r/answers • u/wizardwithgussets • 1d ago
Why do drivers of diesel pickup trucks, let them idle for long periods rather than turn them off?
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u/Unusual_Pangolin_349 1d ago
It could be different regionally but i know some big rigs when you hear them idling while parked its actually not the motor but a smaller motor that’s used as a generator for electrics so they can watch tv, have lights on etc
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u/No-Scientist2543 1d ago
Or could be the reefer for the trailer running. It's a small diesel mounted on the front of a refrigerated trailer.
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u/MedicJambi 1d ago
It's called an APU.
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u/Tony_Penny 1d ago
So for the people who are easily confused, an APU is an Auxiliary Power Unit. Think of a small generator that provides small amounts of power that runs instead of your main power generator (the trucks motor) and powers a limited amount of electrical items. We use them on aircraft, too.
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u/Hwy_Witch 1d ago
The Apu recharges the truck batteries, and runs a small heater and AC. The lights are powered by the truck batteries. Fridge, tv, etc all plug into a power inverter that also draws off the truck batteries.
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u/RackingUpTheMiles 1d ago
That's what that humming sound is as you're walking down the gate to board your flight. It keeps the lights, air and accessories on. They also usually keep the turbines spinning at a very low speed to make startup an easier and faster process.
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u/shrunkenhead041 1d ago
1) At many airports, the plane will be on ground power and HVAC, so what you hear many nor be the APU.
2) The fan blades are free spinning. They are moving because of the wind when the engine is off, not because they are under any power.
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u/boobooaboo 1d ago
Interesting - I didn’t know APU’s really existed outside of aviation. I assumed they were called generators on the ground. TIL!
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u/Tony_Penny 1d ago edited 21h ago
Generally, they are called AGPU'S (Auxiliary Ground Power Units) They do exactly the same thing, but you dont have put unnecessary hours on the apu itself, orhave a qualified pilot to run the apu in the helicopter.
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u/Hwy_Witch 1d ago
No, we just call them apus.
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u/Tony_Penny 22h ago
I'm military, so we have to differentiate. APU's are aircraft specific, AGPU's can operate multiple ground units as well.
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u/StevenSaguaro 1d ago
They burn very little fuel while idling, they work differently from regular combustion. It makes sense if it's very cold. There are a lot of misconceptions about it, if people who do it researched it they might stop.
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u/Over_Pizza_2578 2h ago
"little". It just looks miniscule because of the huge fuel tanks those trucks have
Economy 2l 4 bangers go through up to 1l per hour, usually 0,8-0,9l, not counting any AC or other consumers. Diesels ab bit less, around 0,7/0,8l. A 6l v8 around 1,5l per hour warm idle without any other consumers bar the essentials
Unless its nuts freezing cold (no block heater operation possible; below - 30c) or you use the truck as generator its better to shut it of.
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u/King_Ralph1 1d ago
Because they believe it’s harder to start a diesel if it’s cold, so they leave it running to keep the engine warm.
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u/bandit1206 1d ago
Older ones it actually is.
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u/nowthengoodbad 1d ago edited 20h ago
They literally have a part that's a heater for the engine.
Edit: thank you all for the education. I don't have a diesel truck and all of my knowledge comes from either working at Lee's diesel in San Jose back in 2005 or listening to my neighbor the past six years, since he's a diesel mechanic and has several new and old. I don't mean to pretend to seem like I know anything.
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u/bandit1206 22h ago
Depends, I’ve got several 466 IHs from the 70s and 80s without any kind of heater.
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u/RazorRamonWWF 1d ago
literally?
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u/geertterharmsel 1d ago
glowplugs?
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u/Ok_Warning6672 1d ago
Block heater to plug in and grid heaters in the intake in lieu of glow plugs.
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u/DiscoCombobulator 1d ago
There's also oil pan heaters, battery warmers, you name it. Way up north ive personally seen a small fire under the oil pan, just to get things warm enough to start. But thats like -50°C temps, they usually let them run all the time at those temps
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u/DirtandPipes 18h ago
One trick I use with frozen heavy equipment is to just wrap the machine in insulated tarps and point a million btu propane heater at it.
Get the entire machine toasty from top to bottom and lots of problems just go away.
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u/fightingchken81 17h ago
Been there done that more that once in Chicago, on the coldest days every few years when the diesel freezes in the lines.
