This is something I was thinking about last night. Does a diesel have a choke to help it warm up like a gasser?

It is called high idle. If it is turned on it will activate when the temp is around freezing.

Tht's what I thought. Buddy has an 06 Dmax, with 8" lift and says that his milage sucks now (winter) because the choke stays on during the short drive to work. I said I didn't think they have chokes, but wanted to find out so I'm not talking out my a**.

Thanks, Jason

your buddys mileage sucks because of the winter blend fuel. if he is from az like you maybe not. how cold does it get there? that and high idle are the only things that would change during the winter months. unless he is warming it up longer than during the summer.

there is a "CHOKE" or butter fly in the intake to allow the EGR into the cylinders......

it is normally vacumm operated.. CA emissions LB7 and all LLY and newer trucks have soemthing simmilar...

His high idle wouldn't be on while in gear anyway. Winter Blend Fuel...

It has been "cold" for us, low to mid 30's in the morning depending on where you live. I do not think we have the winter blend, that was going to be my next question. My milage has not changed, so I do not think we have it. He does not warm up his truck before going to work. I know that for a fact.

A diesel engine is a constant compression engine, they have no throttle plate. A gas engine is a variable compression engine, the throttle plate restricts the air intake flow. PLAY SAFE!

Also the High Idle I believe adds fuel to help it warm up faster.

I did a little test yesterday. My truck idles around 1100 for high idle and EGTs are around 600-650*.

When it was cold, I manually raised the idle w/ my foot to around 1100-1200 rpms. EGTs would only get up to about 400-450*.
Then I let off the pedal and waited for the High idle to kick in (about 30 seconds) and it went up to 1100 rpms and EGTs went to 600-650*.

Adding fuel will cause EGTs to raise so that's why I think our mileage sucks in the winter time for those who use High idle.

This is something I was thinking about last night. Does a diesel have a choke to help it warm up like a gasser?

The LMM has something of a choke on it:

"Another emissions addition to the Duramax is an electronically controlled intake throttle upstream from the turbocharger. While the engine is idling or the truck is traveling at low speeds with no load, the engine computer will limit the amount of air allowed to enter the engine to intensify the load. This will increase the temperature of the exhaust, which helps maximize the performance of the passive catalytic converter and the diesel particulate filter."

That was taken from here (http://www.dieselpowermag.com/news/0702dp_last_gmc_duramax_diesel/index.html).

there is a "CHOKE" or butter fly in the intake to allow the EGR into the cylinders......

it is normally vacumm operated.. CA emissions LB7 and all LLY and newer trucks have soemthing simmilar...

The LMM has something of a choke on it:

"Another emissions addition to the Duramax is an electronically controlled intake throttle upstream from the turbocharger. While the engine is idling or the truck is traveling at low speeds with no load, the engine computer will limit the amount of air allowed to enter the engine to intensify the load. This will increase the temperature of the exhaust, which helps maximize the performance of the passive catalytic converter and the diesel particulate filter."

That was taken from here (http://www.dieselpowermag.com/news/0702dp_last_gmc_duramax_diesel/index.html).
thanks;)

:bow: Thanks for all of your replys!

Also the High Idle I believe adds fuel to help it warm up faster.

I did a little test yesterday. My truck idles around 1100 for high idle and EGTs are around 600-650*.

When it was cold, I manually raised the idle w/ my foot to around 1100-1200 rpms. EGTs would only get up to about 400-450*.
Then I let off the pedal and waited for the High idle to kick in (about 30 seconds) and it went up to 1100 rpms and EGTs went to 600-650*.

Adding fuel will cause EGTs to raise so that's why I think our mileage sucks in the winter time for those who use High idle.

After thinking about this for a while, I'm confused about one thing.

