Car Advice

Myth-busting: You use more fuel at start-up than idling?

This is one that divides opinion even in the face of reason; do you use more fuel at start up than when you idle your car?

THIS IS A FAIRLY simple one to bust but it’s surprising how many people believe that turning your car on will use more fuel than when you’re idling. And this relates to stop-start system which some people will tell you are dangerous and don’t actually save much fuel.

Stop-start systems we’ve dealt with before and their usefulness, depending on what your commute is, can be measured in a few mills of fuel to, over the course of a year, a litre or more. Some systems start up faster than others and so, yes, some can be deemed ‘dangerous’ with that sensation of feeling stranded if you’ve moved into an intersection and the cars stopped and then takes an age to start-up…but that’s not for this article.

In the old days turning on a vehicle did use more fuel than if it was idling for a minute or two once warm because old cars had carburettors. The carbie was designed to get the fuel-air mixture just right and it works very well but, a cold start up generally required the driver to adjust the choke which controlled the amount of air that flowed in, so, first thing in the morning you’d pull the choke out for a fuel-rich start-up and thus burn more fuel than when the engine was up to temperature the choke could be pushed back in and the vehicle would then return to its regulated fuel to air ratio. Until you hit the throttle when more air would be sucked in along with more fuel causing the vehicle to accelerate.

Before the tech-heads get up and arms the aim of this article is not to explain the intricate details of how a carburettor works because most people reading this would fall asleep. It’s just to explain that an engine with a carburettor did use more fuel at start-up than at idle…

But then along came fuel injection (in part to save fuel and reduce emissions via catalytic convertors – careful control of fuel to air was required for them to do their job) and ended that dead. See, fuel injection is an electronically controlled system that regulates the flow of air to fuel so that the right mixture is delivered correctly from the moment you turn the key or press the starter button on your car.

I stumbled across the video below which was produced by The Massachusetts Department of Energy Resources. It does a good job of busting this myth.

If you know you’re going to be parked up somewhere waiting for more than 30 seconds, then even with the efficiency provided by fuel injection, in that you’re not burning more fuel than is necessary, it’s a good idea to switch off your engine.


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Azmodan
Azmodan
2 years ago

My wife’s Mazda 3 stop start saves a lot of fuel, at least 1litre/100km in Sydney’s appalling traffic. Wish the LS3 engines in the Commodre had start-stop and cylinder deactivation.

Alan
Alan
2 years ago

My PRIUS Gen4 has STOP/START. It now feels really strange when I’m in another car and the engine keeps just running when we stop.

Rye an
Rye an
2 years ago

On my old cars you had to pull the choke OUT for a cold start.
In any case, facts don’t hold much sway.
I give up.

Brad Dickens
Brad Dickens
2 years ago
Reply to  Rye an

That’s what I thought

PracticalMotoring
2 years ago
Reply to  Rye an

Yep, I arsed it about. Apologies but I was only little when choke cars were around 😉 – Isaac

sagemike1
sagemike1
2 years ago

Great article about a very smart application of engineering technology. It has some clear benefits – just takes some getting used to!

PracticalMotoring
2 years ago
Reply to  sagemike1

Sure does. Doesn’t help that some systems are great and others a little too slow to start up. – Isaac

Isaac Bober

Isaac Bober