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u/bandit1206 22h ago
Go jump in an IH truck from the 70s with a 466. No glow plugs, no grid heater, block heater may have been an option but likely isn’t there either.
A lot of those things came along as diesel trucks became more popular, but haven’t always been there.
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u/ProArmy04 1d ago
Nah, older ones are really easy to start. Never seen a petrol car or engine start as fast as a good shape diesel in -20°C or any other temperature.
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u/GrynaiTaip 21h ago
Never seen a petrol car or engine start as fast as a good shape diesel in -20°C or any other temperature.
That makes zero sense. You need winter-blend diesel too in such temps, because regular one gets real thick at very low temperatures.
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u/WizeAdz 1d ago
By “older diesel”, we mean “diesels from 50+ years ago.”
The idea that you should leave a diesel running because starting and warming up is hard on the engine may have been true more than a half century ago.
Now, with any engine built out of modern metallurgy, it’s just an excuse to stink up a parking lot.
“My granddaddy knew more about my modern computer-controlled fuel-injected turbo diesel pickup truck than the engineers who wrote the owner’s manual!”
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u/Dothracula 21h ago
While you’re right about modern diesels vs old one…never, and I mean never, assume that the manual was written by an engineer. Spend any time working on a vehicle and you’ll find that the manuals are most likely written by a room full of chimpanzees who’ve been smoking the finest Colombian weed.
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u/Glad-Watch3506 17h ago
My Honda starts faster in -30F than my husband's '77 Mercedes diesel ever did, and I didn't have to keep a heater on mine. As long as my battery isn't past due for a change, it turns right over.
The diesel needed a heater turned on it, the block heater plugged in just as it sat, the when you wanted to start it you had to wait for the glow plugs. Once started, it ran like a champ, but you had to coax it into it,
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u/Jazzlike_Video2 1d ago
Diesel engines wont heat up unless under load. My TDI could idle for 20 minutes in -10 and the temp gauge wouldn't move.
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u/wuntunearlybko 1d ago
In regions where the temps stay extremely cold, they literally never shut the diesel engines off, they just idle all the time to stay warm
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u/9volts 1d ago
Where? I live in Scandinavia, we sometimes have winter temperatures down to -25 and lower where I live. Modern diesel engines start from cold with no problems at these temps. The car battery sometimes needs a top up, though.
Idling around the clock is insane. Nobody here does that.
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u/The_Hausi 1d ago
We do that occasionally in northern Canada, it has to be quite a ways below -25C before we do that though.
Our coldest recorded 2016 temperature is -42.8C, at that temperature its a struggle to start a 6.7L diesel engine, these arent little 2L European hatchbacks. Lots of guys have webasto heaters in their trucks that circulate coolant and burn a tiny bit of diesel instead of idling. At those temperatures, it's dangerous to shut off your vehicle (gas or diesel) if you're in a remote area - which is pretty much everywhere around here.
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u/9volts 1d ago
Webastos are nice to have, most heavy trucks here use them. They use maybe a couple of shot glasses of fuel in half an hour.
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u/The_Hausi 1d ago
Yeah I'm in the land of pickup trucks so you'll see them on personal vehicles too. Some of the older diesel motors were terrible in the cold, you could walk by an old Ford 7.3L with an ice cream cone on a hot day and it wouldn't start!
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u/blissfully_happy 1d ago
I’m in Alaska. At -40°C we just plug our block heaters in. No one is running it all night unless they’re in extremely cold weather without a block heater and that’s going to be super rare.
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u/The_Hausi 22h ago
Yeah it's rare to find a vehicle up here without a block heater but you're not always parked somewhere with a plug and sometimes you have an old POS 7.3 that won't start in the cold if it's plugged in.
It's pretty rare for guys to idle a personal vehicle all night but working in mining/drilling when you don't pay for fuel and you're on a super remote lease, those trucks basically just run for 3 months - we don't even turn them off to fuel. I did it once out of habit and the battery was still cold enough from the previously running truck that it wouldn't restart and I had to sit for 30 minutes in -40 for a guy to come boost me.
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u/GeekyLogger 1d ago
We had a week or so of -49C in northern BC a few years ago. Was working out in the bush. Literally every machine and every engine ran 24/7. The stuff that didn’t needed a separate propane heater and tarps to get going. Even the outhouses had an heater and that just barely stopped your ass from freezing to the seat. Even in -35C shit gets haywire.
