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MPG display 06 Explorer

mr.explorer

Active Member
Joined
March 25, 2009
Messages
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City, State
Clearwater, Florida
Year, Model & Trim Level
06 Eddie Bauer
I have noticed that my onboard MPG display is always approximately 1 MPG more then if I take the miles divided by gallons when I am at the gas station. I also notice that when it says I have used xx amoutn of gallons, it's never the same amount that I put in.

Does anyone know how the computer calculate MPG and is it accurate?
 



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My computer always shows about 1 mpg less than what I calculate by hand at the pump. I assume mine is lower because it measures fuel flow during idling, which we do a lot in AK to warm up our rigs in the winter.
 






Mine usually shows .5-1.0 MPG higher than the hand calculated number.
 






That's interesting. My mpg is sometimes correct, sometimes not. But the amount of fuel used is typically pretty close. Makes no sense to me, becuase it should be using Fuel-Used to calculate average MPG.

What I do know is that as long as you trust your odometer, and the gas pump isn't lying, the old manual calculation will be the most accurate.
 






Trouble is I don't think it can know exactly what constitutes "fuel used", since this is an injection system that has a bypass and return. It knows what fuel pressure pumped is, and it knows the air mass flow rate, but I don't think it can measure what fuel is returned to the tank or recircs in the fuel rail. It knows what it tells the PCM to allow the injectors to open, but that's a minute fraction of an amount of fuel.
 






Actually, I didn't know fuel returns to the tank. I knew about recirculation, but I thought it at least stayed up in the engine bay somewhere. If the measurement is taken upstream, but fuel can be returned, then it true fuel economy would always be better than indicated. Since that's not always the case, the measurement system must not be perfect.
 






So why does fuel return to the tank? You learn something new everyday. :P
 






I'm guessing it's an emissions requirement, so no unburned fuel is able to evaporate into the air. But I would think that unburnt fuel could remain pressurized in the system.
 






Recirculation is a cooling feature for the pump. Lets say you are at idle using very little gas, you don't want it dead headed. The pump runs at a single speed, pumping about the same about of fuel all the time. The flow keeps the motor cool.
 






According to the Service manual the fuel system is return-less. Fuel pressure is maintained by a variable operation fuel pump controlled by the PCM and a pressure sensor in the fuel rail. The fuel pump is cooled by the fuel in the tank that is covering the pump.

I think the message center MPG is measured through the fuel level sensor providing information on fuel level to the PCM and the PCM calculates fuel used by distance traveled. The PCM provides info to the message center display through the HS-CAN network.

I think the distance to empty uses an average MPG over the last 500 miles of usage and calculates the distance to empty based upon the fuel level sensor reading of remaining fuel.

Good luck.
 






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