Chevy and GMC Duramax Diesel Forum banner

In very cold weather, should I idle until the RPM settles down?

17K views 66 replies 25 participants last post by  Duramaxdan59 
#1 ·
I always assumed that it was good for the engine to idle for a few / couple of minutes at start-up, to let the engine RPM settle down.

Right now the temps here in South East Wi range from 17-30 deg. In the morning the truck RPM is about 850 when first started. After couple of minutes it settles down to just below 500rpm.


I am reading conflicting advice on the net, with some experts claiming idling an engine is bad. While others say idling for extended period of time is bad, but there is no definition of what entails "extended period of time", though I assume it means well more than start-up idle, so over 15 mins.


So is it good or bad to let my LML idle for a couple of minutes , until the RPM has settled down?
 
#3 ·
imho, it depends on what you are going to do with the truck.

In cool (not cold) weather I usually wait for it to idle down (mine trucks have all lived outside, and I use remote start, so that doesn't mean I'm sitting in the truck waiting for it)

You should enable the 'elevated idle' function.

When it's warm, it will do nothing.
When it is cold enough (ambient and engine coolant temp) it will run the idle at 1050 rpm. It also sets the torque converter, fan and turbo to create more heat. This also warms the transmission oil.


If your truck lives outside, remote start and the 'elevated idle' are your friends in the winter.


Remote start will run the truck for 10 minutes. It can only be used twice and then a key cycle is required before it can be used again.
(the fob can be used to extend the first cycle -- but the time extension counts as the 2nd cycle)


There is a bunch more info about 'elevated idle' in the duramax supplement.
(Don't confuse this with the 'high idle'; or 'fast idle' function which is for a different purpose, and needs to be activated manually)
Once coolant temp reaches about 150F, the 'elevated idle will shut itself back off.


Elevated idle is supposed to only work below 32F. This was true on my LML, but my L5P seems to engage elevated idle a few degrees above 32F, although I haven't been able to sort out what the limit is.
 
#9 ·
I have no idea if my truck has the "elevated idle" option. It does not have remote start, far as I can tell.
Will I be able to use a scanner tool to find out if it has elevated idle?

I am also a but confused on what elevated idle does. Do you mean it runs the engine at 1050 rpm all the time when it is idling, and this RPM never drops? And how does it set the torque converter, fan and turbo to create more heat? Does this higher heat generation subside once the truck has been on the road a few miles, or does it always maintain a specific minimum temp? Does it noticeably affect fuel economy?
 
#5 ·
I just discovered fast idle on my L5P last night.

Usually sleeps in the garage, but on a trip yesterday it sat out all day. 35F when I started it and it idled up to about 850 or so for about 10 minutes. Had the truck a year and never seen this before.

the high idle option is not listed on my build sheet and I have not made/changed any settings for this, so assume it is something built into all trucks, like the coil spring choke on a Rochester Quadrajet. (Ahh, the good old days...)
 
#6 ·
I remote start and let mine idle for 10 mins in the mornings-and plug it in over night. Been in the 20’s lately.

All other times, even in summer, I start and wait at least 30 seconds if it’s been sitting. I just feel it allows for fluids to get to the right place.

If your truck is emissions intact then maybe just make sure you have a good long drive sometime through the week.
 
#10 ·
Knowing I'm not the one going to lunch at 4pm...it does make me feel better. :teehee
 
#8 ·
lol
 
#12 ·
Lunch @2....still

40* and rain all day....I've been hibernatin' anyway. marx
 
#17 ·
no, daylight savings is silly, however it must be weird to have to consult a calendar to calculate what time a football game is on in local time... in some places in the state, but not everywhere.



"......."..........
Very wrong. Continued idling WILL EVENTUALLY “soot up” the DPF in a LML to the point where no “re-gen” can clean it, resulting in a “limp mode”.

You can then either buy a new exhaust assembly for about $2,700, or, if you are near one of the shops that regularly service the DPF,s on the “big rigs” they can cut yours out, cut it apart & clean it, re- install, so that once you find a shop with a sophisticated programmer to take out the “limp mode” you will have a new running truck for about $700 (that is, provided you don’t subject it to extensive idling)
its almost like you intentionally read what you want to out of people posts just so you can tell them they are idiots.
 
#21 ·
Collins,

Like Mikey52 stated, your truck came with the "elevated idle" which you won't even notice unless it gets cold enough and then it will idle up to approx. 850-950 about 30-45 seconds after started and stay up until the temp gets up some then it will drop to your regular idle speed unless you put it in drive or touch the accel pedal. I think that you can disable it in the DIC options but why? It is a passive system that requires no input from driver.

The "High/fast idle" is RPO UF3 and a dealer can reprogram your truck to have it. It is controlled by the cruise buttons. Go to gmupfitters.com and read the bulletin #82d. It will explain the functions.

