Should you warm up your engine in winter before driving?

Why Warming your engine by idling is not good for it

If you’ve ever been told to warm up your engine on a cold winter morning, you’re not alone. It’s a ritual many of us learned from parents or grandparents, but let me set the record straight: modern vehicles don’t need this kind of warm-up. In fact, idling your car for ten or fifteen minutes to warm up your engine can cause do harm than good. Let’s dive into why this outdated practice no longer applies and what you should do instead.

The Myth of Engine Warm-Up

The advice to warm up your car originates from the days of carbureted engines, which were common before the 1980s. Back then, idling to warm up your engine was essential to ensure proper oil flow and ensure proper fuel vaporization for smooth operation, especially in cold weather. However, times have changed. Today’s vehicles use advanced electronic fuel injection systems that vaporize fuel much better that carbureted engines.

Why Idling to Warm Up Your Car Is a Bad Idea

Those who advocate warming your car by idling rely on two outdated beliefs: 1) Oil doesn’t flow when cold, 2) Metal pistons and piston rings must warm up and expand, or they’ll get damaged, and 3) The air/fuel mixture leans out as soon as the oxygen sensor reaches operating temperature.

1) Modern oil builds enough pressure to operate your engine as soon as the oil pressure light turns off. In addition, oil warms up faster and flows faster when the engine is under load than when idling. Idling warms the coolant, but it’s less effective at warming the oil because the load is so low.

2) Engineers aren’t stupid. Contrary to popular belief, they actually do know about expansion-contraction rates for pistons and piston rings. If cold pistons and piston rings needed to warm up to seal the combustion chamber, a cold engine would never even start. Yet they do. And despite the fact that pistons and pistons rings work best when they’re at full operating temperature, carmakers still recommend driving right away after a cold startup, albeit with a light foot.

3) Heated oxygen sensors do heat up quickly. That part is true. But it’s also misleading. All that means is that the computer goes into closed-loop status much faster on modern engines than on engines from the 80’s. However, the air/fuel mixture is determined by engine temperature and ambient air temperature, and the faster the engine heats up, the faster it will move toward a leaner mixture. It’s a proven fact that engines warm up faster when driven than when idling.

Idling to warm up your engine delays catalytic converter operation and loads the catalytic converter with unburned fuel

The minimum light-off temperature of a catalytic converter—the temperature at which it begins to operate effectively by converting harmful exhaust gases into less harmful emissions—varies depending on the specific type of catalyst material used. However, for most modern catalytic converters:

The typical light-off temperature is around 400°F to 600°F (approximately 200°C to 315°C).

How quickly the converters lights off depends mostly on engine load. Under low engine load, the exhaust gases heat up more slowly, delaying light-off and coating the converter structure with unburned fuel. Higher engine loads by driving generate hotter exhaust gases, helping the catalytic converter reach light-off more quickly.

In other words, if you idle to warm up your engine, your catalytic converter will run hotter once it lights off to burn off the extra fuel. That extra heat can reduce the life of the catalytic converter

Idling a cold engine causes some oil wash and oil dilution, especially on engines equipped with gasoline direct injection

Oil wash and oil dilution cause more engine wear

Contrary to what you might think, idling in the winter is not just unnecessary—it’s harmful. When you idle a cold engine, it runs on a richer fuel mixture for longer periods than if you drive to warm it up. Longer periods of idling with a rich mixture cause some oil to wash off the cylinder walls and piston rings, resulting in more wear. Plus, the unburned fuel and moisture from combustion blowby mix with the crankcase oil, causing oil dilution and reducing its ability to lubricate. So idling to warm up your engine in cold weather actually causes more engine wear. This is especially true of late-model engines with gasoline direct injection.

Here are the cold hard facts about warming up your car in cold weather

• Idling to warm up a modern fuel-injected engine is NOT recommended by any carmaker
• Idling warms up the coolant, but driving is what warms up the oil
• Driving with light acceleration for the first mile or so is the fastest way to warm up the oil and the coolant and the fastest way to get heat in the cabin
• You have enough oil pressure to prevent engine wear as soon as the oil pressure light goes out.
• Idling actually degrades your oil faster and causes EXTRA wear due to fuel dilution and soot production. This is especially true on gasoline direct injection engines
• Idling is not necessary, and it’s wasteful

Why idling to warm up your car is bad for the environment and bad for your engine

Idling to warm up your engine pollutes the environment— big time

• Idling a cold engine for 30 seconds produces more pollution than a warm engine produces in 100 miles of driving (proven with actual testing). So idling a cold engine is really bad for the environment.
• Idling also doesn’t allow the catalytic converter to reach “light-off” temperature, so none of that engine generated pollution gets treated by the catalytic converter; it goes right out the tail pipe.

