97 850 T-5 : Computer Mileage lower than Actual?

Discussion in 'Volvo 850' started by Joe S, May 7, 2007.

  1. Joe S

    Joe S Guest

    97 850 T-5 with 200K Miles.

    Several months ago I was getting the trip MPG to show 32-34 MPG for a
    highway run on my way home from work. Kind of a game for me to
    maximize this.

    First thing that happened was the fuel pump failed. I got a
    replacement and put that in, along with a new fuel filter.

    Several weeks later it threw a code that the front O2 sensor was
    "slow". I cleared and got it to repeat a couple more times over the
    next couple of weeks, so I replaced it.

    Somewhere in this time, I don't recall exactly, the computer mileage
    started showing 6-7 MPG less than it used to for known stretches, and
    the average will show in the low 20's, HOWEVER, the actual mileage by
    fill-up calculation still gives me 26-27MPG which is competitive with
    what it was prior to any difficulty.

    It has also thrown a "fuel rich" code a couple of times.

    (After disconnecting the battery for the fuel pump replacement, I was
    not aware of the fuel adaptive "trip", and do not know if that was
    satisfied by accident. I will go through that at first opportunity.)

    So, my basic question is what might cause the computer to be showing a
    very different MPG than it's actually delivering?

    Thanks for any advice/experience/knowledge,

    Joe S, May 7, 2007
  2. My guess - the fuel pressure regulator is running too high. Here's why I say
    The computer knows a lot of things wth great precision. It knows the vehicle
    speed, usually agreeing exactly with the odometer you'd use for mpg
    calculations yourself. It knows the engine speed - how often the injectors
    fire. It knows how long it is telling the injectors to stay open. But it has
    to assume the fuel pressure, which affects the amount of fuel that is
    actually injected as compared to the amount of fuel that should be injected.
    More pressure = more fuel.

    The computer should be accurate to about 5%, and I'd allow 10%. Beyond that
    something is out of whack.

    Michael Pardee, May 7, 2007
  3. Joe S

    Joe S Guest

    Hmmmm....if I am getting good *actual* mileage, then the pressure
    regulator would be fine, would it not? If anything, the computer is
    thinking that more fuel is getting sent through the system than is
    actually going.

    (If anything, I was worried that the new fuel pump was not pumping as
    strong as the old one was because I think it takes maybe a half-second
    extra cranking to start now.)

    The MPG computer might make such a "low mpg" mistake if it thinks the
    wheels aren't turning as fast as they are....say if it thought I was
    in 2nd gear? (I don't know if/how this could happen)

    But I suspect there is something else going on, especially as regards
    the "rich" code I've gotten twice.

    Joe S, May 7, 2007
  4. You're right - I got the pressure backward. I can't help it... I'm old ;-)

    But still, I'm thinking of the fuel pressure. The ECU goes mostly by
    reckoning and uses the front O2 sensor to correct the mixture within limits.
    It sounds like the reckoning is wrong. If it isn't fuel pressure (I know -
    it isn't that common a problem in so new a car) it must be a sensor being

    The thing is that the fuel economy computations aren't affected by the ECU
    having to lean over backward to keep the O2 sensor happy. And the
    computation vs your own calculation is unaffected by how fast the wheels are
    turning. For example, if you put honkin' oversize wheels and tires on it the
    odometer would show you travelling fewer miles but the display would think
    the same thing.

    Michael Pardee, May 8, 2007
  5. Joe S

    Joe S Guest

    If I did put huge tires on....I would actually be going on a trip of
    27 miles and burning a gallon of gas. The computer would figure out I
    burnt that gallon of gas, but could only *assume* that I went I went
    (say) 23 miles because it doesn't know how far I really go on a
    revolution of the wheel.

    Which makes me think....is there some calibration that can be done to
    tune the MPG computer if you do put on big wheels?

    Joe S, May 8, 2007
  6. I would be more concerned about the speedometer. Speed is controlled by
    law. MPG is just an interesting fact. I can always compute my MPG by
    knowing my trip distance from Mapquest and the Gallons from the Weights
    & Measures approved pumps at the gas station. But it is not fun to
    calibrate the speedometer from the mile markers on highways. What is
    even more of a frustrating, odometers don't always agree with
    speedometers (that is they don't have the same error as one would hope
    Stephen Henning, May 8, 2007
  7. Joe S

    me Guest

    I had similar codes on my 96 850 "oxygen sensor slow and I think the other
    was lomg term fuel trim rich"

    It ended up being a cracked vacuum hose under the intake.


    this site shows it very well
    me, May 9, 2007
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