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By Jim Kerr

How long should you idle your vehicle before you drive away? There seem to be many opinions out there and there is even legislation in some parts of the country that limit the amount of time you are allowed to idle your vehicle. This usually becomes an issue in the winter, as many Canadians want to climb into a warm vehicle; there are also drivers who idle their vehicles in the summer to cool the interior down with the air conditioning. So how long do you really need to idle a vehicle so it will operate properly?

Tires, wheel bearings, seals, differential and transmission oils and the engine all need to be warmed up before the vehicle operates its best. Tires and transmissions can take several kilometres of driving to warm them up, depending on outside temperatures, so idling the engine doesn’t help. You need to drive the vehicle, but before that, you probably want to warm the engine up a little.

Dramatic changes occur when starting a cold engine: combustion chamber temperatures go from below zero to over 800 C in an eye-blink. Parts expand rapidly with the temperature changes, but not all at the same rate. The engine block stays cold for several minutes, while the cylinder head and pistons warm up much faster. Modern gasket and fastener technology allows parts to expand at different rates, while piston designs help control their expansion. One advantage of warming an engine up is to allow the parts to expand uniformly before placing heavy driving loads on them. This reduces wear and stresses.

Oil also needs to be warmed up. At temperatures near zero, the oil is thick but still pours easily. When temperatures drop to –20 C, the oil pours more like molasses. Winter grade oils such as 5W-20 and 5W-30 viscosity will flow much easier, so they lubricate better at cold temperatures. When outside temperatures drop to –40, as they do in parts of the country, you can leave fingerprints in summer grade oils. Winter grade oils help but still flow very slowly until warmed up. During the warm-up period, parts may not receive the best lubrication so you want to keep engine speeds and loads low.

Okay, so how long does it take to warm up the engine? On a zero degree day, oil will be flowing and you can start driving slowly in 30 seconds to a minute. If you have to get out on the highway or freeway immediately, let it warm up a couple more minutes. At temperatures of –20 or below, you are best to let the engine warm up two to three minutes before driving, but then drive slowly for a few kilometres to let the transmission and other rotating parts warm up too. Usually when temperatures drop below zero, the ability of the heater to clear the windshield of frost determines the warm up time. Once it is clear, you are set to go.

There are some exceptions where you should warm up the vehicle longer. Turbocharged engines require additional lubrication for the high speed bushings inside the turbocharger. The engine temperature should be starting to rise before you start driving so the oil will flow freely to these bushings. Diesel engines also take longer to warm up. Idle speeds on diesel engines are controlled by the amount of fuel injected. At idle, there is very little fuel burned and not much heat produced, so it takes longer to warm up. You may even have to drive for a little while to warm up a diesel.

Block heaters are a good way to keep your engine warm when temperatures drop, but they are not really needed above about –18 C. Even at colder temperatures, plugging in a block heater for more than four hours is a waste of electricity, as the engine won’t get significantly warmer if plugged in longer.

Thermostats are important to getting the engine up to operating temperature quickly. The thermostat stops coolant from circulating through the radiator while the coolant is cold. If even a little coolant bypasses the thermostat because it is sticking or opening too soon, the engine takes much longer to warm up. This causes the computer to inject more fuel in the cold engine, so fuel economy drops and engine wear increases. Most engine wear occurs during cold starts.

Enough of winter: think of summer. It is nice to let the engine idle and the air conditioning cool the interior, but it isn’t necessary. If the engine has been started for 30 seconds, you are ready to go driving.

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