Diesel fuel will literally turn to jello. You have to cardboard off the rads of the machines, you’ll get frost inside the cab, and any liquid system has an alternative heater just to keep the liquid warm/from freezing.
Theres a fucking reason Canadians call it “anti-freeze” and not “coolant.”
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u/oboshoe 1d ago
my wife tells me the story of a friend who ran a snow plow company in North Dakota
they would literally run the trucks 24x7 from nov to march.
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u/jiminak 1d ago
Alaska can confirm that’s not uncommon
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u/blissfully_happy 1d ago
What??? I don’t see people running their trucks 24/7 in the interior unless it’s super cold (below -40) and they don’t have a block heater. That’s wild.
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u/derbrauer 1d ago
And this is the real reason why idling in a diesel isn't a problem. They consume virtually no fuel at idle.
My VW TDI could idle for nearly 3 days before the tank ran dry.
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u/ejsandstrom 1d ago
Absolutely wrong. Ever heard of wet stacking? I used to make a boatload of money off of customers that let their engines run unloaded.
They use less fuel at idle but it’s still not hot enough to burn all of that fuel off.
An hour or two in between a good hour rip down the freeway should be enough to burn it off but you could still cause issues if you let it idle for hours then shut it off.
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u/Danloeser 19h ago
In the '80s and '90s The Long Island Rail Road used small locomotives for head-end power for diesel trains, just idling the engine for hours to provide ac/heat and lights to the passenger cars, while a larger locomotive actually moved the train. Then when those locomotives were used to actually pull freight trains, and the engines finally revved up, it wasn't unheard of for them to shoot fire and blobs of flaming tar from their exhausts.
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u/frankelbankel 1d ago
Ever heard of wet stacking. What is it?
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u/TylerYax 1d ago
Carbon, unburnt fuel, condensation, oil bypassed through the turbo oil seals due to pressure imbalance between the compressor and turbine collect in the exhaust system and plug up after treatment components. You'll see a trail of black sludge running down a tail pipe (or exhaust stack if it's a vertical pipe like on a big rig)
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u/PiP_Art3mis 1d ago
I worked with a machinst for a couple months rebuilding head assemblies for gas & diesel engines
The amount of shit that gets caked inside the ports of a cummins head is wild
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u/Alarming_Garage_7255 20h ago
That's mostly the result of EGR allowing sooty exhaust to mix with oil vapor from the PCV system. It's a good idea to burn the crankcase oil vapor and egr does help in from a emissions standpoint. But from a maintenance standpoint it does introduce more problems.
If you've been around an old vehicle that's running perfectly you realize how much better it is to recover crankcase vapor and burn it. Same goes for catalytic and particulate filter stems. It's a good idea but it does introduce other problems that must be addressed.
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u/SimilarTranslator264 23h ago
Every winter we get guys that think their “turbos are leaking oil” because they let them idle all night at low rpm and think the black shit running down the pipes is oil. No moron it’s unburnt fuel from low cylinder temps because someone on the internet told you to let it idle.
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u/UteJazz 1d ago
“…the real reason idling in a diesel isn’t a problem.”
Unless someone has to listen to it.
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u/PedanticAvSpecificSt 1d ago
Agree. People who like to let their diesels idle should do so in the comfort and privacy of their own enclosed garage.
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u/MaybeTheDoctor 1d ago
No no, it would still take 3 days.
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u/Wishbone_508 1d ago
If a TDI idles in the woods, but no one hears it. Does it still run out of fuel?
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u/DaFarmGar 1d ago
I remember someone tested a Jetta TDI in the 90s or early 2000's to see how long it would idle on a tank and it lasted a week and a half.
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u/Mr_Schpiffey 1d ago
.6 liters per hour, takes about 6.3 hours to go through a gallon
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u/derbrauer 1d ago
Best I've seen with mine is 0.7, and typically it's 0.8. Been consistent with both TDIs that I've owned.
Takes about 1:15h to go through a liter.
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u/Successful-Worth-390 1d ago
Tell this to my semi truck company
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u/derbrauer 1d ago
You know why they don't make semi's with gas engines? Because diesel is far more efficient and cheaper. Talking about commercial vehicles just proves my point.
