MPG gauge unstable

[mamij]

Hi guys,

This problem started a few days ago.
I am having an issue with the economy gauge.
Basically what is happening is the gauge is not staying stable, even when the car is at a stable speed the gauge will go between 1cm of where it should be staying just going less then more then less then more.

As it does so the car seems to loose speed and acceleration. It looses power as the gauge reads lower then it will randomly increase and the car will accelerate faster, all the time like this, even with a stable foot.

Happens with both engine cold and hot.

I have read the gauge should be vacuum controlled.

I have checked all of these things, and found no problems:

-CCV and hoses
-ICV and hose
-Rubber boot
-Fuel pressure regulator hose
-Swapped Ecu
-throttle body and gasket


This is more or less all the vacuum hoses I have. Besides that I did the trick with Carb cleaner, sprayed everywhere, still no solution.

Does any of you know if there is a vacuum hose leading from the instrument cluster, to the throttle body, or don't these cars have this?

Any other ideas?

much appreciated,

Mario

[johnf]

To avoid wasting the efforts of people who try to help out, you might want to edit your cross-post to reflect the first responses and your clarification found here (http://www.bimmerfest.com/forums/showthread.php?t=777803).

Having summited Himmelbjerget, I would say some parts of Denmark have enough topography to influence the consumption gauge. :)

[mamij]

To avoid wasting the efforts of people who try to help out, you might want to edit your cross-post to reflect the first responses and your clarification found here (http://www.bimmerfest.com/forums/showthread.php?t=777803).

Having summited Himmelbjerget, I would say some parts of Denmark have enough topography to influence the consumption gauge. :)



Some parts yes, but NOT the highway!
the highway roads here are pretty flat, and very easy to keep the mpg gauge steady.
thats why I know this is a problem and not normal, what I am having

[johnf]

I am not an engine guy, but from reading that other thread and your response, I really suspect your mpg gauge is just fine and that it is responding to a problem from elsewhere.

The entirely electronic consumption gauge responds to:




the distance pulses from the rear differential that also drive the speedometer
the rpm signal from the Digital Motor Electronics (DME) and
the fuel injection time signal from the DME.
Vehicle speed, engine speed and fuel injection – I really suspect one or more of those inputs is anomalously changing. Rather than focus on the consumption gauge, what can you tell us about what the car is doing that relates to the moving car and rotating engine? For example, does the car travel at a steady speed (what does the speedometer do)? Does the engine feel rough?

[mamij]

I am not an engine guy, but from reading that other thread and your response, I really suspect your mpg gauge is just fine and that it is responding to a problem from elsewhere.

The entirely electronic consumption gauge responds to:


the distance pulses from the rear differential that also drive the speedometer
the rpm signal from the Digital Motor Electronics (DME) and
the fuel injection time signal from the DME.

Vehicle speed, engine speed and fuel injection – I really suspect one or more of those inputs is anomalously changing. Rather than focus on the consumption gauge, what can you tell us about what the car is doing that relates to the moving car and rotating engine? For example, does the car travel at a steady speed (what does the speedometer do)? Does the engine feel rough?

Sometimes there will be a bit of a rough idle, rpm would drop and feels like engine wants to die, but doesnt die.
Steady speed all the time, speedometer is steady, but the mpg gauge moves back and forth, car accelerates and loses power, along with the mpg gauge moving back and forth.

- - - Updated - - -

Problem solved :). Disconnected the o2 sensors, and the problem went away.
I have two precat o2 sensors, only one of them affects the mpg gauge, and looks like it needs replacement.

[johnf]

... Problem solved :). Disconnected the o2 sensors, and the problem went away.
I have two precat o2 sensors, only one of them affects the mpg gauge, and looks like it needs replacement.
Excellent. I would report your success on the other thread to help everyone out.

[mamij]

Excellent. I would report your success on the other thread to help everyone out.

Sure will do.
Now I atleast know that the o2 sensors can affect the gauge, so weird.