does the 80 shut down injection while coasting?

[e9999]

This was vaguely discussed some time back but I don't remember any conclusive information and did not find anything while searching.

Does anybody know for sure whether or not the FZJ 80 shuts down its fuel injection system while coasting to conserve fuel?
In principle that would be a good idea, of course, assuming the engine would not run rough and there would be no issue reigniting things up when needed.
IIRC, I worked on a euro Diesel a long long time ago that did that. I imagine it's probably not unheard of if not downright common in at least Diesel engines and probably some gas engines (no idea).

Anybody knows for sure?

[jonheld]

There are several manufacturers out there that shut off 3 of the 6 injectors when the ECU says it's not needed. I have never heard that with regards to the 1FZ-FE. Certainly not with the 3FE, but it would be interesting to try. If I'm not mistaken it can be done manually with a switch to ground. However a well performing 3FE has "just" enough ponies to keep it moving through the air. With 3 less cylinders firing I would need to go downhill both ways.

[Hants]

There is a fuel cutoff relay that stops fuel (most times) when you release the throttle.

I don't know the details of how it works.

On my rig, it shuts down most(all?) of the fuel when I let off the gas. ScanGauge goes to 9999 MPG. Sometimes, it doesn't, though (ScanGauge goes high, but not to 9999). I haven't figured out any pattern.

Some new vehicles variably shut down individual cylinders when full engine output isn't needed (low-load cruising down the highway). Said to be unnoticeable (except at the gas pump).

[alaskacruiser]

Hants- do you have the old or new Scangauge? Mine is the new (Scangauge II) and I've never seen 9999MPG when I let off the gas. I usually see something like 60-80MPG. I also have a '97 LX, BTW.

[e9999]

Hants, what does it read mpg wise when you're idling at a stop? should be zero to make sense, or is it 9999 as well?



back to the basics: the thing is that for this to work, it has to know that there is a reverse load on the engine. Simplest way to know that would be RPM greater than nominal idle with throttle closed...?


interesting, read someplace that some manufacturers did away with the cutoff feature upon introduction of OBDII.... no idea if true...



and here is a vid of the scangaugeII reportedly showing 0 consumption when coasting in gear (not a LC, a CIVIC):
YouTube - Fuel injection shut-off while coasting in gear
and interestingly, the SgII does show a finite consumption and zero speed when stopped and idling... more instructive to show gph than mpg for this...

here's another thought: if you were to coast down in gear and record the speed or gph and rpm (unfortunately the current SGII can not do that, I'm told), and if the injection is indeed cut off at high rpm (yet to be confirmed), you'd probably be able to tell at what rpm it comes back on by looking at the data (either a reduction in deceleration rate or the gph going back up)


still more: just read that there is a "cutoff" function on the scangauge. Called to clarify what that does. Apparently, there is a bit in the ECM that tells whether you are either in injection cutoff mode or under high acceleration (could be either one depending on circumstances). So they ask you to set a threshold for the TPS so that the Sg2 can tell whether the engine is under actual cutoff and display same as zero consumption. Strangely, the ECM will still report a hypothetical injection rate, fuel mixture etc even though the injectors are cut off. A bit murky and room for errors, there...

[Hants]

Originally Posted by alaskacruiser

Hants- do you have the old or new Scangauge? Mine is the new (Scangauge II) and I've never seen 9999MPG when I let off the gas. I usually see something like 60-80MPG. I also have a '97 LX, BTW.

I have the II (with X-Gauge).

[Hants]

Originally Posted by e9999

Hants, what does it read mpg wise when you're idling at a stop? should be zero to make sense, or is it 9999 as well?

0, IIRC (I haven't really paid attention lately, and its not a DD). Back-to-back runs of 0 throttle (tps @ 9) on the freeway, and most times it will go to 9999MPG, but some times not. Same with city driving. I gave up trying to figure out the pattern. Its more useful to me monitoring instantaneous MPG to "today" MPG, and adjusting my driving habits (or not!).