If High Idle does add extra fuel, how does it keep the rpms down with nothing else to regulate the throttle?
With no throttle body, adding extra fuel would just cause the engine rpms to keep raising but it doesn't.
With me holding the pedal to keep a constant 1100 rpms should be the same as the High Idle feature but what is causing the jump in EGTs?
:confused:

The LMM has something of a choke on it:

"Another emissions addition to the Duramax is an electronically controlled intake throttle upstream from the turbocharger. While the engine is idling or the truck is traveling at low speeds with no load, the engine computer will limit the amount of air allowed to enter the engine to intensify the load. This will increase the temperature of the exhaust, which helps maximize the performance of the passive catalytic converter and the diesel particulate filter."

That was taken from here (http://www.dieselpowermag.com/news/0702dp_last_gmc_duramax_diesel/index.html).
your right. i forget about those in the new motor. good info.

is the 08's the only ones with this kind of "choke"

is the 08's the only ones with this kind of "choke"
yes.. it is part of the EGR system

Diesels are also sensitive to operating temperatures. The engine is designed as a working engine, so it has a powerful cooling system. It operates best when up to temperature and if ambient temps are low enough, the cooling system will keep it inefficiently cool, even if the thermostats are closed. That's a big reason for the winter front grille covers, to help get and keep the engine up in a more efficient operating temperature range.

Your friend is not experiencing a "choke" situation, but he WILL experience restricted and less efficient operation until the Allison heats up. That impacts mileage in cold weather, too, because the Ally will top gear inhibit until fluid temps get up to 100 degrees.

After thinking about this for a while, I'm confused about one thing.

If High Idle does add extra fuel, how does it keep the rpms down with nothing else to regulate the throttle?
With no throttle body, adding extra fuel would just cause the engine rpms to keep raising but it doesn't.
With me holding the pedal to keep a constant 1100 rpms should be the same as the High Idle feature but what is causing the jump in EGTs?
:confused:

When your high idle comes on turns off when the truck is warmed up you should notice your tranny is warmed up too. I was told the truck uses to tranny to put a slight load on the engine to help it warm up, that plus it restricts the exhaust flow to help it all warm up faster.

I don't know for sure about the tranny being used, but I do know the Duramax insert in the owners manual says it restricts the exhaust flow to help it heat up faster, it turns off as soon as you touch the brake, throttle, or put it in gear.

When your high idle comes on turns off when the truck is warmed up you should notice your tranny is warmed up too. I was told the truck uses to tranny to put a slight load on the engine to help it warm up, that plus it restricts the exhaust flow to help it all warm up faster.

I don't know for sure about the tranny being used, but I do know the Duramax insert in the owners manual says it restricts the exhaust flow to help it heat up faster, it turns off as soon as you touch the brake, throttle, or put it in gear.


High idle only works in Park (maybe neutral too, never tried), how is the tranny suppose to put a load on the engine in park?

How would it restrict the exhaust flow?

ok, about the high idle feature. i had a reply to me not long ago about how it really works. First, when it first kicks on, it raises rpm's to 1100rpm. Then it also engages 3rd and 4th gear while keeping the parking pawl engaged which puts a load on the engine warming the trans up and the motor. It also partially closes the turbo vanes which puts a slight restriction in the exhaust which helps warm it up faster. Which is why when high idle first kicks in if you listen, after the rpm's increase, then within a few seconds you'll hear the sound change and sometimes the truck will slightly "sway" or jerk, when it engages the 3rd and 4th gears in the trans and then you will hear the turbo whine start up in the exhaust. Once the engine has warmed enough or the intake air temperature gets warm enough, then it will idle it down and go back to normal. If it needs high idle again soon, it only raises rpm's to 1100. the trans and turbo are only used to initially warm it up. Pretty cool design feature if you ask me.

High idle only works in Park (maybe neutral too, never tried), how is the tranny suppose to put a load on the engine in park?

How would it restrict the exhaust flow?

I think with the putting it in gear comment the book was referring to the 6 speed manual tranny.

I'm not sure exactly how it restricts the exhaust flow, but the book says it, I'm still new to duramaxes.

