Do You Need to Warm Up Your Car?

Reader Contribution by Richard Backus
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Should I warm up my car on cold mornings? Does doing so help or hurt my car? Does doing so waste gas?

Although you might think warming up your car is best on a cold morning, doing so is a bad idea, and not just because it wastes gas.

Cold engines typically want a rich (more fuel) mixture to run well. More than likely, your car uses electronic fuel injection. If your car’s engine is cold, sensors relay that information to a computer, which signals the fuel injectors to stay open longer, allowing more fuel into the engine to help it run while cold. As the engine warms up, the computer signals the injectors to let in less fuel and everything returns to normal, so to speak. Importantly, the faster your engine warms up, the quicker it assumes its most efficient level of operation.

The problem is, letting your car sit and idle is the slowest way to bring it up to operating temperature because it’s generally sitting in your drive at just above idle speed. And this method of warming up also invites other problems. Modern cars are equipped with a multitude of devices to help them run clean and efficiently, including a catalytic converter (sometimes three of them), a device in the exhaust system that works to oxidize unburned hydrocarbons and reduce carbon monoxide and nitrogen oxide levels in the exhaust stream. A cold engine emits a far higher percentage of unburned hydrocarbons and much higher carbon monoxide levels than a warm engine.

Unfortunately, the average catalytic converter can’t process 100 percent of unburned hydrocarbons even in the best of times. The catalytic converter needs high exhaust temperatures — generally a minimum of 400 to 600 degrees Fahrenheit — just to start functioning. Because of the catalytic reaction, the higher the pollutant input, the higher the operating temperature of the catalytic converter after it starts working, typically about 1,200 to 1,600 degrees. Because a cold engine emits high levels of unburned hydrocarbons, it can cause the converter to run hotter than desired. If this occurs too often, these high temperatures can cause the converter to physically collapse internally, or plug. This doesn’t happen at once, but over time, yet the end effect is the same: poor mileage and significantly dirtier exhaust.

There are a lot of myths concerning the risk of damage if you don’t thoroughly warm up a cold engine, but most of them are just that — myths. Modern engines are built to a much higher standard than engines of 25 years ago, and you can’t compare a 2012 Prius — or even a 2002 Camry — to a 1987 Toyota Tercel. The best bet? Even if it’s just 10 degrees outside, start your car, let it run for 30 to 60 seconds to get all the fluids moving, then drive off gently. Don’t race the engine or accelerate suddenly. Compared to idling in your driveway — where you get zero miles to the gallon — your engine will warm up faster, your exhaust system will get up to temperature faster so the catalytic converter can do its thing, and you’ll use less fuel. Which is what you wanted all along anyhow, right?

Certainly, ambient conditions will rule. If it’s extremely icy or foggy, you might have to let your car warm up a bit more to clear the windshield. And drivers in extremely cold climates will want to give their engines a little more time to warm up, if only because moving off too quickly if it’s, say, 20 degrees below zero outside, can actually damage your engine. If it’s below zero, try to limit your warm up to no more than five minutes before you drive off into the frozen wilderness.

— Richard Backus,
Gas Engine Classics and Motorcycle Classics magazines

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