There is a 3rd system found on WT's and cab/chassis trucks if optioned with the PTO system. It has a PTO switch and the wiring but does not have the actual PTO mounted to the trans. This system allow you to have 3 different speeds all the way to approx 1800 RPM.

My last 3 trucks all had this installed and I never needed it once. Don't even need the high/fast idle-- I have let my trucks idle for 2 hours at a time in winter and dead of summer(Texas type) and never needed any higher idle. Guess if you were using a big electric load it might be handy?

Hope this explanation helped you out
 
#24 ·
SAMBOR - thanks for the info.
In the owners manual I have read about the fast idle on the PTO trucks, and also about the RPO option for fast idle. I knew I didn't have either of these so I was surprised that cold day when it obviously idled up. As you say it only lasted a few minutes. Nice to know.


LMMWASHINGTON -
My wife had an Acura sedan a few years ago when her mom was taken to the hospital in another town and she went down be with her. My wife called me late one night and said she went to the hospital about 7AM and when she came back to the car about 7PM it was idling - apparently she forgot to turn it off. It did idle so quietly you couldn't hear or feel it, plus her mind was elsewhere. She was worried about whether it was damaged.

I searched around the internet and found dozens of personal stories of folks leaving cars running this long and longer. I just changed the oil when she got back and it was fine.
The 850-950 rpm for 30-45 seconds is not the 'elevated idle' function kicking in. (even though it does seem to occur when cool, but not cold enough to activate the 'elevated idle'.

Elevated Idle will run at 1050.
Fuller description in my previous post above..
 
#22 ·
Lmao this reminded me of a time I had my wife’s old car at work for some reason and I forgot to shut it off. It idled for a solid 10 hours before I got in it to go home. Killed like quarter tank.
I run heavy equipment for a trade and man I have seen some serious abuse from ether in a frozen skid steer in -40 to running an entire rock crusher off of a single cat c27 generator when it’s normally on 2 for 3 weeks just maxed out 22 hours a day the turbos would glow red hot.
Anyways in my trade you always idle equipment for 10 minutes or so maybe 20-30 in the winter before we even hop in them. In the cold states like North Dakota it’s common for trucks to run days on end or equipment. I keep it plugged in when I can and always remote start even in the summer (I got a 2008 gmc) I recommend high idle 😉 it’s amazing my duramax will be nice and warm to operating temp seats hot ready to roll even in -30 before my first start has timed out and truck shuts off bout 10 minutes.
 
#23 ·
SAMBOR - thanks for the info.
In the owners manual I have read about the fast idle on the PTO trucks, and also about the RPO option for fast idle. I knew I didn't have either of these so I was surprised that cold day when it obviously idled up. As you say it only lasted a few minutes. Nice to know.


LMMWASHINGTON -
My wife had an Acura sedan a few years ago when her mom was taken to the hospital in another town and she went down be with her. My wife called me late one night and said she went to the hospital about 7AM and when she came back to the car about 7PM it was idling - apparently she forgot to turn it off. It did idle so quietly you couldn't hear or feel it, plus her mind was elsewhere. She was worried about whether it was damaged.

I searched around the internet and found dozens of personal stories of folks leaving cars running this long and longer. I just changed the oil when she got back and it was fine.
 
#25 ·
If you reread my post you will see I said 30-45 seconds AFTER starting it will idle up to the elevated idle speed. Maybe not quite that long on some trucks but it isn't an immediate upspeed
 
#26 ·
And both of my last trucks including the new 4500 idle at 950 when the elevated idle is on. Pick something beside 100 rpm to quibble about
 
#27 ·
And I stated it won't activate unless its cold enough. I think collins knows that it takes more than "cool" to come on.
 
#28 ·
You also need to make sure you know the difference between elevated idle and high/fast idle. Maybe you need to look at my post again. It explains it well as all of my trucks have had those systems
 
#30 ·
Good for you on the rpm. 3 of mine were 900-950. And as for required criteria- reread my post
 
#33 ·
Engine Idle Variations this refers to idle speed variations that are not caused by 'Elevated Idle' or Fast Idle' functions

Under certain conditions the engine
idle speed can vary or be elevated.
Change in idle speed is normal and
does not indicate a problem. Normal
conditions that can raise idle speed
are low voltage, DPF regeneration,
air conditioning compressor loads,
and engine warmup. These speeds
can range from approximately 600
to 1000 rpm.