Warming your engine in winter wastes gasoline — a lot more than driving it to warm it up

Most late-model vehicles are built with smaller engines that use turbochargers and direct fuel injection. Because they are smaller and hold less coolant, they warm up faster. Idling these engines until the cabin is warm just wastes fuel and damages your engine.

Idling degrades your oil much faster

If you read your owner’s maintenance guide, you’ll see that warming your engine in winter is regarded as severe service, requiring more frequent oil changes. Here’s why.

All engines produce blow-by gasses that escape past the piston rings and into the crankcase. Blow-by from a cold start contains raw fuel that didn’t get burned, oil-wash from the cylinders, water (a by-product of combustion), and soot.

Since it takes longer to warm up an engine by idling than it does by driving, you add more of those contaminants to your oil by idling. Exhaust gases, fuel, and water form acids in your crankcase that rapidly degrade the anti-corrosion additives in your oil. Plus, at idling speeds, your oil isn’t filtered as quickly as when you’re driving. So less soot gets filtered out.

Idling warms the coolant but does a poor job of warming the oil

Scientific studies show coolant is warmed by combustion heat, while oil is warmed in the bearings. Idling produces little to no load on the bearings, so the bearings generate little heat during idling. See this article on oil flow in cold weather

Cold start advice from the Energy and Transportation Sciences Division of Oak Ridge National Laboratories

“Light-duty vehicles are ready to be driven at startup, and neither medium nor heavy-duty vehicles need long warm-up periods. According to many original equipment manufacturers (OEMs), idling can actually be damaging to an engine and vehicle components. This is because idling can produce sulfuric acid, which can eat away at the engine and other components.

Additionally, idling results in lower in-cylinder temperatures combustion, which can produce additional soot, create buildup in the engine, and cause unnecessary engine wear. Based on the potential for engine damage, drivers should avoid idling whenever possible. Generally speaking, more than 5 minutes of idling is excessive and should be avoided.” — Summary of OEM Idling Recommendations from Vehicle Owner’s Manuals
Energy and Transportation Sciences Division June 2016 prepared for the U.S. Department of Energy

What does your owner’s manual say about warming your engine by idling?

These carmaker comments were taken from the Oak Ridge National Laboratories Summary of OEM Idling Recommendations from Vehicle Owner’s Manuals.

BMW: Drive away without delay. Do not wait for the engine to warm up while the vehicle remains stationary. Switch off the engine during longer stops, e.g., at traffic lights, at railroad crossings, or in traffic congestion. Drive at moderate speed for a short distance first, especially in cold weather.

Cadillac: Avoid idling the engine for long periods of time.

Chevrolet: Do not warm up the car. Even on the coldest mornings, the vehicle is ready to go in just 30 seconds. In fact, vehicles reach optimum operating temperatures faster when driven instead of idling. Operate the engine and transmission gently to allow the oil to warm up and lubricate all moving parts.

Ford: Don’t idle for more than 30 seconds. Today’s engines don’t need to be warmed up. Turn the engine off in non-traffic situations, such as at the bank and fast food drive-up windows when idling for more than 30 seconds.

Honda: Avoid excess idling.

Hyundai: Remember, your vehicle does not require an extended warm-up. After the engine has started, allow the engine to run for 10 to 20 seconds before placing the vehicle in gear. In very cold weather, however, give your engine a slightly longer warm-up period.

Infiniti: Allow the engine to idle for at least 30 seconds after starting. Do not race the engine while warming it up. Drive at moderate speed for a short distance first, especially in cold weather.

Lincoln: After idling for a few seconds, release the parking brake, apply the brake, shift into gear, and drive.

Mazda: After idling for a few seconds, release the parking brake, apply the brake, shift into gear, and drive.

Mercedes: Do not warm up the engine while the vehicle is stationary.

Nissan: Allow the engine to idle for at least 30 seconds after starting. Do not race the engine while warming it up. Drive at moderate speed for a short distance first, especially in cold weather.

Toyota: Avoid lengthy warmup idling. Once the engine is running smoothly, begin driving—but gently.

VW: Do not let your vehicle warm up while standing; instead, start driving right away after making sure that you have good visibility through all windows.

* Unless you’re still driving a carbureted vehicle. Then warm-up is important. See this article on why warming up a carbureted engine is important

©, 2018 Rick Muscoplat