Have you ever been sitting in your turned-off car in a gas station parking lot and you hear the rumbles of a nearby diesel truck? They’ll park the car, and seemingly forget to turn it off as they leave. Is this just a case of forgetfulness, or is something else going on?
Diesel engines prefer to stay running instead of constantly getting turned off and on. In addition, the fuel has a higher flash point and it’s more difficult to combust. This means that it’s a safer fuel around sparks that would otherwise explode gasoline. With big rigs, the diesel engines were built to run 24/7, so it’s never an issue when they’re idling.
You want to know why people leave their diesel trucks running, and I don’t blame you. I have some experience with this topic, so I want to give you a few common reasons. Spoiler alert: it has nothing to do with forgetfulness.
When Do Diesel Trucks Get Left On?
I’ve lived and visited plenty of areas that had more pickup owners than car owners, so I know a lot about this topic. The sound of diesel running became almost a background soundtrack for a lot of my life.
If you’re not familiar, there are plenty of times when people leave their diesel trucks idling. This goes beyond just idling for a minute before leaving for work in the morning — diesel owners might leave their trucks running while they pump their gas, run into a store, or have a conversation nearby.
If you didn’t know any better, you would probably assume that the driver was forgetful and simply left their truck without realizing it was still running. That’s not the case.
In fact, a lot of lifelong diesel truck owners will frequently leave their trucks running as a habit.
The most popular case in my experience is a truck driver leaving their truck running while they’re in a gas station parking lot. They’ll park, leave their truck running, run in, get some food and coffee, then come back out and leave.
This is a confusing idea to a lot of people. Let me explain what’s going on here.
Why Do People Leave Diesel Trucks Running?
Let’s get right into the answer. The following are a few reasons why people leave their diesel trucks running at certain times.
Diesel Vehicles Can Be Stubborn When it’s Cold
A cold diesel engine is a lot harder to start than a cold gas-powered engine. If you wait in a Home Depot parking lot in the Northeast during winter, you’ll hear the constant sound of trucks turning over for a while before finally starting.
This is just something to expect when you own a diesel, and it’s not a big deal.
In order to avoid the time wasted trying to get a diesel started, the truck owner will just leave their truck running. Of course, the engine keeps the temperature regulated as long as the truck is running unless the thermostat is stuck of course.
Diesel Starters Use More Electricity
Another difference in starting a diesel truck is that added electrical requirement. Turning over a diesel engine takes more power than a gas-powered engine. As such, the starter needs to put a bigger toll on the battery.
If battery life is dwindling, then a diesel simply won’t start. However, it takes a lot less battery power to keep a truck idling.
The simple solution here is to swap out the battery, but idling is a fine fix until the truck owner gets around to doing the swap.
Keeping the Cabin Comfy
When a truck is turned on, it keeps the HVAC running inside the cab. Who doesn’t like to sit in a comfortable cabin with an ideal temperature whenever they have the option? Some diesel owners will keep their truck idling for that simple reason.
A vehicle’s HVAC system works a lot better when the vehicle is running. There are options to run your AC even when the engine is off by putting the car in some sort of Accessory mode. In this case, energy is being siphoned from the battery without the ability to recharge.
An On/ Off Cycle Isn’t Great for Engines
The theory is that car engines don’t like to be turned on and off repeatedly. Since the startup process for diesel vehicles is a little more involved, it supposedly does even more damage to the engine over time.
People are pretty split about this idea — other people will say that engines are built to turn on and off easily.
At any rate, some diesel truck owners will just keep the truck running as a means of minimizing the on/ off cycle for their engine.
Keeps the Engine at Peak Temperature
Every car engine has a temperature range where it can achieve peak performance. In fact, auto manufacturers design a whole coolant system just to keep a running engine within this temperature range.
Whenever your car is turned off, the engine will cool off and typically dip below the “peak performance” temperature range. This means that there will be another performance ramp-up once the truck starts. Until then, you’ll be wasting fuel and experience a worsened performance.
By keeping the engine running, you’re keeping the temperature within this perfect range.
Big Trucks are Meant to Run 24/7
If you’re not wondering about pickups, but your question is specifically why people leave their tractor-trailers running, then the answer is a lot easier: these big trucks are designed to run 24/7.
Some semis have refrigerated goods in their trailer. The refrigeration only runs when the truck is on, so they need to keep their vehicle constantly running.
Since truckers often sleep in their vehicle, they need to keep it running in order to keep the cabin HVAC on.
With these vehicles, idling is the least impactful thing on the engine. The bigger stress is getting them started after turning them off.
No Gas Vapors to Blow Up While Refueling
I remember the first time I saw a truck idling when it was refueling. As a kid, I knew very little about diesel vehicles so I ran over to the driver in a panic.
After he was done laughing at me, he explained that diesel vehicles don’t have a risk of blowing up if they idle while refueling. He even stomped on the gas pedal and revved the engine to prove his point.
That memory stuck in my head. Regular fuel has gas vapors that are highly explosive. Even a small spark can set them off, which then ignites the fuel itself and blows up — this is why there are so many “No Smoking” signs near gas pumps.
If you want to see a great example of this, take a look at this video. The creator puts a lit match up to a puddle of gasoline and diesel and shows the differences.
It Keeps the Battery Charging
Your battery works in junction with an alternator. The alternator is a small engine that feeds off of your main engine’s power. As long as your vehicle is running, the alternator is firing.
The alternator’s only job is to constantly recharge your battery. Without this piece of equipment, your car’s battery will die so much sooner, and you’ll be replacing them very often. Since batteries are so heavy and the reinstallation process is so uncomfortable, it’s best to avoid this.
When a truck is idling, the battery is getting a little trickle charge. As long as the big electrical components like stereos and HVAC systems are turned off, the battery will recharge.
Some Issues with Keeping a Diesel Running
Even though a diesel truck can keep running, it doesn’t mean that it has to. In fact, there are some issues to keep in mind regarding idling diesel trucks.
It Wastes Fuel
As gas prices go up each year, you probably want to avoid the gas station as much as possible. When you idle your truck, you’re wasting fuel and therefore wasting money.
Even though your truck isn’t moving, diesel is still being used up. It uses a lot less fuel as compared to driving, but it still adds up.
This also means that your miles per gallon will be a lot worse if you idle between trips.
Hurts the Environment
Not to be “that guy”, but idling also has a negative impact on the environment. When any non-electric car runs, carbon emissions are emitted. This adds to the collection of greenhouse gases that ruin the Earth’s ozone and contribute to global warming.
In an effort to minimize our impact on the environment, it’s a good idea to turn off your car between drives. Even if you’re just parking for a few minutes, that’s a few minutes that you can keep your car off instead.
Annoying for Nearby People
A big complaint you’ll hear about idling diesel cars and pickup trucks is from neighbors and nearby people. A lot of diesel pickups are pretty noisy as they run.
If a driver leaves their truck running for more than a few minutes, people in the area can start to get annoyed and say something.
I actually grew up near someone who would idle their diesel for about 30 minutes every morning before leaving for work. He did it every day until one day someone apparently left a pretty threatening note on his truck.
I’m not suggesting that you change your way of life just to appease your neighbors, but turning off your truck might be a good way to avoid a potential problem in the neighborhood.
Although it isn’t the best idea to keep a diesel running, there’s nothing overtly wrong with it. As you just saw, there are a number of reasons why a driver might leave their diesel truck running at different times. For more of your car questions answered, be sure to explore the rest of my site. I also have a handy list of car products that you might benefit from.