Troubleshooter: How long should you warm up your vehicle?

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The leaves have fallen, the first frost has covered the land, and along has come the great seasonal question: how long do we need to warm up our engines before heading out to challenge winter roads? When the windshield and other glass panels are frosted enough to prevent proper visibility, it’s a no-brainer — you idle the vehicle long enough to clear all the windows before putting it in gear. But what’s the rule of thumb when vision is already crystal-clear?

Of course, unnecessary idling is something we’ve all been asked not to do. Internal-combustion engines in any type of road vehicle all have one thing in common: when they’re idling, their emission control systems are turned off.

A silky-smooth, quiet idle is a very unnatural condition for a gasoline or diesel engine. Ignition timing and fuel injection rates are altered to produce the smoothness everyone wants, but emission controls like exhaust-gas-recirculation and leaner fuel mixes conflict with this mission and are simply disabled by onboard computer controls. And no matter how fuel-efficient your vehicle may be, it gets zero kilometres per litre of fuel when it’s idling in Park. Modern engines can use 2-4 litres of fuel per hour of idling.

The biggest risk to an engine is putting it under load by engaging the transmission and moving off before all its oil has a chance to circulate to all of the lubricated components. This is when wear can accelerate, sowing the seeds of premature issues. Fortunately, even at the coldest outdoor temps, most passenger- and light-truck engines’ full oil pressure and flow is realized within a minute of startup.

An engine can reach normal operating temperatures much faster under load than just at idle. Reaching that point brings welcome heat into the cabin and allows the engine controls to lean out the fuel mix quicker, meaning better fuel economy.

But it’s not just the engine oil you have to worry about on cold starts and take-offs. Transmission and power-steering fluids have to be up to pressure and flowing freely to avert premature wear and performance problems. Depending on the types of systems involved, the time and temperature factors can differ.

As a rule of thumb, wait until the fast idle settles down to its normal engine speed (usually around 1,000 rpm for most engines). Once you do take off, start with gentle acceleration (no Andretti moves), then head safely on to your winter commute without delay.

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