How Long Should a Vehicle Idle?
Warm up (while parked) for just 30 seconds
The overall stationary warm-up period is just 30 seconds, even in winter. The U.S. Dept. of Energy states it best: "Minimize idling your car to warm it up. Most manufacturers recommend driving off gently after about 30 seconds. The engine will warm up faster being driven, which will allow the heat to turn on sooner, decrease your fuel costs, and reduce emissions." EXCEPTIONS: do not drive unless defrosting is adequate; in temperatures below 10 degrees F (-12 degrees C) warm up while parked for one to three minutes (due to thickened motor oil); for infants and frail elderly passengers, allow up to several minutes.
Drive gently to continue the warm up
Driving gently to continue warming up places virtually the same load on a cold engine that is idling, yet allowing a gain in fuel economy. But more importantly non-engine moving parts like the transmission, bearings, catalytic converter, steering, and tires warm up with the engine when driving; they will not do so when idling. Note for vehicles equipped with a temperature gauge or light indicator: this indicates the engine is warming up. It will go off as soon as the engine reaches normal operating temperature. This does not mean, however, that a vehicle must remain stationary during this period. It just means not to exceed warm up RPMs; driving gently will allow RPMs to stay low until warm up is complete.
How long should a warmed up vehicle idle?
The Dept. of Energy Argonne National Laboratory concludes: "Idling for more than 10 seconds uses more fuel and emits more CO2 than engine restarting". However, the break-even time to offset any potential incremental maintenance costs to the starter or battery is 30 seconds. So, as a guideline, if you're stopped out of traffic for more than 30 seconds turn off the engine. TIP: Keep blankets or afghans in your vehicle to use when parked in cold weather to help avoid excessive idling.
What about engine wear?
MYTH: "Letting a car idle is necessary for the engine!"
Prolonged idling causes engine wear. According to Oak Ridge National Laboratory Operations Best Practices Guide: Idle Reduction, "Excessive idling can create engine wear and carbon soot buildup in the engine and components" and "Excessive idling can affect the life of engine oil". It also shortens the life of spark plugs and exhaust system. Many auto manufacturers owner's manuals advise limiting warm-up idling, such as 2013 BMW 3 Series, 2015 Ford F-250 Power Stroke diesel, 2015 Ram 1500-2500-3500, and all 2015 Nissans. Excessive idling places vehicles under a Severe Duty maintenance schedule. John German, an automotive advanced technology and efficiency expert with a combined 21 years with Chrysler and American Honda, further explains why engines don't need long stationary warm-ups. NOTE: avoid using remote vehicle starters which encourage excessive idling.
The key: idling behavior
"I will not freeze in a cold car (or sweat in a hot car)!"
Sacrificing a little "idling comfort" has huge benefits for everyone! Avoiding unnecessary idling of one average-sized vehicle for 10 minutes daily on an annual basis means it will not burn around 26 gallons of fuel, reduce CO2 emissions by more than 500 pounds, improve our air quality, and save around $100. MULTIPLY BY MILLIONS OF IDLERS TO REALIZE THE POSITIVE IMPACT.
Warm up (while parked) for 3 to 5 minutes
The overall stationary warm-up period is three to five minutes. In colder weather, more idling time may be necessary, especially for non-garaged vehicles. Block heaters and other idling reduction equipment can reduce cold weather warm-up idling.
Overall idling guidelines
The Oak Ridge National Laboratory Operations Best Practices Guide: Idle Reduction provides medium- and heavy-duty idling guidelines:
Medium-Duty Idling Summary
• Avoid excessive idling (Idling beyond 5-15 minutes is
• Extended idling wastes fuel and causes reduction in fuel
• Excessive idling can create engine wear and carbon soot
buildup in the engine and components
• Idle time at cooldown is only required if vehicle operated
under extended, high power conditions (as recommended in
Heavy-Duty Idling Summary
• Avoid excessive idling (idling beyond 5-15 minutes is excessive)
• Avoid idling for more than 3-5 minutes at start-up
• Extended idling wastes fuel and causes reduction in fuel economy
• Excessive idling can create engine wear and carbon soot buildup in the engine
• Idling produces sulfuric acid which can eat into engine surfaces and
• Idle time at cooldown is only required if vehicle operated under extended, high
power conditions (as recommended in manual)
Engine manufacturers on idling fuel economy and engine wear
• Caterpillar Inc. "How much is idle time costing you" document states, "Turn off trucks that are waiting more than 5 minutes to load or unload." and
"Restrict morning warm-ups to 3 to 5 minutes."
• Caterpillar Global On-Highway representative states, "There is no additional wear when shutting the truck on/off several times a day. There are
benefits in fuel economy and wear/durability when shutting the truck down rather than idling."
• Cummins Inc. MPG Guide states, "...avoid unnecessary engine idling. The vehicle gets its worst mpg when the engine runs and the truck doesn’t
move. Every hour of idle time in a long-haul operation can decrease fuel efficiency by 1%."
• Daimler Trucks North America/Freightliner Trucks: "The belief that idling a diesel engine causes no engine damage is wrong. Idling produces
sulfuric acid, that is absorbed by the lubricating oil, and eats into bearings, rings, valve stems, and engine surfaces."
• Detroit Diesel notes that along with other fuel-efficient habits, it’s critical to educate drivers on the role of shorter idle times. “Optimizing driver
habits alone can improve fuel economy by up to 30%”.
• IC Corporation's engine manual states that "...Excessive idling reduces fuel economy, and may decrease oil life."
• Kenworth Truck Co. representative states, "Starting and stopping the engine is actually easier on the engine than prolonged idling."
• Navistar/International Trucks: "When a truck is stopped and idling, it is achieving zero miles per gallon. In fact, it burns through about a gallon
per hour, decreasing overall fuel economy by 1 percent." "Unnecessary idling increases cost of ownership and also results in excessive engine
wear and additional pollution."
For additional information on medium- and heavy-duty diesel idling, visit the Idle-Free VT Fleets page.