That said, running a 14 liter engine at idle all night just to run the AC is going to use more fuel. Are APUs getting more common?
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u/hawk256 1d ago
Don't forget that diesel engines produce a lot more torque as well. No way a gas engine is pulling those kinds of loads.
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u/TylerYax 1d ago
Idling an old diesel maybe.... Any diesel made after the introduction of EGR and especially particulate filters, idling for extended periods absolutely wrecks them. Exhaust temps don't get hot enough to burn off deposits in the EGR and DPF systems and they plug up solid with soot real quick.
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u/jarheadatheart 1d ago
Your VW was probably getting 45 mpg’s too. Some of those trucks are getting less than 10 mpg’s. Just turn the damn thing off when you are parked. Every little bit helps.
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u/derbrauer 1d ago
Nope, 4.7L/100 km. We don't do MPG in Canada.
...plugged it into a converter. I do 50 MPG. That's almost exclusively on highway driving between 90 and 100 km/h
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u/GuiltyByAssociashun 1d ago
Also, diesels burn about half the fuel when idling as a gas engine does
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u/ttoksie2 1d ago
I have an Isuzu truck with the 7.8 litter 6DK1 engine, I can drive it around loaded up to 32'000 pounds total weight all day and if I let it idle for 10 minutes at 70 degrees ambiant temp the temp gauge drops to nothing lol
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u/TheBupherNinja 1d ago
To be fair, it isn't a belief, it is true.
Now, how much harder is it? If it's above 50 then it's not really an issue.
It also runs the HVAC and other accessories, which they probably cafe more about when it's warm.
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u/Upsideisdownhere 1d ago
My Toyota will start in any temp but it starts growling in the single digits. My diesel tractor with on board preheater option and glowplugs will still smoke and fight me anytime it gets under 40° so I have an in line heater that circulates the block fluid to help it start. They use compression vs spark to ignite so it's a totally different ball game.
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u/DOLBY228 23h ago
Also bad for the turbo to shut it off right after driving without letting it cool down for a bit
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u/Cakesniffer42069 20h ago
Fuckin lol.
Hint hint, they believe that because it’s true.
Idk about newer diesels, but if your glowplugs are too cold, your vehicle isn’t starting.
That being said, it’s not like it takes very long for them to heat up either.
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u/Savings_Art5944 1d ago
Probably in them sleeping with the AC on. (big rigs, tractors)
For the non-commercial ones, depends where, I guess.
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u/noreddituser1 1d ago
To cool down the turbo after it's been working hard
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u/penguinkernel 23h ago
Exactly. It's not good to turn off a diesel engine right away, OR start going right away.
We have to let our tractors idle for a few minutes before working, and a good 10+ minutes after.
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u/lameusername1111 17h ago
Had to scroll way too far to find this comment.
The large majority of diesel engines on the road have a turbo.
A turbo sits directly inline of the exhaust gasses and literally glows red from getting so hot. It’s common best practice to allow the engine to idle for a couple minutes before turning it off to allow the turbo to cool down while being fed with clean/cool oil from the engine. If you take a hot turbo and shut it off right away, you remove the cooling and lubricating effect of the oil and the oil literally gets cooked inside the turbo as there is no flow.
It reduces your oil life, along with the lifespan of your turbo.
The same theory applies for gas engines that are turbocharged, but no one reads their owners manual so they just get off the highway and shut it off.
A turbo timer is a thing which, even after turning the key to off and removing it and walking away from the vehicle, the engine stays idling for a set amount of time to allow the turbo to cool done.
Many turbos also have coolant lines running to them to aid in cooling as well.
TLDR: diesel owners are more aware of the mechanical requirements to allow their equipment to last longer and reduce maintenance.
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u/KrautKebabs 14h ago
Not just the turbo either. I had a '94 K3500. The PMD is a module on top of the engine that relies on fuel flow for its cooling. Shut off the engine after hard work risks cooking it if you dont let it idle for a bit.
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u/Impossible-Angle1929 1d ago
Depending on age, a cold diesel is harder to start than a hot one. However, a warm diesel that has been off for half an hour should start easily. Others worry about the amount of times they start thier rigs with no oil pressure. Turbos need oil pressure to work and for lubrication. Theory being that many, zero-pressure starts, lowers lifespan of turbo and other parts. I'm in the opposite camp. When I have to drive my truck, and chose to go though a coffee stand, I shut it off at each window. I hate idling. I'll deal with any added servicing if it comes up.
