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What would cause the MPG Gauge to malfunction?

72K views 30 replies 12 participants last post by  Shawnkazz 
#1 ·
Here what it is doing. I noticed when I drive the MPG Gauge reading goes crazy. Its constantly going up and down in a not so normal way. Also when I come to a stop the gauge shoots up to the end like 12mpg and then back to 50mpg.
It starts to work normal again when I been driving the car for a bit.

Im wondering if it has anything to with MAF. Car has shut off a few times when driving and coming to a quick stop. I check for codes and its always the MAF. It wont trigger the Check Engine Light but it does store the code.

I also have the swoosh sound on the gauge but that has been going on since day one.
 
#5 ·
I don't think the whoosh sound is normal. The vacuum line under the dash panel gets chaffed over time by the steering column and develops a hole (vacuum leak). The solution is to cut the damaged part out and install a plastic T-fitting. AFAIK, this fixes the whoosh sound and/or any vacuum leaks. I haven't had a chance to get to it yet.. maybe today I will!!
 
#8 · (Edited)
It is normal that when you stop, the gauge shoots to the 50MPG limit. Logic says it should go to the 0MPG end of the scale.

As for the wild movements, these are normal as well. The gauge looks at the instantaneous conditions of load, pedal position, and so on, and reports what the then current consumption rate is. If the current conditions change, the gauge responds instantly. The idea is, you may want to know what your foot is doing. You get the best gas mileage if you can get the needle to seldom move as opposed to constantly change.

You have to clear the code for it to go away. If you look at the codes and don't clear whatever comes up, then it will keep coming up everytime you look.
 
#9 ·
Thanks for the replies guys... The swoosh sound Im not worried about. Like I said thats been going on since day one and I searched on this forum and found out its a common problem and the fix for it.

My main concern is the gauge going up and down which I know is normal but like I said its doing it in a not so normal way. It going crazy, more then usual. Up down Up down....Even when the pedal is not being pushed. When you come to a stop its suppose to shoot back to 50mpg. It does but not right after shooting really fast to 12mpg and then right back to 50mpg.

It never did this before, so I know something aint right. Im wondering if my secondary air pump isnt working and thats why it happens only when the car is cold. Reason why I think it isnt working is because I haven't heard it turn on. I will have to check to confirm. Also I will check that vacuum hose underneath the steering column. Maybe its broken really bad and thats why its getting the false reading as well as making the swoosh sound.
 
#17 ·
Let me see if I can explain this...

The fuel gauge under the tach gives the instantaneous result of the current conditions. You are driving along at (whatever) and let off the gas in preparation for the coming stop sign. The gauge shoots to 50 or better. This means that if the current conditions could exist to infinity, the car would give 50mpg or better. Of course, the contitions cannot exist into infinity, and as the car slows the fuel consumption remains the same but the speed is so low as to not allow the car to give 50mpg anymore, so the needle falls. The slower the car goes,the more the needle falls. As you approach 0mph, the needle does a counterintuitive thing, to jumps back to 50mpg, which is an impossibility because the car is not going anywhere. Logic says the needle should be buried at the low end of the scale, not pegged at the high end. But this is a programming error in the vehicle's computer code, it is not an error of the gauge.

The car knows how much gas is being asked for because it knows where the throttle plate is. It knows how fast the car is going because of various speed sensors. If the car is rolling along at relative high speed, and the throttle is closed, then it is moving fast (and therefore far) on no fuel demand, the indication is that the current cnsumption is 50+mpg. The fuel demand remains constant, and the car slows as you prepare to stop, eventually there is a point where the car cannot continue to give 50mpg because the inertia of its own movement simply cannot be sustained. When this happens the gauge falls. The only thing "wrong" is that when the car stops moving, the needle swings back to 50+mpg.

You have noticed, and asked about, a normal set of events and the gauge is behaving normally. There is nothing to fix. No wires to check. No hoses to tape up.
 
#18 ·
JD, the OP says his gauge is at 12 mpg while stopped, right? If so, that is not normal. And AFAIK, the MPG gauge is vacuum-controlled (similar to a boost gauge on turbo cars) so what do you mean there's nothing to fix? Wouldn't a compromise of the vacuum line mean the needle isn't pointing where it should be? Maybe I'm reading your post wrong or just misunderstanding the OP's problem.
 
#23 · (Edited)
I read that the needle swings to 12 before the stop, then swings back to 50. This is normal behavior.

The needle is not vacuum controlled. It behaves like a vacuum gauge, but it isn't one. If it was a vac gauge, then it would not drop to 12 and then swing back to 50 when the car comes to a full stop. It is an electronic gauge that gets input from the computer as the computer calculates the speed vs. the fuel demand. As the speed changes and the demand remains a constant, the needle swings. If the vac was driving the gauge, then the speed would not have any bearing on the indication.
 
#25 · (Edited)
There is no reason to divide by zero. If the car is not moving, the fuel consumption is the fewest miles per gallon possible, not the most. The gauge should be programmed to be buried at the minimum reading, not pegged at the maximum. It's a simple line of code that somebody overlooked. Indeed, perhaps it is a setting that the GT1 can make, but for whatever reason the setting is set by default to what we see instead of what logic says we should see.

MY BAD
I shoulda said that to Jason...
 
#27 ·
Mine has been doing the same thing recently, but it doesn't make it all the way to 12, it goes to 30, then back to 50. (when i am coming to a stop, and it should be resting at 50) Started this a few weeks ago, really annoying. worried it could be a bigger issue...
EDIT: My car is a 2003 325i
 
#31 ·
Here what it is doing. I noticed when I drive the MPG Gauge reading goes crazy. Its constantly going up and down in a not so normal way. Also when I come to a stop the gauge shoots up to the end like 12mpg and then back to 50mpg.
It starts to work normal again when I been driving the car for a bit.

Im wondering if it has anything to with MAF. Car has shut off a few times when driving and coming to a quick stop. I check for codes and its always the MAF. It wont trigger the Check Engine Light but it does store the code.

I also have the swoosh sound on the gauge but that has been going on since day one.
Here what it is doing. I noticed when I drive the MPG Gauge reading goes crazy. Its constantly going up and down in a not so normal way. Also when I come to a stop the gauge shoots up to the end like 12mpg and then back to 50mpg.
It starts to work normal again when I been driving the car for a bit.

Im wondering if it has anything to with MAF. Car has shut off a few times when driving and coming to a quick stop. I check for codes and its always the MAF. It wont trigger the Check Engine Light but it does store the code.

I also have the swoosh sound on the gauge but that has been going on since day one.
I've recently had the same issue going on with my 2006 330ci where the needle fluctuates and I can feel the car is trying to adapt when slowing down or accelerating. I'm at a complete loss as to the cause
 
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