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E39 (1997 - 2003)
The BMW 5-Series (E39 chassis) was introduced in the United States as a 1997 model year car and lasted until the 2004 when the E60 chassis was released. The United States saw several variations including the 525i, 528i, 530i and 540i. -- View the E39 Wiki

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  #1  
Old 12-10-2013, 07:12 AM
BioGuy BioGuy is offline
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How does the needle MPG gauge work?

Does it pull figures based on engine vacuum, or digital calculations... .?

I ask because yesterday (based on the needle gauge in the cluster) it looked like I was getting a little lower than normal fuel economy. The 'digital' gauge in the MID read normally.

I should note that is was also pretty chilly yesterday- about 2 degrees on the commute home.

It seemed like after about 30 minutes or so, the needle gauge started reading 'normally' again.

I guess I was wondering if the cold temps had any effect on the operation of that gauge, and more specifically- where does that gauge pull its data from?
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  #2  
Old 12-10-2013, 08:25 AM
pshovest pshovest is online now
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Mein Auto: '10 535iA, '00 528iTM
Speedo provides MPH.
DME provides fuel flow signal that can be converted to GPH, gallons per hour.
MPG gage displays MPH/GPH ====> instantaneous MPG

Cold temps certainly do reduce MPG, particularly during warmup, but it's not clear you can detect this on MPG gage.
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  #3  
Old 12-10-2013, 08:31 AM
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kevbot13 kevbot13 is offline
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Location: Middletown PA
 
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Some answers over from another forum..

Quote:
Originally Posted by bmwbob89 View Post
Fuel injector open time and speed signal. No vacuum.
Quote:
Originally Posted by edjack View Post
The reading is calculated from the fuel injector pulse width and instantaneous speed. The only possible useful function is to make you drive as though there's a fresh egg between your foot and the go pedal.
While other people say it's just a vacuum line...I'm not really sure though. All I know is I don't trust it that much.
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  #4  
Old 07-30-2014, 10:43 PM
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bluebee bluebee is offline
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Quote:
Originally Posted by kevbot13 View Post
While other people say it's just a vacuum line...I'm not really sure though. .
This thread today says it's electric ...
> E39 (1997 - 2003) > Insantaneous MPG guage reading.
Quote:
Originally Posted by tygr1 View Post
The MPG gauge in my cluster just under the tach; does it read MPG based on a vacuum signal?
Quote:
Originally Posted by JimLev View Post
Where did you find that answer?
It's an electric gauge, probably getting it's signal from the DME.
Quote:
Originally Posted by edjack View Post
The "gauge" reading is derived from a measure of the injector activity. It's an amusing readout, nothing more.
Quote:
Originally Posted by crowz View Post
Its an electric gauge that reads the computers value for mpg. This calculation varies a bit from car line to car line. The reason the gauge swings to the lift sitting still is because it cant calculate under a certain speed. Math issue
Quote:
Originally Posted by crowz View Post
Umm I can play with the values in the computer and make the gauge read whatever I want
Quote:
Originally Posted by crowz View Post
Actually I thought of an easier way to "prove" it to you for grins and giggles.

Drive your car and notice the mpg reading. DO NOT change the amount of pedal your set at. Aka keep the pedal the same with it showing say 30mpg or whatever. Grab the parking brake and pull it up slowly. Do this to fast and you might wreck so CAREFULLY add some braking with the parking brake. Watch the mpg reading. Its a mechanical brake and does not effect vacuum at all. So your adding load without changing the motors vacuum levels.
Quote:
Originally Posted by doru View Post
I believe your MPG gauge functions correct.
If it goes to the right when stepping on the brakes, and the speed is substantially reduced, the mpg gauge goes almost all the way to the right, and once stopped, it goes to "0" (all the way to the left)
Quote:
Originally Posted by mjalloul11 View Post
As doru said. It is working corect. There are no vacume tube to the cluster it is all electric.