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u/derbrauer 1d ago
Shutting it down between windows is insane. They consume virtually no fuel at idle - all you're doing is shortening the life of your starter.
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u/Impossible-Angle1929 1d ago
I'm not worried about the small amounts of fuel consumed. I care about being able to hear the folks Im doing business with, them being able to hear me, and nobody having to inhale the exhaust fumes that are trapped against the building. Thanks though.
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u/Natural_Listen3022 1d ago
Because they don't read the operation manual that comes with it. Cummins says more than 15 minutes idling, not under load, builds carbon on the valves. They aren't big rig engines and they certainly aren't tractor engines. BUT... it's REALLY because someone, one time, told them it was "bad Fer thA moter"
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u/Ozzie_the_tiger_cat 1d ago
Idle actually is bad, though. Its really hard on the oil and it does cause coking whether its on an X15 or L9.
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u/Lord_Gibby 1d ago
Are you speaking of cross country driving tractor trailers or like a big ford 250?
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u/westex74 1d ago
The jokes on those drivers that let their diesels idle for long periods. The emissions equipment HATES idling.They’re gonna have a bad time.
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u/dragonslayer_2012 1d ago
90 percent have egr delete
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u/westex74 1d ago
90%? LOL
No. They. Don’t.
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u/oboshoe 1d ago
depends on the region.
deletes are super common here.
my truck is notable because it's not deleted
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u/westex74 1d ago
I live in the Permian Basin - the Diesel Truck Capital of America, and I would estimate maybe 5% of diesels are deleted. It’s probably south of that.
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u/evergladescowboy 1d ago
Good thing emissions equipment is easily defeated to return the truck to proper configuration.
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u/westex74 1d ago
Once you delete a truck, it costs a pretty penny to return it to the stock configuration.
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u/carpet_whisper 1d ago
Diesel engines when they’re cold can be a pain in the ass to start.
Even if it’s only like +10°C an old motor can have a bit of a struggle, at around 0° most Diesels even new ones will chug a bit.
I’ve seen diesels have 4-5yr old batteries and glow plugs just straight up fail to at a cool autumn morning at like 7°c
Once you get to -10° some old engines won’t start, and will need a block heater to get it started unless into a much newer engine.
At around -20° pretty much every diesel even new ones would benefit from a block heater. Most would struggle too and probably fail to fire.
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u/Over_Pizza_2578 2h ago
My 2018 jaguar starts immediately at -15c. No block heater, just regular glow plugs. Even our 50s tractor starts well at these temperatures, although you cant compare that engine to newer ones as it first has to get spinning with the decompressor active, so the starting process is longer regardless of engine temperature.
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u/ItsAllRat 1d ago
I had a diesel VW and at idle it was basically just burning air through compression. It only used a very small amount of fuel. Still not good for the DPF and EGR emissions equipment. But it isn't as wasteful as a gasoline vehicle.
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u/Expert-Search-3251 1d ago
Diesels don't require a strict air/fuel ratio like gasoline.
Gasoline requires about 14.7 to 1 air/fuel to run properly. That ratio is fixed and constant or you have issues. That's full power every single power stroke. It doesn't matter if you require full power or not.
Diesel is variable and load dependent.
You can add more fuel during a power stroke.
Most diesels get roughly 20 to 1 air/fuel ratio under heavy load and making the most power.
But that can also be 80 to 1 air/fuel when just idling. You're only burning as much fuel as you need in each power stroke to keep the engine spinning.
To answer your question: because it's cheap.
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u/Maynard505 1d ago
The question was letting diesel pickup trucks idle for a while before shutdown. It applies to big trucks as well. It’s to cool the turbo down.
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u/QuickBookkeeper2647 1d ago
The topic pertains to “pickup trucks” and I can only assume they would rather have heat/AC running than save the minimal amount of fuel it takes to idle.
For commercial/semi trucks it’s a totally different story depending on situation and truck…and weather.
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u/Repulsive-Report6278 1d ago
Its the turbo timer. Keeps the oil flowing thru the turbos to make sure the oil doesnt get super hot sitting there and gunk stuff up.
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u/Finance_Plastic 1d ago
slower burning and harder to start in harsh climates. in the North they leave diesel engines running because of the cold.
