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

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

 

My guess - the fuel pressure regulator is running too high. Here's why I say
that:
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.

Mike

 

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

 

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
goofy.

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.

Mike

 

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

 

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
for).

 

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.

http://www.volvospeed.com/Repair/vacuum_lines.php

this site shows it very well