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Discussion Starter #1
Ok. I hope all your rigs do this, of similar years ect. While going steady at a medium to high rate of speed say 55 or 70. Cruise on or not, and you just let off. Everything goes quiet, as if the engine stalled. The tach still reads, EVERYTHING functions normally, but I hear no injectors or combustion at all. Until maybe 1500-1700rpms. And if going say 55 or whatever and slow down very quickly the injectors/combustion goes intermittently until 1400 and down to 600 or so rpms. It also does it while going slow like 35 and let off again until the rpms drop to say 1400.

What is that?! I think it is friggin awesome but I don't understand it 100%. I understand engine braking but are the injectors actually "turning off"? And if so does the engine keep running just on compression/centrifugal force? How's it work. Also if it is shutting injectors down, cutting them off whatever to make them not inject does it do that to all 8 or is it the miraculous "cut off 4 cylinders for economy" type of thing like all the new gasser SUV's and trucks. One more thing, this "coasting" effect, is that what the COAST button on the steering wheel does?

PLEASE someone explain this to me.
I just want to know how this system works, it's very fascinating. And I just can't figure out HOW it works, and what it's SURELY doing.
 

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I know exactly what you're talking about. Always kind of wondered about that when I had my 99.
 

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Discussion Starter #3
I know exactly what you're talking about. Always kind of wondered about that when I had my 99.
:nod: Glad you are understanding my lengthy description there. Its so weird. I hate not knowing what's going on...
 

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All the 7.3 SD's I drive do this. The coast button on the steering wheel is for the cruise control. Go and drive the truck turn cruise on and then after it sets press and hold the coast and the truck will slow down. Press and hold the set/accel button and the truck will speed up. When you step on the brakes and turn CC off press res and it will turn back on.
 

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Discussion Starter #5
All the 7.3 SD's I drive do this. The coast button on the steering wheel is for the cruise control. Go and drive the truck turn cruise on and then after it sets press and hold the coast and the truck will slow down. Press and hold the set/accel button and the truck will speed up. When you step on the brakes and turn CC off press res and it will turn back on.
So it is coasting. I thought so. Just doing that would duplicate what I was saying with the different speeds and rpm controlled by Mr. Right Foot.

That answers one question, now why and more importantly how is what I'm wondering. :dunno:
 

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Discussion Starter #6
All the 7.3 SD's I drive do this. The coast button on the steering wheel is for the cruise control. Go and drive the truck turn cruise on and then after it sets press and hold the coast and the truck will slow down. Press and hold the set/accel button and the truck will speed up. When you step on the brakes and turn CC off press res and it will turn back on.
Oh and thank you for sure. You not only answered one question but gave me a much better understanding of how to work the cruise on it. I got how to set it and shut it off LOL. It's alot different than the ole Buick. :icon_ford:
 

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Discussion Starter #7
Nobody knows? Really? Over 40 views and only two people wanna say something, whats up with that? :dunno:
 

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You know what's funny is that a guy on DTR posted in a thread I'm subscribed to about the exact same type of thing sometime last night or early this morning. lol.....
 

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Discussion Starter #9
You know what's funny is that a guy on DTR posted in a thread I'm subscribed to about the exact same type of thing sometime last night or early this morning. lol.....
Wow that IS weird. Great minds think alike! What did he find out? Anybody on there know?
 

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so is this a automatic or a stick? I would assume if it's an automatic that the torque converter has just disengaged and the truck is coasting at idle..my '06 SD does the same thing..lightly touch the accel and it comes back to life and pulls fine-it's just a coast mode as I see it...with fuel getting to be $4+ a gallon,just sit back and enjoy.
 

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The engine keeps on "running" because it is still spinning. When you remove your foot from the "throttle" entirely you are essentially communicating to the PCM to return the engine to idle. Diesels don't actually have throttles like gasoline engines. The throttle pedal on a gas engine in one way or another (mechanical or servo) controls a butterfly-type throttle valve to actually limit the amount of air going into the engine and then the computer simply ensures a correct "air/fuel ratio" for whatever amount of air is entering at a given time. This "air/fuel ratio" ensures complete combustion at an appropriate temperature. Not enough fuel is termed lean ... and it burns hot. Too much fuel is termed rich ... and it burns cooler.