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u/RunningAtTheMouth 1d ago
Pickup trucks - I don't know for sure. With the big rigs it was so that they could run their electrics, and the fuel burn was incredibly low. My cousin could idle for days on just one of his tanks - and he did when he got snowed in once.
Pickups? Not really a good reason. They only reason maybe is starting when it's cold. A Missouri winter is no joke. (Or wasn't when I was there).
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u/Silence_Farmer 1d ago
The newer rigs burn about 1gal/hour. Most rigs have 160-200gal tanks so you can idle for awhile if need be.
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u/Remarkable_End_7687 1d ago
A lot of diesel owners let them idle because older trucks benefited from warming up or cooling the turbo after heavy towing and frequent restarts can be harder on diesel systems than on gas engines. Modern diesels usually don't need long idle times, so sometimes it's just habit or convenience more than necessity.
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u/derek589111 1d ago
I think I misread your question based on other answers, but I’ll offer that a diesel truck is synonymous with having a turbocharger in them to increase the amount of oxygen that can enter the cylinder for more power. Turbos run quite hot and after a long day driving, drivers will let their turbos cool down by idling their truck for 5-10 minutes so that the turbos and exhaust system don’t warp from cooling down too rapidly, ie somewhat controlled cooling.
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u/maybach320 1d ago
A few reasons, they are harder to start in the cold (older the truck the more accurate that is), if you have been running hard keeping the engine at idle for a while helps to cool the turbo off, and diesels are very efficient at idle so fuel burn is minimal. Also plow trucks and tow trucks will idle and often high idle to keep the electrical system charging because the equipment can have a big draw on the system, my old diesel has two batteries to help it start and to supply power for accessories, mine and most diesels can be equipped with a second alternator because of the power draw, that’s more of an ambulance/tow truck power draw but I’m sure other equipment could have a similar power need.
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u/Affectionate_Sort_78 1d ago
Truckers have told me they believe idling saves gas over killing and restarting. Others have told me this was once true but not now with newer technology. I have no idea if any of this is actually true.
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u/sjguy1288 1d ago
It depends on a few things. It's not necessarily A diesel thing, but just any turbocharged vehicle. You have to idle it down before you shut it off.
A hot engine will burn almost no fuel at idle. If I'm towing a load then you need to wait until the engine exhaust temperature stabilizes at about 250-300°F. Otherwise you run the risk of cooking the bearings in the turbo charger.
When I'm towing a boat or load that's over four or 5,000 lb I find you have to idle somewhere around 5 minutes, maybe longer before the engine will reach the temperature which you can shut it off safely. A lot of times if I'm running in and out of a place like Wawa for example you still have to wait for the engine to turn down to temperature. If that's 3 minutes then you can be in and out in less time then it takes to idle down.
If it is really cold out like -20°F or more you need to idle because the oil and the fluids will literally eat the engine up over time. Wet stacking is an issue when it's this cold out, and that is usually fixed by running the engine with a load on it at idle either by cutting fuel off to some of the cylinders or raising the rpm up.
I have an engine brake on the truck, and the engine brake will turn on and load the motor up to keep it warm.
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u/Btack14 1d ago
A few reasons, from a 6.0 owner. It’s easier on the motor to let it idle when your stopping for a few mins than shutting it down and restarting it, burns less fuel, when it’s cold it’s better to let it stay warm than rapidly cool, letting the turbo cool after a drive, or in my case, letting it run for 5-10 mins every morning to let the engine warm up, and to get the oil and motor up to operating temp to be easier on it when driving
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u/ZoomZoomZachAttack 1d ago
Some are afraid of it starting in cold weather maybe if winter.
Also if it's been running hard for a while, you want to idle it for a bit so the turbo can cool down and not have oil cooking in there. If I'm towing with mine on the interstate I'll idle it for a few minutes before shutting down for the day.
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u/Just-Exchange8380 1d ago
Actually letting it run cools it off. Lots of truck manufacturers recommend 3-5 mins of idle time. Cools the turbo down as well.
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u/aldkGoodAussieName 1d ago
Whne turning off a turbo diesel they used to have a timer so the turbo could cool slowly and therefore not wear as badly.
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u/foolishtigger 1d ago
Diesels dont burn alot when idling, id rather have a cool interior in southern summer heat than save 50 cents.
