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

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.

this site shows it very well

opportunity.)