With diesel engines, the intake is wide open at all times... there is no throttle valve. The "throttle pedal" would be most aptly called a "demand pedal". Assuming for simplicity that there is no turbocharger boost involved ... a naturally-aspirated diesel will pull in a specific flowrate of air for the given RPM. The "demand pedal" simply controls the amount of diesel that is being dumped into the combustion chamber. The "air/fuel ratio" is not important in diesel engines as it is in gasoline engines. However, when too much fuel is being dumped in for the amount of air ... you produce black smoke (wasted fuel). With a turbocharger involved ... as the turbo builds up boost more air is entering the engine and more of the diesel can be turned into power before it is wasted as black smoke. The power output of your diesel is basically limited to however much diesel it can pour in (for a given RPM and boost pressure) before the point of producing smoke.

To specifically address your question ... with no throttle valve on your engine ... there is no engine braking. The "engine braking" effect is from the shut throttle on a gasoline engine restricting flow on the intake cycle. It takes more for the engine to pull air in then it does to exhaust it so it slows down.

The only "engine braking" to speak of on a diesel is due to mechanical friction. With a free intake, free exhaust, and the "compression stroke" taking as much work as the "power stroke" (w/ no combusting diesel) gives work ... a diesel will only slow down by putting a load on it or due to friction of the piston rings, main bearings, etc.

When you take your foot off the "throttle" the diesel engine stops getting diesel. It doesn't need _any_ diesel because you told it to return to idle, it is trying to slow down ... and adding diesel would not help it slow down. It gets quiet because it is not getting any diesel and nothing is combusting ... it is pulling in air; compressing air; expanding air; and exhausting air. It starts receiving diesel and making noise again to "catch itself from falling" past idle. It cuts in diesel fuel before it gets all the way down to idle to avoid falling right past it and then it cuts in just the right amount of diesel at idle to maintain it.

I hope that was coherent, understandable, and helpful.
 

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It's normal, your injectors stop injecting since your engine is coasting along and will begin firing once the load/demand comes back.
 

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I wanted to address the "engine braking" that some pretend diesels have ... it took some background info :)
 

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Discussion Starter #15
Well you went further than i needed you to BUT that is very good for anyone else who needs to know. What a great explanation.

So basically other than incorrect terminology I was right? Shuts off the injectors so it is only an air mover, and the lateral force of 8 pistons converted to centrifugal force of spinning the crank keeps it spinning (or running ... technically) , but the compression it makes slows it down, and it catches at around 1400 to idle which is around 650. And so when I brake hard it does that but has to catch sooner, still trying to slow down and that's why it is a quick on/off/onon/off/on/off/on. (Explaining why the pedal is oh so touchy) That is what I thought engine braking was BASICally, the engine slowing down, however means, it slowed down the rest of the vehicle. Now I know it isn't called that, but i know how it works. Very good wording of the demand pedal, i like that. That ... setup/system/technology is why I LOVE diesels and love driving them, just so much smarter IMO.

Now I can tell my friend that he's completely wrong and maybe have him read this. He claims that the force of one piston coming back down isn't enough to make an opposite compression stroke and visa versa so there's no way the injectors will shut off or it wouldn't "run" as in it would just stop spinning. :doh: He swears by and worships Duramax... :rolleyes:
 

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Discussion Starter #16
It's normal, your injectors stop injecting since your engine is coasting along and will begin firing once the load/demand comes back.
i thought so. That's so cool. I <3 diesels.
 

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Discussion Starter #17
That was one of the main reasons I originally joined. I wanted to get that taken care of, find out about boost numbers, and make a thread about my bodywork. FINALLY, my purpose is fulfilled, I don't feel so clueless. Lol with that bodywork thread, I feel as though I've made my dent on the org, it's huge.

I'm definately staying on here for as long as possible, I love this place, you guys are awesome. :icon_ford:
 

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Agree'd... good job scraph...
 

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Discussion Starter #20
Yes it was. Very thorough. You should teach an engine theory class. You're doing better right there than my auto teacher in high school. :clap:
 
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