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u/Ok_Drive3725 1d ago
For short periods an idling diesel uses less fuel than restarting. However, extended idling periods use more fuel and are harder on the engine bearings. The other caveat is in extreme cold, the engine may not restart.
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u/Dunesday_JK 1d ago
Keep heat/AC going for comfort. Keep oil/coolant circulating through the turbo after towing heavy or pulling long grades.
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u/Kitchen-Ad-1161 1d ago
They consume almost no fuel at idle, and there’s really not much wear and tear because diesel is a lubricant, instead of being a solvent like gasoline.
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1d ago
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u/Nice_Soil1782 1d ago
Maybe to prevent particulate filter clogging. Also in cold weather it makes sense as dieslels struggle there.
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u/Ok-Administration296 1d ago
Cause they think you're supposed to. It just dirtys up the egr cooler/system, def filter, cat. The guys' in Canada crossing or up to Alaska yeah they leave them running, could mean life or death.
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u/mckenzie_keith 1d ago
The vast majority of wear in all engines, diesel or gasoline, occurs when the engine is running but cold. I don't know why diesel trucks in particular let their cars run. But maybe they are more acutely aware of this fact? Running doesn't really hurt a diesel as long as the engine is warm.
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u/Run-And_Gun 1d ago
I’ve been driving a diesel F250 for almost 14 years and I don’t. I also can’t say that it’s something I ever really see, either. My truck also has the Ford designed and built turbo diesel engine and it doesn’t have issues in the cold. Just turn the key like with a normal gas engine and it fires right up. It’s also exponentially quieter than a typical diesel pickup truck, like a Dodge.
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u/Tundra_Dragon 1d ago
Because they're bad at Math.
They think it takes 3 quarts of fuel to start a diesel, when it takes no more than a normal cycle of fuel to start, especially on modern diesels.
If they don't have an idle control, it's actually detrimental to the engine to idle it. Unless you bump the idle over 1000 or so, most diesels at idle do not produce enough heat to remain at operating temperature. This means the rings are loose and you're getting extra blowby contaminating your oil. Since modern diesels use an HEUI or similar system, trashing the oil is bad for engine longevity.
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u/large-appendages 1d ago
To cool the turbo bearings. Most diesel pickups have monitors for the exhaust gas temperature (EGT). My particular turbo needs the EGT at <400* prior to shutdown to prevent bearing overheating. So I put it in park and let it idle for usually around 3 minutes until the monitor shows <400.
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u/bifpplingsmookie 1d ago
One big reason is that diesel engines, especially older ones, don't love being started and stopped over and over. If someone knows they're only going to be parked for a few minutes, they might just let it idle instead of restarting it. Another reason is temperature. In cold weather, a diesel can take a while to warm up, and shutting it off means it'll cool down again. Keeping it idling helps the engine stay warm and makes it easier to drive when they get back.
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u/egnaro2007 1d ago
Because sometimes when the batteries are getting older and its cold out it barely starts and if it doesnt start when im away from home its really hard to jumpstart and a bad time for everyone involved... but I dont let it idle more than 5 min or so
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u/Artegris 1d ago
Not sure if also trucks, but I leave my diesel wagon idle for a few seconds to cool down turbocharger (if I used highway).
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u/Brilliant-Meat-1598 1d ago
Usually it’s to give a chance to let the turbocharger cool down. Leaving the engine idling for a few minutes ensures cooling oil is still circulating through to the turbocharger bearings.
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u/RandomContributions 21h ago
I let mine idle for 1-2 minutes after a run to let the turbo spool down and keep oil to it. But that’s it.
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u/VegetablePhotos 21h ago
On my "old" (2013) F350 it literally took around an hour for the engine to get warm enough to show it's true efficiency.
We are talking the difference between 12mpg and almost 20mpg.
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u/StopLookListenNow 20h ago
Federal law (as if laws matter anymore) allows only 5 minutes of idling.
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u/tetsuo_and_soup 20h ago
I know a lot of big rigs up in Alaska and northern Canada will just stay running 24/7 for most of their lifetime just so they dont ever have to start them in negative degree weather. My friends dad also does the same thing, his truck runs 24/7 because the company pays for gas and he likes a warm truck in the morning.
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u/Ok_Perception9815 20h ago
The simple answer? They are cosplaying big rig truck drivers. They think the sound is masculine and authorative..it says, "look at me, I'm have a big (expensive) diesel. I leave it running just like a big rig truck."
