Rick's Free Auto Repair Advice

Idling versus restarting

Idling versus restarting your engine — it’s wasteful and costs you more

This question has been around for a long time and there are lots of myths and just plain bad advice about which uses more fuel; idling or restarting your engine. So let’s get to the facts.

Starting a cold engine uses a lot of fuel

When you turn the key to start a cold engine, the vehicle computer looks at these sensors: engine coolant temperature, ambient temperature sensor, the mass airflow sensor or manifold absolute pressure sensor. With the data from those sensors, the computer knows how cold the engine is, how cold the incoming air will be and what the barometric pressure is. It can then calculate how much fuel to add to the air charge.

It takes a lot more fuel to start a cold engine than it does to restart an already warm engine. Why? Because the cold engine “quenches” the combustion flame. In other words, the cold engine acts like a heat sink. Until the cold metal is up to operating temperature, around 200°F, the computer must continually add more fuel to compensate for the energy loss of the cold metal.

How much fuel does an idling engine use?

If the engine is up to full operating temperature, every two minutes of idling consumes as much fuel as takes to drive a mile. If you’re doing deliveries and you let your engine idle, you’ll use a lot more fuel than you would if you shut off the engine and restarted it.

An idling engine isn’t running at peak performance

Engines don’t cool efficiently when idled for long periods. The water pump doesn’t pump as much coolant and the radiator has no ram air to remove heat from the coolant. At low idle speeds, the oil pump doesn’t move as much oil, so the oil accumulates more heat, causing more wear. So overall, idling causes more wear than stopping the engine and restarting.

But what about the wear on the starter from restarting?

Again, if we’re taking about restarting a warm engine, the starter and engine wear is minimal. The engine is already up to operating temperature, so there’s far less friction loss from the oil. They cylinders are already hot so the fuel ignites faster and there’s minimal quench loss. Starting a warm engine causes minimal starter wear because the engine turning resistance is a fraction of what it is compared to cranking when cold.

Just watch a UPS driver do their routes. They ALWAYS shut off the engine and restart when they get back in. They’ve done extensive studies and confirmed that it’s less expensive to stop the engine and restart it, than it is to let it idle

Will you have to replace your starter more often if you restart your engine rather then let it idle

If your engine is in good operating condition, with good spark plugs, a good battery, and fresh oil the added starts will eventually wear out your starter slightly faster than if you idled your engine. But the cost of a new starter will be less than the cost of the wasted gas and the wear and tear on your engine caused by idling.

©, 2021 Rick Muscoplat

Posted on by Rick Muscoplat

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