Myth dispelled?

[92EcoDiesel Jetta]

I have read here that when you let off the throttle at speed and coast down while in gear that the IP injects zero fuel, until it reaches idle where it starts injecting fuel again to maintain idle, so it is better to coast down in gear rather than to shift into neutral for better MPG. I was really curious if this is true and rigged my Mac Tools diesel pulse adapter by unsoldering the LED, ran an extension inside the the car. As you can see, the LED continues to flash, i.e. fuel continues to be injected while it is coasting down in RPM. Does this dispel the myth that no fuel is injected while coasting down in gear with no throttle?

CLICK TO PLAY VIDEO!

[Rising]

I have actually been thinking about this for weeks whenever I'm driving my car and I'm so glad to have this confirmed through actual experimentation.

I think what was previously observed was that egts seem to fall faster in gear vs. Neutral coast. However I believe this observation can be explained by two things: probe reaction time and engine efficiency at different rpms.

Probe reaction time is the minor player here, however I believe it is reasonable to accept that all probes no matter how small have some degree of latency and will react faster to a higher volume of air rushing over them than a lower volume. More molecules transferring energy to or in this case away from the probe will always result in faster reaction even if temperature was equal. I believe this plays a very minor role.

The big one for me is engine efficiency. At different Rpms, due to complex fluid dynamics I don't understand, engines are capable of more or less efficient flow. This is why @ n/a diesel smokes more at high rpm where air flow is getting choked out (resulting in richer mixture) and doesn't make as much torque at idle as it does at 2000rpm. My understanding of this is rudimentary so please excuse it. However I believe that an engine operating at a more efficient rpm(able to ingest more air per cylinder stroke) compression braking would result in a leaner mixture with the throttle resting against the idle screw than say the same engine operating at a relatively inefficient 700 Rpm idle with the throttle in the same position. It would also produce lower egts because it is running LEANER. But this doesn't necessarily mean less fuel is used... Just more air mixing with the same fuel.

 Since the throttle arm(a horrible term I seem to be unable to escape in this post.. Please excuse my terminology) is in the same position in each situation, fuel injection quantity per injection should be equal, however while compression braking at 2000 rpms you are injecting twice as often as if you were idling, so in spite of your overall leaner fuel mixture and lower egts you actually using twice as much fuel as had you pushed your clutch in and idled for that short period, and that isn't taking into account the losses in speed from engine braking.

I have recently switched methods as of late and have been seeing an improvement in my fuel mileage. Could be placebo but I believe that combined with other methods of fuel conservation coasting in neutral is superior. 

Feel free to correct any of my  misunderstandings. However I believe this explains in corroboration with the above evidence why we see lower egts compression braking but could still be using more fuel total.

[92EcoDiesel Jetta]

Your explanation of why egt seem to fall faster in gear vs. neutral coasting has a corollary in what I observe with coolant temperature. Coasting in neutral  down a long hill produces higher coolant temp vs coasting down the same hill in 5th. It has to do with the amount of flow, in one case air flow, the other coolant flow.

[745 turbogreasel]

I have read here that when you let off the throttle at speed and coast down while in gear that the IP injects zero fuel, until it reaches idle where it starts injecting fuel again to maintain idle, so it is better to coast down in gear rather than to shift into neutral for better MPG.Going down a hill yes, but on the flat you willlose speed and have to burn fuel to re accelerate. the f I was really curious if this is true and rigged my Mac Tools diesel pulse adapter by unsoldering the LED, ran an extension inside the the car. As you can see, the LED continues to flash, i.e. fuel continues to be injected while it is coasting down in RPM. Does this dispel the myth that no fuel is injected while coasting down in gear with no throttle?
No, you are going to have to rev a good bit higher than that.

I have actually been thinking about this for weeks whenever I'm driving my car and I'm so glad to have this confirmed through actual experimentation.

I think what was previously observed was that egts seem to fall faster in gear vs. Neutral coast. However I believe this observation can be explained by two things: probe reaction time and engine efficiency at different rpms.

Probe reaction time is the minor player here, however I believe it is reasonable to accept that all probes no matter how small have some degree of latency and will react faster to a higher volume of air rushing over them than a lower volume. More molecules transferring energy to or in this case away from the probe will always result in faster reaction even if temperature was equal. I believe this plays a very minor role. How slow do you think my probe is? I  have plenty of hills here that take 5+ minutes to descend at 40-50 MPH in N at 800 RPM(325ish EGT), or  3rd at 3200+(200EGT).

The big one for me is engine efficiency. At different Rpms, due to complex fluid dynamics I don't understand, engines are capable of more or less efficient flow. This is why @ n/a diesel smokes more at high rpm where air flow is getting choked out (resulting in richer mixture) Up to here it's not complex  fluid dynamics, but the adjustment of levers and springs. and doesn't make as much torque at idle as it does at 2000rpm.Physics is in play here My understanding of this is rudimentary so please excuse it. However I believe that an engine operating at a more efficient rpm(able to ingest more air per cylinder stroke) compression braking would result in a leaner mixture with the throttle resting against the idle screw than say the same engine operating at a relatively inefficient 700 Rpm idle with the throttle in the same position. It would also produce lower egts because it is running LEANER. But this doesn't necessarily mean less fuel is used... Just more air mixing with the same fuel. This would be closer to true with a non load compensating governor, but those drive  pretty weird.  we have a lot of parts  moving downstream of the throttle arm. 

 Since the throttle arm(a horrible term I seem to be unable to escape in this post. Please excuse my terminology) is in the same position in each situation, fuel injection quantity per injection should be equal, however while compression braking at 2000 rpms you are injecting twice as often as if you were idling, so in spite of your overall leaner fuel mixture and lower egts you actually using twice as much fuel as had you pushed your clutch in and idled for that short period, and that isn't taking into account the losses in speed from engine braking.