Sent from BimmerApp mobile app
Quote:
Originally Posted by edjack View Post
Same gauge, different calibration. The two fuel consumption measurements are inversely related.

Watch the MPG gauge when decelerating. It moves all the way to the the left, which is "undefined," or "infinity," because the car is moving w/o fuel (the injectors are turned off during deceleration with the foot off the gas pedal; therefore, gallon is zero).

The gauge receives MPH and injector pulse width inputs.

Lobby your Congressmen to convert to the metric system.
Quote:
Originally Posted by poolman View Post
The gauge works off of manifold vacuum--when you turn the car off you also should hear a release of pressure--or vacuum bleeding off of the manifold --this happens a few seconds after you turn the car off--bet that the drop on the gauge is in time with the hiss that you will hear out under the hood after turning off the engine.
Quote:
Originally Posted by mjalloul11 View Post
There are no vacuum hose at the back of the cluster ,and if it is electeric should the gauge drop when i turn off the car?
Quote:
Originally Posted by poolman View Post
I have no idea of whats behind the cluster that hooks to the gauge--but if you go out under the hood and disconnect the small vacuum hose that is under the larger one on you air bonnet going to the throttlebody--you will find the gauge inside the car will not work afterwards---
Quote:
Originally Posted by edjack View Post
As I understand it, the gauge is tied to fuel injector duty cycle.
Quote:
Originally Posted by bmw_n00b13 View Post
It's computerized, and directly tied to the fuel injection system. It's a common phenomenon when stopping; I think it's because the car uses almost no fuel while braking but uses a bit while idling to keep the engine turning over.
Quote:
Originally Posted by edjack View Post
The fuel is completely cut off while coasting. It turns back on at about 800-1000 RPM.

The MPG gauge is fully to the left when coasting, which is an "undefined" area - no calibration marks. As the car slows, the gauge will swing over to a finite reading. With a manual trans, one can actually feel the cut-in.

I don't know how the l/km gauge works, probably opposite to the MPG meter.
Quote:
Originally Posted by RaceCarDriver View Post


Might be a stupid question, but what MPG is mark between 20 and 40 mpg? It's been bugging me not knowing!
Quote:
Originally Posted by nike001 View Post
....30?
Quote:
Originally Posted by gtxragtop View Post
25. I validated this on my OBC
Quote:
Originally Posted by andyffer View Post
25-30.

On the way back from Tucson, 31.5mpg. The comp told me 29 but I went into the hidden OBC to get the accurate numbers and did the math
The way only got 26 or so but it was pretty fun! Disappointed I have that damn whistling at 100+ mph. I dont have all my jackpads and my windshield molding is total crap and Im not sure if the vents were all open or not so... Gonna start testing some things..
See also:
> E39 (1997 - 2003) > MPG Gauge
> E39 (1997 - 2003) > Over active MPG energy gauge
> E39 (1997 - 2003) > The Marks on the MPG Gauge/Econometer
> E39 (1997 - 2003) > MPG gauge
etc.
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Please read the suggested threads, where the best always add value to those threads, either by pictures or by descriptions, so the next person with the same problem stands on your shoulders.
See also: E39 Bestlinks & How to easily find what you need

Last edited by bluebee; 07-31-2014 at 08:58 AM.
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  #5  
Old 07-31-2014, 08:41 AM
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bluebee bluebee is offline
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Unfortunately, there's not much about the "fuel economy meter' in the (attached) IKE instrument cluster electronics training PDF:
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Please read the suggested threads, where the best always add value to those threads, either by pictures or by descriptions, so the next person with the same problem stands on your shoulders.
See also: E39 Bestlinks & How to easily find what you need
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  #6  
Old 07-31-2014, 11:09 AM
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bluebee bluebee is offline
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Another reference of information was suggested today by Ed:
> E39 (1997 - 2003) > MPG Gauge
Quote:
Originally Posted by edjack View Post
Quote:
Originally Posted by bluebee View Post
Nice catch:
-> E39 (1997 - 2003) > Question about the Vacuum Gauge on the Dash

