How to Get the Most Fuel Economy

Messages
3,183
Location
Western S.C.
... When decelerating, the engine RPMs are higher and more air is being pulled through the engine. In order to keep the fuel/air ratio in the range that prevents excess NOx emissions, the computer has to add extra fuel. With carburated engines, the extra vacuum causes additional fuel to be pulled through the idle circuit.
Not necessarily.

In my carbureted Mazda, a solenoid valve shut off the idle circuit whenever the car was coasting in gear with the accelerator released and engine speed above idle. That's with 1980 technology.

When coasting at lowish speeds, the Prius effectively gives a choice between engine-off coasting in neutral, and engine-off coasting with light (if you stay off the brake pedal) regenerative braking. At higher speeds, the choice is between fuel cut with engine being rotated at idle speed by the wheels, or same plus regenerative braking. Using regenerative braking is wasteful, except when you need to slow down.
 

OVERKILL

$100 Site Donor 2021
Messages
46,064
Location
Ontario, Canada
Everyone says that, but do you have any evidence? Other people saying it somewhere else online? Even the engineering explained guy assumed this is true without actually testing his cars. My scangauge reads what the ECU is doing in real time through the OBD port, and in my cars true fuel cut off is only at rpms way above what you use in top gear. At 60 mph rolling down a hill in top gear every car I've owned uses about 30% more gas coasting in gear than idling in neutral, plus you are using engine braking to loose speed. It seems crazy that's what they do for emissions, but that's what they do.:(
I believe SUV's and trucks have slacker emissions standards and they do cut fuel completely more often, I haven't tried the scanguage in the Outback yet, but our 2003 Tracker would completely cut fuel when coasting down to near idle speed. I don't recall if it would do that before the engine was at normal temps though.

It's been around for a very long time, of course implementation is going to vary wildly. I got a chuckle about this revision from the Ford EEC-IV strategy (1987):
9-046 Modify Decel Fuel Shutoff Logic to add Cal Switch to use it at Part Throttle for enrichment via FN374 (N).

Earlier entries:
7-667 Do not do decel fuel shut off if any FMEM flag is set.
7-525 Add Closed Throttle check to DFSO; Add 2 breakpoints to FN131
7-574 Add C/L Fuel delay after DFSO
7-577 S/W - Correct temp. register usage in DFSO.
8-016 Allow clutch input to be selectable for DFSO logic & for ISC logic
6-486 Ramp decel fuel "SHUTOFF".
6-486B Allow decel S/O at commencement of decel.

Anyways, to get into the logic here:
Screen Shot 2021-03-19 at 8.50.54 AM.jpg

Screen Shot 2021-03-19 at 8.51.59 AM.jpg
Screen Shot 2021-03-19 at 8.52.19 AM.jpg

Screen Shot 2021-03-19 at 8.52.42 AM.jpg


Screen Shot 2021-03-19 at 8.53.09 AM.jpg

Screen Shot 2021-03-19 at 8.53.40 AM.jpg

Screen Shot 2021-03-19 at 8.53.55 AM.jpg
 
Messages
9,745
Location
Ontario, Canada
It's been around for a very long time, of course implementation is going to vary wildly. I got a chuckle about this revision from the Ford EEC-IV strategy (1987):
9-046 Modify Decel Fuel Shutoff Logic to add Cal Switch to use it at Part Throttle for enrichment via FN374 (N).

Earlier entries:
7-667 Do not do decel fuel shut off if any FMEM flag is set.
7-525 Add Closed Throttle check to DFSO; Add 2 breakpoints to FN131
7-574 Add C/L Fuel delay after DFSO
7-577 S/W - Correct temp. register usage in DFSO.
8-016 Allow clutch input to be selectable for DFSO logic & for ISC logic
6-486 Ramp decel fuel "SHUTOFF".
6-486B Allow decel S/O at commencement of decel.

Anyways, to get into the logic here:
View attachment 50060
View attachment 50061 View attachment 50062
View attachment 50063

View attachment 50064
View attachment 50065
View attachment 50066
Interesting! I wouldn't think DFCO strategies would vary too much between cars having to follow the same emissions standard of their era, but maybe they do? I'll have to hook up the scangauge and see what the subaru actually does on decel, the dash says 0.0L/100km but we'll see. It feels like it does cut fuel but its not a passenger car for emission standards either so I think it does cut fuel often.
 

OVERKILL

$100 Site Donor 2021
Messages
46,064
Location
Ontario, Canada
Interesting! I wouldn't think DFCO strategies would vary too much between cars having to follow the same emissions standard of their era, but maybe they do? I'll have to hook up the scangauge and see what the subaru actually does on decel, the dash says 0.0L/100km but we'll see. It feels like it does cut fuel but its not a passenger car for emission standards either so I think it does cut fuel often.

Back when this program was written very few manufactures were doing SEFI, so I'd expect it to be a significantly more complex strategy than from other marques that used simpler systems :)

This is a 4x Catalyst system with air injection, charcoal canister and EGR.
 
Messages
2,288
Location
WA
What if you are late for a meeting or something and stuck behind a fuel saver?

big picture ... would your impatient driving result in more fuel usage which balances out the fuel savings of the person in front of you?

collectively speaking, we may be back at zero-sum!
 
Last edited:
Messages
17,948
Location
NH
What if you are late for a meeting or something and stuck behind a fuel saver?

big picture ... would your impatient driving result in more fuel usage which balances out the fuel savings of the person in front of you?

collectively speaking, we may be back at zero-sum!
Sometimes I wonder what it'd be like everyone viewed the speed limit as the target speed to travel at. Would traffic be better or worse, would the roads be clogged up, or things actually move smoother.
 