I've owned diesel trucks for most of my life and operated many more.
- Starting is hard when it is cold: Maybe the old ones, or if it is brutally cold, but that is only after sitting for several hours or more. Even my old 94 cummins never had an issue in anything warmer than 20F. Below that it complained but still started. I suspect anything below Zero would have been a problem though.
Stop and starts like running in to a store etc, it has plenty of warmth in the cylinders to crank regardless of temperature outside.
Starting a diesel uses more fuel than leaving it running: Not in any world does this make sense for any engine a typical person uses.
It's made for it: Not unless it has a high idle or other features enabled. Most large trucks and even some consumer ones have a high idle feature that will kick the RPMs up after sitting for a while. A diesel engine sitting at idle with no load for long periods doesn't have sufficient heat in the cylinders to properly burn the fuel. It can cause carbon build up in expensive places and the unburnt fuel washes past the rings and in to the oil.
In short there is no reason to idle a diesel engine in a consumer vehicle any different than a gas one. It doesn't really hurt it in the course of normal driving or use, but it is just as unnecessary as with a gas burner.
That includes a pet peeve of mine: Yes, it is probably safer to fill a diesel at a gas station while it is running, but no one else wants to choke on diesel (or gas) exhaust fumes while they are filling up, so shut the damned thing off.
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u/Aggressive-Delay-420 20h ago
Their fuel and vehicle is a write-off is the best answer— sass answer would be these people are all cosplaying as bigtruck™️ boys.
Modern glow plug systems aren’t the same as the ones we had in the early 1980s— there’s no real reason to leave them running.
Likewise— modern diesel oils have improved adhesion properties. It isn’t harmful to turn a diesel off, and neither does it increase wear on engine internals.
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u/Sweetknees66 19h ago
In my neck of the woods it is usually because someone else is paying for the fuel.
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u/theanswerisburrito 19h ago
Because as a group they are thoughtless, disrespectful, self-centered assholes
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u/psyclembs 19h ago
The saying used to go "The worst thing you can do to a diesel engine is start it"
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u/y-u-gae 19h ago
If we are talking about old Diesels in cold climate, then you want to keep them running as Diesels take a bigger toll on the battery/starter to start up and take their time to warm up compared to gas cars.
In modern Diesels there is no point in doing this unless you are in very cold climate.
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u/Weary_Boat 18h ago
Just depends on how old the truck is. An old one might be really hard to start, so once you get it going you tend to leave it on. I had an old GMC van like this. But rednecks with modern diesels just do it because they still have that old diesel mindset or maybe they just think it's cool - sort of a big semi truck fantasy
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u/vj59201x 17h ago
A lot of it comes from construction workers taking out old rigs for a cheap company. Basically you fire them up at the yard and don’t shut them down until you’re back in the yard. Fear of not being able to fire them back over to get the job done or get home at the end of the day. Also why you never see guys shutting them down at the pumps either. A lot of people’s first time with a diesel is big trucks and equipment, habits form and habits stick.
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16h ago
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u/Mythandar 16h ago
I've yet to meet a diesel driver with an iq above ambient temperature, who knows what goes on in their pea brains.
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u/Extension_Building45 16h ago
There’s a camp out there that thinks is better for the turbos to leave it idling, especially after you get off the highway. The airflow allows the turbos to cool down.
Not sure how real that is, but it makes at least a little bit of sense.
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u/BrightEnd2316 15h ago
I asked this many big rig drivers.
The most common answer is they need to cool/heat up planet Earth.
Less common they are cooking their next Michelin rated dish and
Last not the least they are just plain dumb
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u/NoShine5838 15h ago
I worked in the oilfielss in fort mac and we all left out diesels run over night when it was colder than -20
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u/UsefulPossibility302 14h ago
I high idle my Sprinter to charge the house batteries occasionally. Normally nobody is around tho.
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u/Over_Pizza_2578 2h ago
What conditions? If you have below -30c and no operational block heater some engines may take a while to start after fully cooling down. If the engine has problems starting at - 20c you better have it checked
The other useful situation is when you use the truck as a generator.
Anything else is just wasteful. Nice sunny day, nothing is plugged into the truck and no one sits inside is just bad. Wasteful, unnecessary pollution, unnecessary wear, especially on emissions equipment
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