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Old 09-09-2011, 03:33 PM   #1
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Default how does my car determine how much fuel ive burned? just curious.

so as many of you know ive got the gas guage that reminds me of the front of kitt from knight rider any time im under 1/2 tank. i know its the sensor on the sending unit, i just dont have the $ or the desire really to fix it. but driving home on what *i think* is barley enough gas i wondered, how exactly does it measure how much gas im burning? i know the maf has something to do with it, but on topgear i remember when they tested it on a jag or something it measured the width of the injector spray (it was ungodly inaccurate if i remember correctly)

my car tends to be up to a gallon optimistic when i run it pretty dry. say 5 gallons left or so. i was just wondering how it works, so i could *maybe* gauge where its actually at a little better.

these pics are from labor day when i filled up before coming back to memphis.

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it also makes me wonder how far off the instant mpg is... cause there is no way in hell im getting this good of mileage at 80mph lol
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Old 09-09-2011, 03:37 PM   #2
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It expects X amount of fuel to be sprayed with each injector pulse. If the fuel pressure is off, then that amount is different from what it expects.
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Old 09-09-2011, 03:39 PM   #3
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It expects X amount of fuel to be sprayed with each injector pulse. If the fuel pressure is off, then that amount is different from what it expects.
so in theory, if i cleaned and or replaced my injectors and seals would it make it more accurate? im all for fixing things that will actually improve my mpg, instead of fixing things that tell me how crappy it is lol.
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Old 09-09-2011, 03:41 PM   #4
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Has more to do with fuel pressure. Lets say you get X amount at 48psi, well, at 45psi you get a little less fuel. This will throw off the readings a bit.
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Old 09-09-2011, 03:47 PM   #5
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Has more to do with fuel pressure. Lets say you get X amount at 48psi, well, at 45psi you get a little less fuel. This will throw off the readings a bit.
well what im saying is, im reasonably sure one of my injectors' seal has a small leak, because you can smell gas around it and at idle it almost has a missfire, if that makes any sense. ive never bothered with it though because i figured between that and my blowing muffler the backpressure would have more of an ill effect on the fuel economy...

im jjust wondering if a leak that small would have any effect at all?
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Old 09-09-2011, 03:49 PM   #6
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Every little thing adds up. I suggest fixing it.
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Old 10-02-2011, 12:57 AM   #7
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Fuel pressure has a big effect on the accuracy. I just replaced my fuel pump because my pressure was 38 PSI.

With the old pump, the MPG displayed was always lower than I actually got, and gallons used on the DIC was always higher. It'* because the PCM increases pulse width to compensate for the low pressure, and MPG / fuel used computer is using that pulse width in its calculations. Since the injectors are open longer, the PCM thinks it'* using more gas than it really is.

Now that I have a new pump with normal pressure, it'* close to actual mpg / fuel used, but it does tend to be a little optimistic.
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Old 10-02-2011, 07:26 AM   #8
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That'* the stupidest way to calculate fuel useage. Why not instead go by the capacity of the tank (x gallons) the amount actual fuel in the tank upon fill up (y gallons) and then use the gauge sensor to figure out how much actual fuel has been used?

It seems to me they over-complicated the thing.
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Old 10-02-2011, 01:47 PM   #9
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That'* the stupidest way to calculate fuel useage. Why not instead go by the capacity of the tank (x gallons) the amount actual fuel in the tank upon fill up (y gallons) and then use the gauge sensor to figure out how much actual fuel has been used?

It seems to me they over-complicated the thing.
Well then they can't really measure MPG on the fly.

Plus the fuel level senders are not sensitive enough, nor are they very linear-- Mine sits at full for 100 miles, then starts to fall for the remaining 300 miles.

Nearly every automaker that incorporates a MPG computer calculates it with fuel injector pulse width as the primary variable.
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Old 10-02-2011, 05:54 PM   #10
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they should put a fuel flow sensor on it, how much could it be relative to the cost of the rest of the car
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