Messages
1,095
Location
Northern, NY
One aspect not covered here is consolidating trips. Driving less saves a lot more fuel than any of the other ideas.

This really applies to those who drive to get a coffee and back home then drive to pick up lunch and drive home then drive to a appointment and drive home, and so on and so forth.

Excellent point. Working from home a few days per week rather than commuting every day has saved me a ton of gasoline.
 
Messages
2,363
Location
South Carolina
Back when this program was written very few manufactures were doing SEFI, so I'd expect it to be a significantly more complex strategy than from other marques that used simpler systems :)

This is a 4x Catalyst system with air injection, charcoal canister and EGR.
My '93 F150 had a TSB come out about a hesitation after decel and re-engaging the throttle (which mine suffered). The TSB required a new PCM which eliminated the MPG enhancement program and a replacement of the EGR. I'm wondering if this was part of what you posted.

I replaced the PCM and EGR with the new part numbers and it cured the hesitation. The hesitation and stumbling was really bad and annoying when getting back on the throttle after decel.
 
Messages
2,363
Location
South Carolina
Sometimes I wonder what it'd be like everyone viewed the speed limit as the target speed to travel at. Would traffic be better or worse, would the roads be clogged up, or things actually move smoother.
Traffic would actually move along quite nicely. It's the difference in speed of drivers that causes the problems.
 
Messages
9,745
Location
Ontario, Canada
What if you are late for a meeting or something and stuck behind a fuel saver?

big picture ... would your impatient driving result in more fuel usage which balances out the fuel savings of the person in front of you?

collectively speaking, we may be back at zero-sum!
Yeah, I don't vary my speed much when I'm in traffic but you can still do a bit of fuel saving while staying with the flow. Most of my driving is on 55 roads that aren't busy so its no issue to try and save some fuel.
 

OVERKILL

$100 Site Donor 2021
Messages
46,064
Location
Ontario, Canada
My '93 F150 had a TSB come out about a hesitation after decel and re-engaging the throttle (which mine suffered). The TSB required a new PCM which eliminated the MPG enhancement program and a replacement of the EGR. I'm wondering if this was part of what you posted.

I replaced the PCM and EGR with the new part numbers and it cured the hesitation. The hesitation and stumbling was really bad and annoying when getting back on the throttle after decel.

An F-150 with the Lo-Po wasn't sequential IIRC, and wasn't MAF. These were batch-fire, which was odd, as the cars were SEFI, even the Lo-Po ones. Of course the trucks were flat tappet longer than the cars too, so there's that.

But yes, that's likely one of the parameters tweaked in the replacement ECM.
 

gathermewool

Site Donor
Messages
8,927
Location
New England
My '93 F150 had a TSB come out about a hesitation after decel and re-engaging the throttle (which mine suffered). The TSB required a new PCM which eliminated the MPG enhancement program and a replacement of the EGR. I'm wondering if this was part of what you posted.

I replaced the PCM and EGR with the new part numbers and it cured the hesitation. The hesitation and stumbling was really bad and annoying when getting back on the throttle after decel.
How does your engine-braking feel now? Similar or is it gone now?
 
Messages
8,490
Location
Champlain/Hudson Valley
1) During my first trip to Los Angeles (after my very first plane ride) I noticed how people on those "L.A. freeways" went 80 mph but when the road bent a little they'd clot up and traffic would bump back. I said to myself, "Why don't these morons just go one speed?".

2) I picked up my current car in Bethesda, Maryland on Monday December 26th. For those who don't know, this is just north of the north pointing angle of Washington, D.C. I used familiar Connecticut Ave, to get to the highway.

At ~4:00PM I began my drive home. I gotta assume there was less traffic due to it being the day after Christmas but there was still some. I was FLOORED that the locals(?) were driving at the perfect slow speed of 45 mph. I prayed, "Just keep doing this and we'll all be MUCH better off. AND THEY DID.
 

gathermewool

Site Donor
Messages
8,927
Location
New England
One thing I’m curious about: fuel economy vs what’s best for the engine. It likely doesn’t matter, but in my 11 mile commute, speeding warms the engine oil up to almost full temp on cold days. It’s left 10F+ cooler while driving around the speed limit.

For obvious reasons I tend to speed on my way to work (no hidey holes for cops) and drive the speed limit or at the speed of traffic, not to exceed five over on my way home.
 

gathermewool

Site Donor
Messages
8,927
Location
New England
1) During my first trip to Los Angeles (after my very first plane ride) I noticed how people on those "L.A. freeways" went 80 mph but when the road bent a little they'd clot up and traffic would bump back. I said to myself, "Why don't these morons just go one speed?".

2) I picked up my current car in Bethesda, Maryland on Monday December 26th. For those who don't know, this is just north of the north pointing angle of Washington, D.C. I used familiar Connecticut Ave, to get to the highway.

At ~4:00PM I began my drive home. I gotta assume there was less traffic due to it being the day after Christmas but there was still some. I was FLOORED that the locals(?) were driving at the perfect slow speed of 45 mph. I prayed, "Just keep doing this and we'll all be MUCH better off. AND THEY DID.
MD’ers are a chill lot. Super annoying. :LOL:
 
Messages
2,570
Location
Caldwell Idaho
Just the opposite in a hybrid which would recharge the battery during coasting.
When I fixed forklifts they had regen on many of the newer models, the batteries still needed charging after the end of then shift. I wonder what difference it actually makes on cars?
 
Top