Computer miscalculates fuel consuption?

bletour · #1 10-23-2008, 12:06 PM

Hey all,

First I'm fairly new to the forum after recently picking up a 2006 4.4i. So far a few minor things (steering rack/pump noise, rear hatch thumps, power mirrors not folding, rattle in driver door) but the X5 is fun to drive for such a big heavy car.

This forum has been a great help with some of the issues (I just did the ATF fluid swap for the steering rack and the noise has been cut in half. I hope the next drain and swap will fix it totally) so I was wondering if anyone has seen this issue.

The "Consumption" MPG reading is WAY off. It has been telling me 22-23 MPG regularly but doing a gallons to miles manual calculation shows it to be more around 18MPG. Not that 18MPG is bad for this type of vehicle but I am wondering how the computer can be THAT far off. That is a good 20% difference. I have tested this over three tanks recently and all are the same. Anyone else see this? Is this an indicator of anything else wrong?
In this day of electronic fuel control I am surprised by this. I know in other cars (I'm new to BMW) I have seen this calculated by fuel injector pulse width and pressure calcs and it is REALLY accurate. Maybe the BMW doesn't do it that specifically?

Thanks,
--Bill

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AzNMpower32 · #2 10-23-2008, 12:18 PM

The consumption figure has probably not been reset in a long while. When displaying average consumption, hold the BC button until the display clears and the data is reset.

bletour · #3 10-23-2008, 01:08 PM

Thanks for the response. I thought that might be it so I did reset it and then watched it over the last couple of tanks. It's still reading way too high.

JustinR · #4 10-24-2008, 10:09 AM

Are the tires/rims stock size?

bletour · #5 10-24-2008, 10:38 AM

Yup... stock tires/rims. Good thinking though... that would possibly explain it.
So no one else sees their calculations off? Interesting. My butt dyno also tells me it is a little down on power. Not a lot but it feels slightly sluggish compared to a couple of months ago when I bought it. Maybe I'm just getting used to the power but it feels like I need to prod it more to merge to the highway and such. Maybe I'll just have to take it in and see if it's throwing any codes.

--Bill

mrkbbd · #6 10-24-2008, 01:19 PM

check your tire pressures

Last5oh · #7 10-24-2008, 01:37 PM

How do the computer calculations deal with the inctances when the car is idling? Meaning stopped (in traffic or for some other reason) with the engine running?

I know that the manual states that ave speed calculations ignore the time when the vehicle is not moving, it this the same for ave fuel economy calculations?

AzNMpower32 · #8 10-24-2008, 01:48 PM

Well first off, the car works and thinks in metric, hence l/100km. For consumption calculations, it simply converts them over to whatever other unit (US mpg, UK mpg, or km/l). But when the vehicle is stopped and idling, the consumption is calculated at 30 l/100km.

mrkbbd · #9 10-24-2008, 03:05 PM

Also check your air filter

bletour · #10 10-24-2008, 03:22 PM

AzNMpower32
You seem to really understand how the system works. What inputs does it use for calculating the consumption? Is it a hard mapped table or calculated dynamically based on other inputs.
I have worked with other systems that basically used the injector size, fuel rail pressure and injector pulse width. I have found those systems to be very accurate. No matter what the outside temp, throttle position, tire size, or any other setting, the system can still calculating almost the exact amount of gas getting put into the cylinder unless an injector is clogged/flakey.
I have seen other systems that just use hard mapped tables but they miss most of the extra variables and are not very accurate.

Thanks for all the suggestions everyone!

--Bill

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