As for the tranny putting a load on the engine, some one on the forums said it. He said it uses third or fourth gear to produce a slight load. Might be kinda dangerous though, what if the parking pin gave out, lol. I don't know if its true or not, but it makes sense b/c after a high idle warm up the tranny is at operating temp too.

*nice beat me to it and explained it better*

Ah that makes sense. I forgot about the variable turbo vanes.

This is why I love this place!
:cool2:

All engines burn more fuel when warming up so if his drive is short that would explain the lower fuel mileage. Especially with a diesel which takes longer to warm up under a no load condition. An air flow restrictor and choke is in no way related. A choke as with a gasser will cause a vac allowing the high speed jets to flow more fuel to provide a richer mixture until the engine warms. An air flow restrictor on the other hand doesn't directly provide more fuel unless commanded by the ECM.

After thinking about this for a while, I'm confused about one thing.

If High Idle does add extra fuel, how does it keep the rpms down with nothing else to regulate the throttle?
With no throttle body, adding extra fuel would just cause the engine rpms to keep raising but it doesn't.
With me holding the pedal to keep a constant 1100 rpms should be the same as the High Idle feature but what is causing the jump in EGTs?
:confused:
Diesels don't work like gassers. There is no throttle body. It regulates RPM's by the amount of fuel added. The intake is open and it will suck in as much air as it needs.

Diesels don't work like gassers. There is no throttle body. It regulates RPM's by the amount of fuel added. The intake is open and it will suck in as much air as it needs.

Obviously you did not read my post well enough. I said "with no throttle body".

id say there are four things that all effect poor gas mileage/warmup. Vane position , high idle,egr function, and also the converter not locking until 95*F. all of these help warm up the truck and anytime you make heat you lose efficiency. Honestly i havent noticed a difference with winter blend, its the warmups that hurt gas mileage

Obviously you did not read my post well enough. I said "with no throttle body".
Okay let me rephrase it.

After thinking about this for a while, I'm confused about one thing.

If High Idle does add extra fuel, how does it keep the rpms down with nothing else to regulate the throttle?
Because it regulates the RPM's by the amount of fuel added.
With no throttle body, adding extra fuel would just cause the engine rpms to keep raising but it doesn't.


With me holding the pedal to keep a constant 1100 rpms should be the same as the High Idle feature but what is causing the jump in EGTs?
:confused:Maybe the high idle feature adjusts more than just fuel output. Like fuel timing or something to intentionally make it run hotter. I'm just guessing at that part.
It's quite possible I'm misunderstanding exactly what you're asking.

The term "choke" in the strictest sense does not at all apply to a diesel. A choke is a mechanical device in a carburetor that, when closed or applied, causes a pressure drop in the venturi of carb to help pull fuel from the float bowl of a cold carburetor/engine. There is no direct correlation with a diesel engine

An electronically controlled diesel provides the necessary cold engine enrichment via fuel flow, injector pulse width and timing. They run a lot richer when cold

O.K Chromer, so the ECM dumps more fuel while the engine is cold to warm it up, just like a re-gen mode in the new DPF motors correct?

O.K Chromer, so the ECM dumps more fuel while the engine is cold to warm it up, just like a re-gen mode in the new DPF motors correct?

I don't think its exactly like the regen, but similar concept. Regen enrichment is designed specifically for high egt's so the entrapped soot will incinerate

Okay let me rephrase it.


It's quite possible I'm misunderstanding exactly what you're asking.

Quote:
Originally Posted by Zick http://www.dieselplace.com/forum/images/buttons/viewpost.gif (http://www.dieselplace.com/forum/showthread.php?p=2337358#post2337358)
Obviously you did not read my post well enough. I said "with no throttle body".

Okay let me rephrase it.

Quote:
Originally Posted by Zick http://www.dieselplace.com/forum/images/buttons/viewpost.gif (http://www.dieselplace.com/forum/showthread.php?p=2335950#post2335950)
After thinking about this for a while, I'm confused about one thing.