Originally Posted by e9999

and here is a vid of the scangaugeII reportedly showing 0 consumption when coasting in gear (not a LC, a CIVIC):
YouTube - Fuel injection shut-off while coasting in gear
and interestingly, the SgII does show a finite consumption and zero speed when stopped and idling... more instructive to show gph than mpg for this...

here's another thought: if you were to coast down in gear and record the speed or gph and rpm (unfortunately the current SGII can not do that, I'm told), and if the injection is indeed cut off at high rpm (yet to be confirmed), you'd probably be able to tell at what rpm it comes back on by looking at the data (either a reduction in deceleration rate or the gph going back up)

The SG will show both GPH & RPM.

[landtank]

I don't think it does and here is why. During my MAF testing I data logged the O2 sensors as well as other ones. If the fuel was cut anytime during the operation there would have been a noticeable spike in the readings from high O2 levels. I don't remember seeing that and I was scrutinizing the logs pretty closely.

[Cattledog]

Originally Posted by landtank

I don't think it does and here is why. During my MAF testing I data logged the O2 sensors as well as other ones. If the fuel was cut anytime during the operation there would have been a noticeable spike in the readings from high O2 levels. I don't remember seeing that and I was scrutinizing the logs pretty closely.

2X

I don't recall any spikes in the O2 senor during testing of the MAF housing and sensor.

[Brian894X4]

My scangauge reads 9999 when coasting from high speed, like going down a hill at highway speed with no throttle. At idle, if I remember correctly, there is nothing displayed next to mpg.

The fuel cutoff switch in my 22RE is located in the AFM. When the throttle is let off and the airplate inside snaps closed due to lack of vacuum, it hits a switch that turns the fuel pump off. This is sometimes a source of problems of the fuel pump not working properly due to lack of power.

I'm sure the early 1FZs are exactly the same way. Not sure about the '95 plus ones like mine that don't AFMs. It's probably all digitally controlled instead of the manual trip switches like the earlier models.

[CivicFerio]

I believe on newer emission controlled vehicles, during down hill or coasting, the injectors spray the amount of fuel as if it was idling. This is done to keep the cat at optimum temperature at all times to reduce emissions.

[RavenTai]

Originally Posted by e9999

back to the basics: the thing is that for this to work, it has to know that there is a reverse load on the engine. Simplest way to know that would be RPM greater than nominal idle with throttle closed...?


interesting, read someplace that some manufacturers did away with the cutoff feature upon introduction of OBDII.... no idea if true...

I have not read anywhere about a fuel cutoff for the FZJ80,
My 1981 280ZX had a Bosch fuel injection, despite the decade between them it was very similar to the OBD1 setup in the 91-94 80 series. it had fuel cutoff, If RPM was >2500 and the throttle was closed via the TPS no fuel was delivered, with a 5 speed in the mountains you could spend quite a bit of time in this state coming down steep grades,

Originally Posted by CivicFerio

I believe on newer emission controlled vehicles, during down hill or coasting, the injectors spray the amount of fuel as if it was idling. This is done to keep the cat at optimum temperature at all times to reduce emissions.

that makes sense and ties in with E's OBDII statement.

[landtank]

Originally Posted by CivicFerio

I believe on newer emission controlled vehicles, during down hill or coasting, the injectors spray the amount of fuel as if it was idling. This is done to keep the cat at optimum temperature at all times to reduce emissions.

I'll agree on this. Our trucks have an idling FT% that is triggered by the TPS when closed. I saw this state every time I let off the gas even when traveling the highway at 65MPH.