I have recently switched methods as of late and have been seeing an improvement in my fuel mileage. Could be placebo but I believe that combined with other methods of fuel conservation coasting in neutral is superior. 

Feel free to correct any of my  misunderstandings. However I believe this explains in corroboration with the above evidence why we see lower egts compression braking but could still be using more fuel total.

Mechanical governor parts take time to move,  and fuel 'maps' area an analog  ramp to prevent  overshoot and sag, and generally make your car drive nice.  This means  just chopping the throttle and pushing the clutch, you may not spend enough time decelerating to achieve fuel cut, especially from a relatively low RPM.
My car doesn't  cut below about 2500 RPM.    It's hard to hear the fuel cut , but  if you are rolling downhill ~3000 RPM, and ease into the throttle, you can hear it light off again.  Listen for a light rattle similar to 7.3powerstroke injectors at idle.

[ORCoaster]

What part of the mechanical system would turn off the fuel delivery?  The only part that does that is the shutoff solenoid and that has 12 VDC on it when running.  So why would you think the injectors would be fuel less when coasting?

What style Mac Tools adapter do you have?  Mechanical or optical?

Watching the LED just means it is sending a signal.  I can get the mech one to do that by clamping it around a 1/4 inch bolt and whacking it with a nail.  Ping!   Light goes on!

[theman53]

It really isn't noticeable in 5th under 3500 for me. Wind it up well and downshift it will go to almost nothing.

[745 turbogreasel]

What part of the mechanical system would turn off the fuel delivery?  The only part that does that is the shutoff solenoid and that has 12 VDC on it when running.  So why would you think the injectors would be fuel less when coasting?

The control collar slides both ways bro.

[ORCoaster]

yeah kind of had that, you duffas, moment on the way home.  Came up to a light and did the downshift thing and thought, Hey foot off accelerator here so that tiny collar is at rest like at idle only compression is way up.  Hmm. Could that be what I was wondering about earlier in the day? 

So use at idle is so little I would think it is nearly nil, but not completely zero.  Zero equals multiple threads to this forum about why doesn't my pump work anymore? 

[shorttimer]

A lot of interesting thoughts, here. If mileage IS the big issue here, then why not activate the fuel shut off solenoid while decelerating with a momentary switch & let off when you wish to continue? The downside is it won't do it automatically.

[ORCoaster]

What if you hooked up one of those mystery switches everyone asks about that are tied to the IP.  As it hits idle it cuts the juice for the solenoid and as soon as you put your foot back into the throttle it turns it on?  You would have to over ride for a wait at the light but a paddle switch on the steering wheel might do that or the actual cut off for downshifts and coasting to give you fuel savings.  Must pay attention or be popping the clutch or hitting the starter all the time.

[92EcoDiesel Jetta]

Mechanical governor parts take time to move,  and fuel 'maps' area an analog  ramp to prevent  overshoot and sag, and generally make your car drive nice.  This means  just chopping the throttle and pushing the clutch, you may not spend enough time decelerating to achieve fuel cut, especially from a relatively low RPM.
My car doesn't  cut below about 2500 RPM.    It's hard to hear the fuel cut , but  if you are rolling downhill ~3000 RPM, and ease into the throttle, you can hear it light off again.  Listen for a light rattle similar to 7.3powerstroke injectors at idle.

So at what RPM and under what circumstance does fuel cut occur? I soldered the LED back into the diesel pulse adapter already but I am willing to unsolder it again to do the test again. Just tell me what exactly I should be doing to get it right this time.

[92EcoDiesel Jetta]

......

What style Mac Tools adapter do you have?  Mechanical or optical?

Watching the LED just means it is sending a signal.  I can get the mech one to do that by clamping it around a 1/4 inch bolt and whacking it with a nail.  Ping!   Light goes on!

Piezo

[92EcoDiesel Jetta]

It really isn't noticeable in 5th under 3500 for me. Wind it up well and downshift it will go to almost nothing.

You did not quote anyone and I am not following what you are trying to say. What is "it"?

[745 turbogreasel]

You did not quote anyone and I am not following what you are trying to say. What is "it"?

Fuel rate.

At 0 throttle, you are requesting ~800 RPM
 -let the clutch out a little, it bogs, and the gov slides the collar up to compensate with more fuel
 -aim down a hill in gear, RPM tends toward 900, idle spring compresses and the collar is slid down the shaft.
 -aim down a steep hill and you are at 4000RPM 0 throttle, the pump says 'OMG I overshot my 800 RPM target by  3200 RPM, holy crap I better reduce fuel a lot!' and totally exposes the spill port
Fuel ramps back on at 2000-3000 RPM to provide a smooth transition


My local off ramp is all downhill and a perfect example.... Downshift to 4th at around 65, I get 4500 RPM, 3-4 PSI, and EGT around 200.
or I go down in N, put allthat heat into my brakes, and have EGT 325-350

You are pretty much looking for a grade steep enough you can maintain 3500 RPM or more coasting in gear.

[theman53]

It really isn't noticeable in 5th under 3500 for me. Wind it up well and downshift it will go to almost nothing.

You did not quote anyone and I am not following what you are trying to say. What is "it"?

745 answered it well.

To add, I don't have a tiny tach but others on here used to and had video explaining and showing the RPM going to almost nothing while coasting. You have to think if on this side of it also, what if it continued to add fuel? What would happen while coasting/engine breaking? It would positively accelerate down the hill instead of holding you back.