That thread is in the bestlinks, but under the digital MPG accuracy thread, and not the analog gauge thread:
- What's the true accuracy of the various methods of mpg calculation (1)

So, I'll add a cross reference for others to find when they're looking at either their digital MPG accuracy or their fuel efficiency analog gauge.
Here are the screenshots kindly supplied by QSilver7 in that thread...
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See also: E39 Bestlinks & How to easily find what you need
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  #7  
Old 09-20-2014, 04:43 PM
4thBMW 4thBMW is offline
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Location: FLORIDA
 
Join Date: Aug 2010
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Mein Auto: 2002 540iT Sportwagon
I find the MPG guage facinating

I got addicted to watching the mpg needle driving X-country I-10, trying to maximize mpg. Also, in another recent post regarding breaking that very small diameter Vacuum hose from the airbox tube to the fuel regulator on the fuel filter under the car while R&R'ing one CPS....I put a temporary fix on that small plastic vac line. Two days later, I discovered that my ever watched mpg needle starts making erratic moves, sometimes backwards for a few seconds and on 40 mpg at idle. I guess my section of paper straw filled JB Weld around the vac tube failed. However, on looking at the "stent" fix, it looked solid and OK (see pic). Maybe not....I'll know when the new vac tube arrives. I hope it is the problem (my JB Weld gave up)as it weirds me out and brought back some cold running irregularities which went away right after replacing the CPS and fixing the vac tube.

>>> Originally Posted by poolman View Post
I have no idea of whats behind the cluster that hooks to the gauge--but if you go out under the hood and disconnect the small vacuum hose that is under the larger one on you air bonnet going to the throttlebody--you will find the gauge inside the car will not work afterwards---
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  #8  
Old 10-26-2014, 06:30 AM
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bluebee bluebee is offline
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There are so many good graphics from QSilver7 that I don't know where to put them, so, I'll add the ones from this thread today so that others can more easily find them:
> E39 (1997 - 2003) > 1997 low cluster
Quote:
Originally Posted by QSilver7 View Post
The instructions for resetting the (low) OBC is in the very last paragraph...after the text that is underlined in blue:
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Please read the suggested threads, where the best always add value to those threads, either by pictures or by descriptions, so the next person with the same problem stands on your shoulders.
See also: E39 Bestlinks & How to easily find what you need
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  #9  
Old 10-26-2014, 02:12 PM
poolman poolman is offline
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Mein Auto: 525i station wagon
I was wrong,,it is electrical. But I watch it , as well the other gauges in my car. It helps to follow the gauge, as I drive certain routes and notice where the gauge readings are from time to time, driving the same section of highway. If there is a small vac leak somewhere,,it shows up , on this gauge first.
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  #10  
Old 04-30-2015, 02:27 PM
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bluebee bluebee is offline
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This came up today...
> E39 (1997 - 2003) > ABS Module and MPG guage

Quote:
Originally Posted by KSUTJH View Post
So, I have the trifecta of lights on now, they were intermittent up to this point, but now they are on all the time and the speedo quit as well. Also the mpg guage thing is pegged all the way to the left, is this another indication of a abs module failure? The previous owner put in a rebuilt one from BBA, they dont want to warranty it unless I get the codes read and sent to them. Of course the codes read both front sensors and the drivers rear. I have taken all the sensors out and cleaned them and put them back in, after I did that was when the speedo and mpg stopped working. Are the mpg guages not vacuum actuated any more?
On a related note, the cruise works, but you can feel it surging, not horribly noticeable, but enough that my wife says it makes her wanna get sick.
Quote:
Originally Posted by edjack View Post
Are the mpg guages not vacuum actuated any more? Never were. The readout reflects a measure of injector duty cycle.
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See also: E39 Bestlinks & How to easily find what you need
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