C
CharlesBrown
Guest
How does the car determine MPG? Does it meter the fuel or is the MPG panel gage just hooked to a vacuum line...what? :confused
Hib Halverson
Technical Writer for Internet & Print Media
Neither.
It derives fuel flow by using the commanded injector pulse width and "knowing" what volume of fuel the injector can flow. Once it knows the fuel flow, using VSS data, it can compute mpg.
The mpg readout is accurate only for stock injectors as stock fuel pressure.
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And, I think, in closed loop programming only - as my experience indicated that it didn't do a very good job calculating MPG if I used a lot of full throttle applications.
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CharlesBrown
Guest
So, you're saying the volume of fuel used to measure MPG is based on the amount an injector is "designed" to flow. Should the injector flow more or less (worn or clogged) there would be no way to know the correct consumption per mile?
boomdriver
Well-known member
I think the trip computer must use other input data to help with that. I have seen mine make adjustments in range and/or avg mpg while driving.
The Instant stays pretty consistant with the pedal unless you run up a hill then as the car slows the instant reflects that in the mpg even though the pedal may be exactly where it was before the hill.
I know mine was dead nutz right on the money accurate to the last mile until I had to pull the sender for a new pump,( cause it don;t like going to the last mile) then the float got tweaked just a hair, so now its off by a small bit. I have a new sender ordered and I'm anxious to see if that restores the accuracy.
It's really just a matter of assigning a numerical value to everything and applying that to a formula that has been stored in the computer.
It also has the ability to display the real time calculations. The coolest part is the fuel level dampener that stops the gauge and the mpg from bouncing all over the place when you go around a corner and sling the fuel to the side of the tank. Even 1980's computers do some amazing calculations.
Not 100% sure about the computer not being able to make very accurate calculations on non-stock injectors, I have larger injectors installed and my mpg readout took a hit right away. I have not done a data scan yet to see what the pulse width is now, but it should be similar with just a bit more fuel flowing. Somehow the computer "knows" there is more fuel being used and indicates that in the instant, avg and the range displays. The O2 sensor has some valuable input as far as fuel used,. but it is not involved all the time on L-98 engines.
The corn fuel also throws it (mpg) off having less thermal energy per gallon, so it takes more than a gallon to get the same results as the computer was programmed to expect. My avg and instant dropped like a rock when ethanol became a common admixture in year round fuel blends. Used to be just a seasonal thing with oxygenated fuel being used the rest of the year. No more, its all ethanol 10% or more and that does mess with fuel estimates.
Ethanol is junk science at its best.
The Instant stays pretty consistant with the pedal unless you run up a hill then as the car slows the instant reflects that in the mpg even though the pedal may be exactly where it was before the hill.
I know mine was dead nutz right on the money accurate to the last mile until I had to pull the sender for a new pump,( cause it don;t like going to the last mile) then the float got tweaked just a hair, so now its off by a small bit. I have a new sender ordered and I'm anxious to see if that restores the accuracy.
It's really just a matter of assigning a numerical value to everything and applying that to a formula that has been stored in the computer.
It also has the ability to display the real time calculations. The coolest part is the fuel level dampener that stops the gauge and the mpg from bouncing all over the place when you go around a corner and sling the fuel to the side of the tank. Even 1980's computers do some amazing calculations.
Not 100% sure about the computer not being able to make very accurate calculations on non-stock injectors, I have larger injectors installed and my mpg readout took a hit right away. I have not done a data scan yet to see what the pulse width is now, but it should be similar with just a bit more fuel flowing. Somehow the computer "knows" there is more fuel being used and indicates that in the instant, avg and the range displays. The O2 sensor has some valuable input as far as fuel used,. but it is not involved all the time on L-98 engines.
The corn fuel also throws it (mpg) off having less thermal energy per gallon, so it takes more than a gallon to get the same results as the computer was programmed to expect. My avg and instant dropped like a rock when ethanol became a common admixture in year round fuel blends. Used to be just a seasonal thing with oxygenated fuel being used the rest of the year. No more, its all ethanol 10% or more and that does mess with fuel estimates.
Ethanol is junk science at its best.
G Winter
Well-known member
My 90 was always exactly 2 mpg to high. I had some bad OEM injectors so I replaced with the Bosch III from FIC . Now the mpg is nuts on to within a tenth on a fill of gas.
Mileage also went up about 1 to 2 mpg.
Glenn
Mileage also went up about 1 to 2 mpg.
Glenn
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