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How much gas does idle burn?

This is a discussion on How much gas does idle burn? within the Technical / Maintenance forums, part of the General Discussion category; I was wondering how much gas my 95 TBI chevy burns while idling. I have some amps in my truck ...

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    How much gas does idle burn?


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    I was wondering how much gas my 95 TBI chevy burns while idling. I have some amps in my truck so I must keep it running if I want to listen to music more than fifteen or twenty minutes. I guess the only way to know for sure would be to fill up then idle for an hour and see how much it burned. Just wondering if anyone has done this or knows about how many gallons per hour it uses. I might try to figure out one day once I actually have the money for a full tank, haha. Thanks for any help.

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    Re: How much gas does idle burn?

    Fill the truck up, run the truck for 10-15 minutes, then fill it back up again.

    No need to run for an hour. Just the same way they do it to test your heart rate, just see how much is used for a short period and extrapolate from there.


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    Re: How much gas does idle burn?

    I know some larger medium to heavy duty diesel trucks use around 1/2 gallon/hour at no-load idle. For comparison a truck that got 15 MPG running 60 mph on the highway burns 4 gal./hour.

    The 15 minute idle would not give you a very accurate answer simply because the meters at the fuel pump are not accurate enough. 15 minutes of idling would probably only use a few tenths of a gallon and you just couldn't accurately measure that small amount of fuel. I do this type of stuff at work all of the time

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    Re: How much gas does idle burn?

    X2. You need a longer burn to determine accurate measurements. That is why it is better to do a full tank run to compute MPG tha a quick trip to the store. Also, how would you determine you had filled it up to the exact same point each time.
    If you really want to play around, get a 1 gallon container, hook it instead of fuel tank and see how long it runs before 1 gallon is used up, although you need some way to keep that pressurized.

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    Re: How much gas does idle burn?

    It "could" be calculated mathematically, just like newer cars have the on-board computer to do it.

    You'd have to know your idle fuel pressure, and the flow rate of the injectors (x2), and then multiply against their pulsewidth. On my Buick (with 6 injectors), I seem to recall the pulsewidth hovers around 3ms, so I'm guessing it's probably about the same for the TBI.

    Since I know some of the numbers for the Buick, I "think" it would be like
    6 injectors @ 83#/hr * 42 PSI * 3ms * [some conversion factors]

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    Re: How much gas does idle burn?

    0mpg.

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    Re: How much gas does idle burn?

    Quote Originally Posted by grayblazer View Post
    I know some larger medium to heavy duty diesel trucks use around 1/2 gallon/hour at no-load idle. For comparison a truck that got 15 MPG running 60 mph on the highway burns 4 gal./hour.
    I'd also guess 1/2 gal per hour, maybe a little bit more. I have a Scan Gauge II that displays gal/hr as one of it's readouts. Won't work on my 92, but works on the other vehicles we've had/have. All come in around 0.35-0.6 gal/hr, depending on the vehicle and load (AC kicking on ups he fuel usage). This has been on
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    Re: How much gas does idle burn?

    Quote Originally Posted by Quyonmob View Post
    0mpg.
    Yup, and that gives us the phrase "Idle wastes more gas than driving". Which unfortunately confueses way too many people out there into thinking that idle BURNS more gas than driving. To this day I can't convince my wife that she'd burn 4-5 times more gas spending an hour on the freeway than idling in the driveway. Her dad always beat it into her that idling was sucking the tank dry, and to her that means the gas gets used up at a faster rate. I've seen this argued on forums a number of times.

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    Re: How much gas does idle burn?

    It doesn't use much that's 4 sure. It does waste 100% of it though. Why not just buy a new battery and fix your alternator?

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    Re: How much gas does idle burn?

    I would have to argue with some of you. Reason being a diesel can burn at a 50 to 1 ratio. Reason being a diesel can run lean without overheating. This is why the "gas" pedal controls fuel on a diesel, not air. A gas engine "at least newer fuel injected one's" will run a 14.7 to 1 ratio. (both warmed up). So to compare a diesel to a gas engine is not even going to be close.
    Something else nobody has even though about is engine temp, air temp, air pressure, humidity, time of year (winter fuel always burns more), ect... All that being said you can get a summer/winter base line.
    If you leave it run for an hour won't be to bad. If you do it daily, your going to burn alot of gas. But like others have said, at idle you aren't pulling in alot of air, so you don't need alot of fuel. But your engine will stay at a 14.7 or around and about air fuel ratio. (warmed up)


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    Re: How much gas does idle burn?

    Quote Originally Posted by grayblazer View Post
    I know some larger medium to heavy duty diesel trucks use around 1/2 gallon/hour at no-load idle. For comparison a truck that got 15 MPG running 60 mph on the highway burns 4 gal./hour.

    The 15 minute idle would not give you a very accurate answer simply because the meters at the fuel pump are not accurate enough. 15 minutes of idling would probably only use a few tenths of a gallon and you just couldn't accurately measure that small amount of fuel. I do this type of stuff at work all of the time
    I never do that, so i will definitely take your word for it.

