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Discussion Starter #1
Long story short- One day my car over heated and I unscrewed the radiator cap to have loads of hot water blow all over my engine (Idiot, I know) :bawling:. So i get it up and running and after that my mpg indicator started acting wierd. I can give it gas and the mpg gauge wont react until I'm at a certain RPM. What caused this? Any ideas? What indicates our mpg gauge to move?:confused:
 

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I could be wrong, but I'll give it a shot since no one else has responded thus far.

Between the DME (engine computer) and IKE/KOMBI (instrument cluster), they take the injector flow rates, the injector on times, the engine speed, and the vehicle speed to calculate the instantaneous fuel consumption.

The flow rate is probably a programmed value (not easily, directly measurable) and may be affected by various engine operating modes and inputs.

I don't quite understand the way in which it's malfunctioning, but if you think it's related to the coolant spill, you might check that the system is fully bled and check for soiled connections.
 

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the needle that says your mpg is just a vacuum gauge apparently, its not accurate at all, i can get mine to say 50 mpg going 45 in 5th gear. thats better than a prius if it was correct lol.
 

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the needle that says your mpg is just a vacuum gauge apparently, its not accurate at all, i can get mine to say 50 mpg going 45 in 5th gear. thats better than a prius if it was correct lol.

NO, the guage is accurate and is NOT vacuum controlled. Vacuum is probably the only thing the DME/guage does not read.
 

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the needle that says your mpg is just a vacuum gauge apparently, ...
NO, the guage is accurate and is NOT vacuum controlled. ...
Both wrong actually. If it was simply a vacuum gauge it would not swing full left when at a stop and coasting in idle would have the same reading no matter your speed, which is not how it actually works. At the same time, there is a vacuum line leading under the dash for that gauge. It doesn't plug directly into the gauge, so the best I can figure is that it plugs into a meter that then adds its information to the rest of the info that the gauge ends up using...

See this thread for pics of said vacuum line: http://forum.e46fanatics.com/showthread.php?t=533035&highlight=steering+wheel+column+vacuum+leak
 

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That vacuum line does not exist on all E46's and what makes you think it is used for the MPG guage anyway. The MPG guage is completely controlled by the DME using sensor data to determine fuel delivery rates relative to vehicle speed. It is a INSTANTANEOUS MPG guage, not an average guage. Therefore, reading in the high 40's and even 50's are plausible when travel under light engine loads. Light engine loads (relative to speed) can happen via: traveling down hill or slight grade, tail winds, drafting, etc. The guage swings to "infinite" when vehicle speeds are not sufficient for proper readings...such as when coming to a (just before) stop, creeping, or idling.

I can assure you, vacuum is not used to control the guage in any manner.
 

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Discussion Starter #9
Wow I really want to thank you guys for all the feedback. I really want to fix this. Ill post a video tomorrow and maybe you guys can see whats happening.
 

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Discussion Starter #11
Haha no im talking about the one with the needle. That indicator.
 

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Since the MPG gauge relies on several sensor inputs, I am guessing one of your sensors is now broken or shorting out causing it to behave erratically, particularly your MAF. Did you rinse the coolant off of your engine after it blew everywhere and do you have access to an OBD-II scanner so you can see if there are any codes pending?
 

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Well if I got the mileage it shows sometimes, I could go weeks with a full tank opposed to a week.
You're correct. If all you did was cruise at 45mph in 5th, you could probably get 600 or 700 miles on a tank.

Unfortunately, you also have to sometimes start the engine, brake, and accelerate. The instantaneous gauge is reasonably accurate, but instantaneous MPG should not be confused with average MPG.
 

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Discussion Starter #15
Here's a short video guys. Hope it helps solve the problem.

Notice how at the start off the gauge needle lags.

 

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Discussion Starter #16
Since the MPG gauge relies on several sensor inputs, I am guessing one of your sensors is now broken or shorting out causing it to behave erratically, particularly your MAF. Did you rinse the coolant off of your engine after it blew everywhere and do you have access to an OBD-II scanner so you can see if there are any codes pending?
When the coolant blew everywhere i just simply dried it with a towel. My dad has a Scanner and nothing pops up. Hmmm :bawling:
 

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It seems normal to me, my old E46 had the same thing.
When taking off from a dead stop the mpg needle did not move right away but moved after a second or something.
Looks absolutely normal to me.
 

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Discussion Starter #18
It seems normal to me, my old E46 had the same thing.
When taking off from a dead stop the mpg needle did not move right away but moved after a second or something.
Looks absolutely normal to me.
Really? When I first got the car it was reacting like in a quickness. Hmm. Does anyone elses move like that?
 

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Discussion Starter #20
bump :craig:
 
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