Elevated Idle this is the one that is standard on dmax

The engine has a cold temperature
high idle feature which elevates the
engine idle speed from base idle to
1050 rpm for pickup models or
1200 rpm for van models when
outside temperatures are below
0°C (32°F), and the engine coolant
temperature is below 65°C (150°F).
This feature enhances heater
performance by raising the engine
coolant temperature faster. For truck
models, there may be two methods
of activating the elevated idle.
To turn this feature on or off:
1. Turn the ignition to ON/RUN,
with the vehicle off.
2. Press the accelerator pedal to
the floor and hold while quickly
pressing the brake pedal three
times in less than eight seconds.
3. Release the accelerator pedal
and start the engine.
If this method is not available, see
the alternate method under Truck
Models in Vehicle Personalization
(Truck Models)

When the engine is started, it will
slowly ramp up to the high idle
speed after a delay of a few
seconds up to approximately
two minutes. For this method to
work properly there must be no
throttle or brake pedal faults.
The engine idle speed will return to
normal once the following conditions
are met:
. Engine coolant temperature
reaches 65°C (150°F).
. Air intake temperature reaches
0°C (32°F).
The high idle speed will be
temporarily interrupted and the
engine speed will return to normal if
any of the following conditions
occur:
. The brake pedal is applied.
. The accelerator pedal is
pressed.
. The transmission is shifted out
of P (Park) or N (Neutral).
. Vehicle speed is detected.
Once these inputs are removed, the
engine idle speed will slowly ramp
back up to high idle after the normal
delay, if the conditions for engine
coolant temperature and air intake
temperature are still met.


Fast Idle Control this is the one that is there if the UF3 option is purchased sometimes called 'high idle'

The vehicle may have this system
which can be used to increase the
engine idle speed.
Fast Idle control will be enabled
when the following conditions
are met:
. The parking brake is set.
. The transmission is in P (Park)
or N (Neutral).
. The vehicle speed is about
0 km/h (0 mph).
. The cruise control Set switch is
pressed and released for Preset
Fast Idle Speed (1200 rpm).
Fast Idle control will be disabled
when one or more of the following
conditions occur:
. The cruise control Set switch is
pressed and released. See
“Cruise Control” in the owner
manual.
. The cruise control Cancel switch
is pressed.
. The brake pedal is pressed.
. The transmission is shifted out
of P (Park) or N (Neutral).
. The parking brake is released.
. The vehicle speed is not
0 km/h (0 mph).
On vans, when fast idle is active, a
FAST IDLE ON message will be
displayed in the DIC.
 
  • Like
Reactions: sambor
#34 ·
I have a 2013 lml and I let it do what it wants when it's cold out. Lol I start it from the bedroom and get ready for work and when I go out its ready to rock and roll I just rolled 70k and no problems with the high idle I figure it's just powering the glow plugs and intake air heater and trying to get that alternator warmed up so I can do its job but I wouldn't worry about it. My grandfather always said if it's not broke dont fix it
 
#35 ·
At 32 degrees, a minute or two is all I let it run before backing out and taking off. Now at -53, you better believe it’s gonna idle for the full 20 minutes that it will on the remote start before I go hop in it lol. I’ve had it be so cold that the trans won’t go into gear until it’s ran long enough to warm up some.
 
#36 ·
Duramax Dan
The high idle is RPO UF3 like redwngr stated. It has only one speed--1000 rpm on trucks. The upspeed on PTO trucks( almost all WT's/ cab and chassis) is 3 different speeds: 900/1200/1900. Last 2 3500's and current 4500 have had this system. If you want to verify the info look at tech bulletin 120F on upfitters site.

By the way, the high idle speed UF3 on anything other than a van is 1000 rpm not 1200. That is the speed for vans.

Guess we have beat this thread to death-- at least until next cold season and new owners didn't get an owners manual.

By the way redwngr, can you edit your post to include the PTO speed info and sticky note it for future use? People seem to confuse elevated idle with high/fast idle
 
#39 ·
Fwiw, my post above is cut/past from the 2015 dmax supplement.

The confusion is why I copied the instructions for both 'elevated idle' and 'high/fast idle' in one post above.
Elevated idle is the one that is for cold weather engine/transmission warm up.


PTO operation is 10 pages of text and tables of info in the Dmax Supplement.
It's usage will be dependant on the PTO accessory that has been added.

Here is a link to the 40 page Bulletin at the gmupfitter site regarding the PTO for anyone that has one and needs more information.
https://www.gmupfitter.com/files/media/photo/1136/UI Bulletin 120f-2018DEC21.pdf
 
#38 · (Edited)
Yup.

That's an example of the UF3 'Fast Idle' that is operated by the cruise control buttons.
Factory installed UF3 should also be 1200 if in a pickup.

Elevated idle 1050 on pickups, 1200 on vans).
 
#40 ·
Thank you for posting the info. It is a long procedure but the basics are similar to the high idle except for the 3 speeds
 
#41 ·
I have always let all my trucks warm up for a bit. I don’t like to run a cold engine cause you could end up breaking something. Just my 2 cents
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top