You’ve probably heard that idling is bad for your car. However, it’s sometimes impossible to avoid letting the engine run idle.
Does keeping your vehicle running while parked lead to issues? If it does cause problems, how long can you leave the engine idling without negative consequences? And what are the effects of extended idling on your car?
Modern cars can idle longer than older cars without issues because of the PCM/ECM programming. It will change timing and fuel trim to help cool the engine, not to mention engaging the fan. Older non-fuel injected vehicles will overheat if they idle too long in some cases.
If you’re in a drive-thru or a traffic jam, don’t sit idling with the car in gear. Bump it up into neutral to prevent the transmission from overheating.
Modern cars can idle longer than older cars without issues because of the PCM/ECM programming. It will change timing and fuel trim to help cool the engine, not to mention engaging the fan. Older non-fuel injected vehicles will overheat if they idle too long in some cases.
– Richard McCuistian, ASE Certified Master Automobile Technician
Idling is affected by the process of combustion. Understanding the basics of internal combustion can help you understand why you shouldn’t let the engine run idle for too long.
Combustion is the process of releasing energy from a mixture of air and fuel. The less fancy term is burning. An internal combustion engine burns a mixture of air and fuel in the combustion chamber inside its cylinder. It converts part of the energy into work.
How the engine achieves combustion varies based on the fuel it uses. Gasoline engines ignite the air-fuel mixture in the combustion chamber with a spark plug. In contrast, diesel engines compress the mixture, relying on pressure to ignite the fuel.
Gasoline and diesel engines usually have at least one fixed cylinder and a moving piston. During combustion, the expanding gases push the piston downward. The piston’s movement rotates the crankshaft, delivering torque to the powertrain and the drive wheels.
After getting pushed down, the piston rises inside the cylinder. It expels the spent exhaust gases through the exhaust valve. The engine repeats this cycle to keep the crankshaft spinning.
Three of the four piston strokes deliver no power at all. During the exhaust, intake, and compression strokes, the crankshaft is driving the piston, which consumes a bit of the power generated by the firing cylinders. On a one-cylinder engine, momentum maintained by the flywheel releases stored energy to drive the piston through the three non-power strokes.
Idle speed refers to how fast the crankshaft rotates when the throttle is closed and the driveline isn’t engaged. It’s measured in revolutions per minute (RPM). The crankshaft’s spinning, driven by the action of the pistons, creates torque that propels the vehicle. As you drive, the combustion process can speed up to help the car accelerate or slow down to decelerate.
When the engine is under load, it’s working to overcome the drivetrain’s resistance and the vehicle’s overall weight to move the car. The powertrain multiplies the engine’s torque to break the inertia of the vehicle sitting still.
An idling engine loses efficiency. The inefficiency shows itself in several ways.
Oil and coolant circulate around the idling engine at a slower rate. They remove less heat, leading to the engine running.
An idling vehicle also has a higher air intake temperature. Usually, fresh air enters the front grille as the car moves forward. Since the vehicle stays still during idling, it doesn’t force as much air as it should. Hot air contains less oxygen. Between the reduced airflow and the hotter air with lower oxygen content, idling engines run rich.
In older engines with high mileage, running idle (or driving) might leak gasoline or diesel past the piston rings.
Degraded engine oil results in the engine running hotter than it should, and fuel-contaminated oil can cause fuel vapors from the crankcase to be drawn into the intake so that the fuel trim is forced to correct for uncommanded fuel delivery. While the cooling system can cool the engine, parts like the radiator fan will wear out faster and break down more often.
Finally, the engine itself can wear out faster if you leave your vehicle idling for too long. The increased heat, reduced efficiency, and running rich strain the engine’s parts.
While idling isn’t ideal, some scenarios might force you to leave your vehicle on idle. Others might seem like a good idea at first.
Getting stuck in traffic is the most common situation. Your car either sits in place or moves slowly for many minutes. You can let the engine idle or turn it off.
If you expect your vehicle to stay immobile for long periods, you can turn the engine off. You can save fuel, reduce engine wear, and improve air quality.
However, refrain from turning the engine off at red lights and stop signs. You must respond quickly to sudden changes in those areas, and it takes time to turn the engine on again.
In newer vehicle models, an auto stop/start system automatically turns off the engine when the latter traditionally runs idle. When you take your foot off the brake pedal, the system rapidly restarts the engine.
During winter or other periods with cold weather, some drivers let their vehicle idle for several minutes after starting the engine or they’ll use the remote start function to warm the vehicle up before they get in it to drive away. The HVAC system draws heat from the engine by way of the heater core if the warm setting is selected to warm the interior cabin.
Unfortunately, a cold engine cannot produce enough heat to quickly or effectively warm your vehicle. If it’s a diesel, it takes even longer to begin to heat the cabin than a gas burner. Idling helps raise the engine’s temperature faster on some vehicles (but typically, driving slowly in a low gear will heat the engine up faster than idling because there are more combustion events per second) providing the HVAC system with the needed heat.
If you want to stay warm during winter without idling your vehicle, wear warm clothing before entering your vehicle. Instead of letting the engine idle, drive your car slowly to help raise the engine temperature.
While idling might help warm the engine, it cannot heat other parts like the tires, suspension, and wheel bearings. Those parts require the vehicle to move. Instead of idling, drive your vehicle slowly to warm these parts alongside the engine.
There are other incorrect ideas about letting the engine idle. Here are some of the common myths and misconceptions about idling:
Leaving your car idling is bad for it. Don’t let your vehicle run idle for more than a few minutes. You will save fuel, keep emissions low, and make parts last longer.
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