Idle time


#1

About 50 years ago my dad told me that if you’re idling for less than 5 minutes you should not shut the engine off because it would take more gas to restart it than you would burn just idling. In those days, however, most car engines had carburators while most of today’s engines are fuel injected. Does the 5 minute idling rule still apply?


#2

That was true 50 years ago. It no longer is. And you’ve correctly stated the reason. Today’s engines don’t need to “prime” the intake manifold with a rich spray of gas. Today’s engines spray exactly what’s needed and only what’s needed right at the intake ports, right behind the intake valves, and only at the proper time in the specific cylinder’s cycle. They waste no gas before its time.


#3

I would adjust your figure from 5 minutes to 1 minute, excluding relatively short red lights and similar situations.


#4

One minute might be right, but frankly I doubt if it is worth the bother and possible wear on the starter. Other than hybrids cars don’t come with starters that are designed to start the car that often.


#5

It was never true. A properly tuned engine required no additional fuel to start, the only energy draw from using the starter was replenishing the battery charge from the alternator. That is accomplised in seconds. So if you’re going to idle more than a few secondes, carbureted or injected, it’s more efficient to shut the engine off.

It’s a matter of convenience. Idling uses so little fuel, though, that shutting the engine off saves practically nothing.


#6

It’s a matter of convenience. Idling uses so little fuel, though, that shutting the engine off saves practically nothing.

I used to believe that also before I took a car for a drive that featured a fuel consumption readout on the dash. Set to average trip mpg instead of instantanious mpg, I could see the average mpg reading drop at the red lights while I was sitting there.
An idleing car engine burns anywhere from .2 to .8 gallons per hour depending on the size of the engine, AC use etc. Diesels burn considerably less.


#7

You’re obviously right, the running engine does use some quantity of fuel.

I should have said that shutting the engine off for a minute or two saves practically nothing, but as I said in the rest of my post, it does save some fuel.