    As for measuring how much gas is burned, i wouldn't use the pumps either. I'd just have it measured and poured in with small quantities until it fills up. If i'm gonna do some testing, i might as well get it as exact as possible.


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    Re: How much gas does idle burn?

    Quote Originally Posted by grayblazer View Post
    I know some larger medium to heavy duty diesel trucks use around 1/2 gallon/hour at no-load idle. For comparison a truck that got 15 MPG running 60 mph on the highway burns 4 gal./hour.

    The 15 minute idle would not give you a very accurate answer simply because the meters at the fuel pump are not accurate enough. 15 minutes of idling would probably only use a few tenths of a gallon and you just couldn't accurately measure that small amount of fuel. I do this type of stuff at work all of the time
    I never do that, so i will definitely take your word for it.

    As for measuring how much gas is burned, i wouldn't use the pumps either. I'd just have it measured and poured in with small quantities until it fills up. If i'm gonna do some testing, i might as well get it as exact as possible.


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    Re: How much gas does idle burn?

    Quote Originally Posted by kcb37 View Post
    I would have to argue with some of you. Reason being a diesel can burn at a 50 to 1 ratio. Reason being a diesel can run lean without overheating. This is why the "gas" pedal controls fuel on a diesel, not air. A gas engine "at least newer fuel injected one's" will run a 14.7 to 1 ratio. (both warmed up). So to compare a diesel to a gas engine is not even going to be close.
    Something else nobody has even though about is engine temp, air temp, air pressure, humidity, time of year (winter fuel always burns more), ect... All that being said you can get a summer/winter base line.
    If you leave it run for an hour won't be to bad. If you do it daily, your going to burn alot of gas. But like others have said, at idle you aren't pulling in alot of air, so you don't need alot of fuel. But your engine will stay at a 14.7 or around and about air fuel ratio. (warmed up)
    I don't think you'll find a fuel injected car that idles at 14.7:1 A/F. Most will be in the high 16's or into the 17's. With no load on the engine you can get away with those much leaner mixtures without temp issues.

    My wife's 07 Passat has direct injection (gas engine). Claims are that it can inject fuel right before TDC and run on the small pocket of gases that haven't even had a chance to cover the full piston / head area. Idle A/F ratios supposibly can be in the 60-65:1 range. It's like a little diesel.

    Any you are right, air temp, engine temp, fuel blend, etc etc will all effect the amount of fuel used. However, when we are talking about an hour of idling it's not going to be anything too significant. Maybe you burn 0.83 gallons instead of 0.71. A big different percentage-wise, but you aren't running the tank dry in 43 minutes vs 3 days.

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    Re: How much gas does idle burn?

    At a 60 to 1 ratio it would overheat the engine. I can tell you this much idling my gas 5.7 to my diesel 6.6 it is a world of difference in the amount of fuel used.
    I will admit I have not read enought nor done enough research to really say what a gas engine runs now. I do know that newer one's or properly tuned gas engines can run leaner then 14.7 upwards of 16 from what I have read. But a 60 to 1 will not happen. (Could be that I am way off, but that just seems completely unrealistic from everything I have ever read) Also we are talking about a 95 which would run somewhere around a 14.7 give or take.


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    Re: How much gas does idle burn?

    Quote Originally Posted by kcb37 View Post
    At a 60 to 1 ratio it would overheat the engine. I can tell you this much idling my gas 5.7 to my diesel 6.6 it is a world of difference in the amount of fuel used.
    I will admit I have not read enought nor done enough research to really say what a gas engine runs now. I do know that newer one's or properly tuned gas engines can run leaner then 14.7 upwards of 16 from what I have read. But a 60 to 1 will not happen. (Could be that I am way off, but that just seems completely unrealistic from everything I have ever read) Also we are talking about a 95 which would run somewhere around a 14.7 give or take.
    It's the direct injection that allows extremely lean ratios under some circumstances. Since the timing of the fuel injection is totally independant of the cam timing, you can inject fuel long after the valves are closed. So after the valves shut, and as the piston is nearing TDC a very small amount of fuel can be injected and almost imediatly ignited. Around the spark plug and center of the bore the A/F may be in the teens. But out towards the cylinder walls it's still just air, so your overall A/F is very lean. With virtually no load at idle heat is not an issue, and you aren't wasting fuel to wetting down cylinder and port walls, a problem with regular fuel injection and the low air velocities at lower engine speeds.

    I'm still trying to find a link. There was a great technical paper about this on another forum.

    Although I still agree, a 95 TBI isn't going to be running that lean. 16-17 is still do-able though. I've ran anchient technology Briggs and Kohler engines into the 17's under power. Never mind that it looks like the mufflers will melt off of them at those ratios though!

    By the way, threw the Scan Gauge in my Jeep tonight. Completely ineffiecient 4.0L I6, idles around 0.48-0.50 gal/hr on the read out. I wouldn't guess that a TBI 350 is too much worse than that. If it were it'd be spitting raw fuel out the exhaust in short order.

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