Calling all mechanics...

At what point does ideling your car use more gas than turning it off?

Do I save gas by turning the car off or letting it idle for 10 minutes?


Re: Calling all machanics…

I read somewhere (it was years ago) when cars had carbs on them instead of fuel injected, that 5 minutes of idling used as much gas as driving 1 mile. Don’t know if that is still true or not.

Re: Calling all machanics…

If you are sitting in the car, and the engine is idling, know that you are getting negative gas mileage. You are using fuel without going anywhere. Many people believe it takes more fuel to start a car than it does to sit idle. How this belief got started is unknown, but it probably has to do with the fact that in many situations, you will expend more energy to reach a level where you can coast or idle. Fuel is used only in the initial ignition of an engine that first second. Anything after that is part of the engine idling. The number varies, but if you are idling the car anywhere from 30 seconds to a minute, you are burning more fuel than is used for the initial ignition. Again, the time range is dependent on the size of the engine. Four-cylinder engines would be closer to 30 seconds, and eight cylinder engines would be closer to the one-minute limit. That means if you are going to sit still for more than a minute, it uses less fuel just to restart the engine.

Re: Calling all machanics…

Shut if off if you are going to be more than a minute or two, provided the engine, electrical and fuel systems are in good condition.

If you are on a delivery, or picking up someone at their house and your car tends to need to be cranked a bit before it fires up it is beter to leave it running just in case. Cops do not like people to shut off their engine if they are stopped for a minor infraction in older vehicles because if it fails to start, they are stuck waiting with the car and driver for a tow.

I dont know how much of this is true, but I have heard that gas engines don’t like to just sit and idle for long periods of time. Diesel engines on the other hand would rather idle than be turned off and restarted, and use practically no fuel when they are just idling (thats how truckers and EMT/Fire people can leave theirs running all the time).

I’m sure you were referring to a vehicle with a gas engine, but I thought i would throw that info out there. Also, like Rick said, if you have an issue such as a weak battery, let it idle. One other point to throw out there: at what point does the wear and tear of shutting it down and restarting it frequently become more costly than the fuel you are trying to save in the process?

Anytime the engine is OFF,you save gas.On a warmed up engine,if you shut off and then in a few minutes turn back on,it will not use alot of gas to warm back up,as say a cold engine.Still uses up fuel though,but minimal.

Alot of the old stories were true for older cars,Yet the new cars today run high pressure efi fuel systems that are pretty much well tuned so even when idling,they sip gas and don’t waste it like the old carburetor cars that would just keep flowing fuel into the intake manifold.

Yet when idling very long times,you waste gas.If you are delivering,keep the car running,unless your car is way out of tune and is burning gas at a high rate.The reason for this is starter life and battery.They take alot of power to turn the engine over.Turning a car on and off each time you stop can foul the plugs,wear our the starter,the solenoid,the ignition switch and or drain the battery some.

When you first start a cold engine,it has to idle higher and use more fuel until it warms up.It runs in open loop,until it warms up,becomes closed loop,and then relies on the oxygen sensor for adjustments.

Thats why some cars with vacuum leaks or efi problems around the intake or engine,run better while the car is cold (in open loop)but warming up,its running on a fixed program with extra fuel coming in.It is ignoring the 02 sensors until it reaches a desired temp.

It makes up for the problems until the 02 sensors take over (in closed loop)and then if there is a problem,it idles like garbage,shakes,stalls and wastes gas,because the 02 is getting some crazy readings across the board and trying to tell the cars computer how to compensate at the fuel system.