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Thread: Fuel Computer after CR2 fitted

  1. #1
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    Default Fuel Computer after CR2 fitted

    Having just fitted the CR2 to my ZAF 1.9 150 CDTi, the power increase is great, but before I either get it RR setup or more range added to the adjustment, I've noticed the Fuel Computer must be completely lying...

    I've not done enough steady miles to confirm through fill ups, but its reporting stupidlyhigh (circa 58-62MPG) @ 80 MPH on the motorway, where as it previously reported 36-40 MPG (The cars only done 2.5K miles, and is still probably running in)
    A trip in this morning that normally I would get 40MPG, exact same speeds etc, and its 64MPG.. yet I can see from the fule guage that with just over half a tank left, I ain't gonna be doing a reported 427 miles that it thinks I will..

    Does anyone know how the fuel consumption is calculated/calibrated etc??

    - Is it Rail Pressure * Injector open time (Fixed Area) thus flow can be calculated? so the principle of the tuning box, i.e. lowering the reported Rail Pressure is causing a wild inaccuracy?
    - Does it take historical data into account and will even out over time?? or does it re-cal it self dynamically, or is it just a simple fixed gain adjustment made at the factory..

    Don't get me wrong, the power is great, smooth and plenty of it, and some innaccuracy is expected, but 60-80% out is a little too much, it renders the trip computer completely useless..

    I've phoned Diagnostix, but I think that discussion is better left for another day..

  2. #2
    [Ex]Admin Duncan's Avatar
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    It seems to be a problem when fitting tuning boxes. This is from other members' comments as I don't have an oil burner!

    It seems that the mpg reading is higher, yet the actual mileage to the tank doesn't reflect this mpg figure. I think the mpg does actually go up with a box (more efficient burning etc.) but the figure on the computer does seem to be somewhat erroneous!

    iirc, Matt did a test where he ran the car with and without the boxe for a week, I think, and worked out his mpg based on the mileage he did and the fuel he put in. Maybe he'll be along to verify - or correct me!! lol

    D

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    I saw a few comments when I searched for MPG on this forum!!! I've seen 20% mentioned as the worse case, most people claimed less then 10% innaccuracy...

    I will see what happens over time, I've done 800 miles since the cr2 was fitted, but becuase of testing performance etc, I've not looked at the fuel computer.. just this week, I am using the zafira for work, and had a full tank in it, and noticed my alarmingly good reported level.. but this doesn't tie in with the rate the fuel gauge is dropping.. I'd esitmate its around 80% difference.. even if I have a 10% real world MPG Increase..

    I am hoping that it may sort it self out to a degree, but am not sure if it's capable of doing this..

    I just hope that it doesn't stay as it is, and render the trip computer useless, as it's apparantly not grounds for returning it for a refund.....
    Last edited by phil-t; 9th February 2006 at 13:43.

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    Regular Member Maverick's Avatar
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    Bottom line is, all makes of tuning box screws up your trip. It can't be avoided.

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    Seems like it is.. I assume they take the reported fuel rail pressure/opening duration to calc flow, so anything that fools this is bound to cause an offset error..

    Luckily, the CR2 is fantastic.. I'll be sending it back to allow for more adjustment, although at the moment its much much quicker then standard, and has made it fun to drive...

    Kudos to Diagnostix for being tolerant with my newbie questions..

    The fuel computer is changing, and touchwood it appears to be calibrating itself to a degree, everynow and again the Range value drops by 50 miles, and the instantaneous values seem to be slowly falling..

    thanks for the replies, I'll hang on and see how it pans out..

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    Regular Member MJED's Avatar
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    Quote Originally Posted by Matt
    Bottom line is, all makes of tuning box screws up your trip. It can't be avoided.
    Thats where you're wrong Matt - The PBox Doesn't! and it doesn't plug into the common rail neither!

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    Quote Originally Posted by MJED
    Thats where you're wrong Matt - The PBox Doesn't! and it doesn't plug into the common rail neither!
    Was thinking the same - our trip is showing 41.9 mpg at an average speed of 29mph over 3400 miles.(3.0 CDTi V6), and that seems to be quite accurate.

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    Regular Member Maverick's Avatar
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    Quote Originally Posted by MJED
    Thats where you're wrong Matt - The PBox Doesn't! and it doesn't plug into the common rail neither!

    OK, I admit I don't know enough about the PBOX, I was basing my statement on the devices that plug into the common rail. So apologies there.

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