If High Idle does add extra fuel, how does it keep the rpms down with nothing else to regulate the throttle?
Because it regulates the RPM's by the amount of fuel added.
With no throttle body, adding extra fuel would just cause the engine rpms to keep raising but it doesn't.


With me holding the pedal to keep a constant 1100 rpms should be the same as the High Idle feature but what is causing the jump in EGTs?
:confused:Maybe the high idle feature adjusts more than just fuel output. Like fuel timing or something to intentionally make it run hotter. I'm just guessing at that part.

It's quite possible I'm misunderstanding exactly what you're asking.

I understand that the rpms are controlled by the amount of fuel that is added.
I was questioning earlier if the High Idle feature did add "extra" fuel why does the rpms not go up? Because the EGTs were higher at the same rpm when I had my foot on the pedal vs letting the high idle do the work.
It's still possible that it does add extra fuel and the turbo vanes adjust to restrict the airflow to keep the rpms lower.

But niceguypmp3 answered my question in post #20.

Remember these engines are electronic on a class 8 and I am sure these also the reason the EGT's goes up is that the engine advances the timing to get the heat in the engine quicker. On a class 8 when it starts up it automatically goes to 900 to 1200 and advances the timing. I hope this explains it better. PLAY SAFE!

ok, about the high idle feature. i had a reply to me not long ago about how it really works. First, when it first kicks on, it raises rpm's to 1100rpm. Then it also engages 3rd and 4th gear while keeping the parking pawl engaged which puts a load on the engine warming the trans up and the motor. It also partially closes the turbo vanes which puts a slight restriction in the exhaust which helps warm it up faster. Which is why when high idle first kicks in if you listen, after the rpm's increase, then within a few seconds you'll hear the sound change and sometimes the truck will slightly "sway" or jerk, when it engages the 3rd and 4th gears in the trans and then you will hear the turbo whine start up in the exhaust. Once the engine has warmed enough or the intake air temperature gets warm enough, then it will idle it down and go back to normal. If it needs high idle again soon, it only raises rpm's to 1100. the trans and turbo are only used to initially warm it up. Pretty cool design feature if you ask me.


PUTS IT IN 3RD AND 4TH WHILE STILL BEING IN PARK DONT THINK THTS POSSIBLE IN SO MANY DIFFFRENT WAYS. MIGHT WANT TO RECHECK THT INFO

I have read somewhere on the GM site or Allison that the transmission puts a load on the engine to increase warm up time. Not sure how it is done but if you listen to it or have an edge attitude monitor (or similar) you can see tell the load is increased.

When i started the truck this morning, i paid attention to when it went on elevated idle. about 15-20 seconds after it idled up, i put my hand on the truck and could feel a slight "jerk" and then the turbo spooled up like usual. i have looked everywhere online and cannot find an official document saying how the allison increases the load on the engine. It probably increases line pressure somehow or redirects fluid which in turn increases the load. Did sound like a far cry that it engages 3rd and 4th gear to do it, but it's possible. With these electronically controlled transmissions, all of the shifts are done with solenoids, so it is very possible for the computer to engage any of the gears while the gear shift and parking pawl remain in park. makes sense because if it all occurs inside the transmission, then when it engages the gear, the drive shaft isnt able to move at all because of the pawl, so the truck and engine doesnt "jerk" the same as if you manually put it in gear. i'm thinking that gm wont say how the trans does it precisely because its patented. would be cool to know though.

I know the manual says the trans puts a load on the engine. If I had to guess how, it would be that they restrict the pump outlet. This would raise the pressure, adding the load, and creating the heat. Basically it would work like a retarder on a heavy haul truck or a scraper, or a rock truck. when you activate them it slows the equipment w/o using wheel brakes. It uses the trans to slow ground speed.

A diesel engine is a constant compression engine, they have no throttle plate. A gas engine is a variable compression engine, the throttle plate restricts the air intake flow. PLAY SAFE!

I think this is correct, at least mine does not have any type of butterfly, mabee inside the egr to allow ex gasses back into the motor but nothing controling fresh air into the motor