[turbocruiser]

Originally Posted by landtank

I don't think it does and here is why. During my MAF testing I data logged the O2 sensors as well as other ones. If the fuel was cut anytime during the operation there would have been a noticeable spike in the readings from high O2 levels. I don't remember seeing that and I was scrutinizing the logs pretty closely.

x3

No spikes in O2 levels, but AFR does immediately go to 16 (or leaner), and load immediately goes to the 12% level which is what idle is or sometimes lower if you are actually coasting down a steep grade with OD off and letting the engine brake against the vehicle. I am almost positive that more or less the amount of fuel injected is at idle levels or something slightly less than idle levels (because idle is always at an AFR of 14.7 or so - richer than the 16 we see without any pedal at all and coasting). I am also almost positive that the fuel pump never stops seeing voltage and that the spark plugs never stop seeing spark - I'd think that at the least the FP would not receive voltage in a non-fueled mode and maybe also that the spark plugs would stop seeing spark. I believe (although might be wrong with this) that all the selectable cylinder engines out there like the Northstar cutoff both fuel and spark. HTH.

[turbocruiser]

Originally Posted by e9999

back to the basics: the thing is that for this to work, it has to know that there is a reverse load on the engine. Simplest way to know that would be RPM greater than nominal idle with throttle closed...?

Let's make sure that the basics are correct though:

There is no such thing as "reverse load" on the engine. Even when engine braking down super steep grades load is never less than about 6 to 8%. Also, the throttle is never totally closed, and RPM could be almost redlined while the throttle is almost closed (engine braking down super steep grades especially after downshifting) and the load would still be 6% or more. Believe it or not load at idle is typically 12% with some small jumps up and down. IOW, the engine "works" to stay on regardless of how easy you make it work.

[landtank]

it's important to realize that "calculated load" is not standardized like other OBDII values. Ford actually has negative load for just this occasion of engine braking. Our systems don't but just wanted to point this out in case people start trying to apply valid information from other manufacturers to our trucks.

[e9999]

Originally Posted by turbocruiser

Let's make sure that the basics are correct though:

There is no such thing as "reverse load" on the engine. Even when engine braking down super steep grades load is never less than about 6 to 8%. Also, the throttle is never totally closed, and RPM could be almost redlined while the throttle is almost closed (engine braking down super steep grades especially after downshifting) and the load would still be 6% or more. Believe it or not load at idle is typically 12% with some small jumps up and down. IOW, the engine "works" to stay on regardless of how easy you make it work.

??
I don't know what you mean by load I guess. I meant mechanical work in or out of the engine. if you are engine braking, how can the load not be reversed if the fuel is cut off and if the losses in the drivetrain are not enough to slow down the truck by themselves (unlikely since you can coast for a long time on flat ground in N)? The fact that the engine rpm increases does mean there is work applied *on* the engine, no?


Also, remember that the guy at SG said that in some systems at least, when cutting off fuel completely, the ECM still produces fake numbers on AFR, fuel injected etc. If true, one would want to be careful about "data" shown on some of these systems.

[619TOY]

First let me say that I know nothing about this and while decipherable, seems to verge on philosophical. My Scangauge creeps up in MPG until it jumps to 9999 when coasting downhill. I know that even if idling, I would not get 9999 miles per gallon. Since the gas pedal has been completely released and I am coasting down a pretty good grade, is the engine even idling anymore? OK, back to my

[CivicFerio]

I stumbled upon this Toyota Yaris thread while researching about toyota engines. It's a very interesting read about DFCO deceleration fuel cut off. Check it out!

Using DFCO to increase your MPG - Toyota Yaris Forums - Ultimate Yaris Enthusiast Site

Quote from the yaris thread:

How can I tell if a vehicle other than the Yaris has DFCO?

Be careful with this test as you may lose Power Steering and Power Brakes during it! Choose the right time and location to perform the test.

On any level or slightly downhill road get up to at least 40 MPH, then start engine braking by keeping the vehicle in gear and letting all the way off the throttle while keeping the clutch disengaged (MT) or keeping the transmission in a gear lower than D (AT). Notice the deceleration force and the sound of the engine.

Now tun off your ignition (This is where you may lose your PS/PB, so be wary!) and watch for any harsher deceleration or change in the pitch of the engine for a few seconds.

If nothing was any different with the ignition turned off then your vehicle uses DFCO. If it decelerated at a different rate or pitch with the ignition off then it does not use DFCO. Do not forget to start your engine back up!