Originally Posted by NachtRitter

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

Sorry about that, I meant Enine OFF Coasting, they do not read the same, or at least not the same on my Scangauge.

[brucepick]

Re. coasting downhill in gear to control speed - with injectors off.

IF your car burns oil in any significant amount, you might not want to do this a lot. The high cylinder vacuum will likely suck in a lot of oil - either through the rings or the valve seals, where it comes through to let it burn oil. You'll have a batch of oil in the cylinders when you fire it up with fuel. If so, you'd just be causing it to burn oil

You should be able to test for this. Try it, with medium-high engine rpms, on a longish hill. Restart it when needed, give it maybe 2/3 throttle, and watch for a blue-white oil cloud from the tailpipe. If you see it, it's from the slug of oil built up in the cylinders in that long coast with high vacuum.

[NachtRitter]

Originally Posted by trikkonceptz

Sorry about that, I meant Enine OFF Coasting, they do not read the same, or at least not the same on my Scangauge.

If you are reading ~2800 mpg with Engine Off Coast (as you mention previously), then I can only guess that you've got a software problem, as euromodder said:

Originally Posted by euromodder

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

When your engine is truly off, you are using 0 gallons fuel (0.0000 etc forever), so your miles per gallon would be some number greater than 0 miles divided by 0, which is infinite. There's nothing else it could be, to the best of my knowledge.

[NachtRitter]

Originally Posted by brucepick

Re. coasting downhill in gear to control speed - with injectors off.

IF your car burns oil in any significant amount, you might not want to do this a lot. The high cylinder vacuum will likely suck in a lot of oil - either through the rings or the valve seals, where it comes through to let it burn oil. You'll have a batch of oil in the cylinders when you fire it up with fuel. If so, you'd just be causing it to burn oil

You should be able to test for this. Try it, with medium-high engine rpms, on a longish hill. Restart it when needed, give it maybe 2/3 throttle, and watch for a blue-white oil cloud from the tailpipe. If you see it, it's from the slug of oil built up in the cylinders in that long coast with high vacuum.

Would it burn any more oil in DFCO than not? Seems that it'd be the same amount of oil getting sucked in whether fuel is coming in or not... the valves still open the same...

[comptiger5000]

Valves open the same, yes, but the throttle plate is closed, causing the engine to build a significant amount of vacuum (more than any other normal condition). The effort of sucking air past a closed throttle plate is what provides the engine braking effect during DFCO.

[RobertSmalls]

Originally Posted by comptiger5000

The effort of sucking air past a closed throttle plate is what provides the engine braking effect during DFCO.

I thought so too, but then I hit the fuel kill toggle switch for DFCO on demand. When I open the throttle plate, all that changes is the exhaust note, not the rate of deceleration.

[user removed]

Originally Posted by NachtRitter

Would it burn any more oil in DFCO than not? Seems that it'd be the same amount of oil getting sucked in whether fuel is coming in or not... the valves still open the same...

Combustion pressure casues the rings to seal the cylinder walls more closely, because the pressure increases the force applied to the cylinder walls by the rings.

This is the same reason you will see a certain amount of blue smoke between gear changes in an engine with significant wear.

DFCO with your foot off the throttle creates highest engine vacuum readings, which combined with no combustion will cause higher amounts of oil to be sucked past the rings and valve seals, as was previously mentioned.

regards
Mech

[comptiger5000]

RobertSmalls - With smaller engines, the effect is less significant, and friction/windage, etc contribute a greater proportion of the engine braking effect. In my Jeep, it doesn't get a ton of braking on downshifts at first. Then, after 1 - 2 seconds when DFCO cuts in and the IAC closes, it's like someone slammed on the brakes (big engine gives lots of engine braking).

It also depends on the cam setup, etc of the engine. Some simply provide more braking than others. For example, the engine braking in my sister's 94 Camry 4 cyl is pretty strong, but dad's 05 Camry V6 has much weaker engine braking, even though the engine is bigger.