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Old 03-07-2012, 01:01 PM   #1
Custom3
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What would cause the MPG Gauge to malfunction?

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.
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Old 03-07-2012, 01:06 PM   #2
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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.
its normal when you start to move and just about to stop the gauge briefly swipe all the way across to the right.
and that wushing sound is normal too.
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Old 03-07-2012, 01:08 PM   #3
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Could be a wiring issue but most likely the gauge itself is malfunctioning. You can try to repair it with a donor cluster or you will have to replace the cluster with a new, uncoded unit. You could also code it off with NCS Expert if it's just annoying you.
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Cheat Sheet:

GT1/ISIS/MoDiC = Factory authorized diagnostic system used by BMW dealerships across the world.

NCS Expert = BMW Factory R&D programming tool available on the internet (not intended for the public and not available to the dealership network)

OEM = Genuine BMW part only available from a franchised BMW dealership.

DME = Engine Management Computer
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Old 03-07-2012, 01:35 PM   #4
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The analog guage is just a funtion of vacuum. So if you are also getting CEL codes for the MAF and you think the guage is acting weird, then both of these point to a possible vacuum leak.
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Old 03-07-2012, 03:05 PM   #5
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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!!
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Old 03-07-2012, 03:17 PM   #6
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Here is the fix for the Hissing. I did this and it works great

http://forum.e46fanatics.com/showthread.php?t=533035
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Old 03-07-2012, 03:28 PM   #7
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^ yep thats the one and now that i think about it, not a t fitting but just a coupling is needed...
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Old 03-07-2012, 03:56 PM   #8
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Quote:
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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.
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.

Last edited by jdstrickland; 03-07-2012 at 03:58 PM.
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Old 03-08-2012, 08:23 AM   #9
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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.
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Old 03-08-2012, 08:26 AM   #10
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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.
like I said, shooting to 12 mpg just before stopping is normal because the computer still reading speed but your engine is still using gas until you fully stopped then it sees zero speed and goes to 50.
mine does it since the day I bought it.
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Old 03-08-2012, 08:29 AM   #11
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even when your driving at 5mph and come to a complete stop


Quote:
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like I said, shooting to 12 mpg just before stopping is normal because the computer still reading speed but your engine is still using gas until you fully stopped then it sees zero speed and goes to 50.
mine does it since the day I bought it.
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Old 03-08-2012, 08:58 AM   #12
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even when your driving at 5mph and come to a complete stop
yes. just when the car is stopped it goes all the way to the right then back to the left when when I am not touching the gas pedal.
I mean think about it, the speed is slow and engine is using gas idling, at very low speed the fuel consumption is almost infinite.
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Old 03-08-2012, 09:17 AM   #13
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I understand that part...it going to 50mpg and the fuel consumption almost being infinite at low speed. I just don't get why it would shoot up to 12mpg when stopping as if I just floored the pedal and was going 100mph.


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yes. just when the car is stopped it goes all the way to the right then back to the left when when I am not touching the gas pedal.
I mean think about it, the speed is slow and engine is using gas idling, at very low speed the fuel consumption is almost infinite.
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Old 03-08-2012, 09:31 AM   #14
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Just theorizing, but are you SMG/Auto or MT?

I'm wondering if because the engine is still applying load against the transmission as your are applying the brakes it would cause what you are experiencing.
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Old 03-08-2012, 09:51 AM   #15
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I understand that part...it going to 50mpg and the fuel consumption almost being infinite at low speed. I just don't get why it would shoot up to 12mpg when stopping as if I just floored the pedal and was going 100mph.
why? like I said, engine idling requires fuel.
say 0.5 gallon per hour just by idling and you are doing 5 MPH. that give you 10 MPG which is why the gauge moves when you about to slow down.
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Old 03-08-2012, 09:57 AM   #16
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why? like I said, engine idling requires fuel.
say 0.5 gallon per hour just by idling and you are doing 5 MPH. that give you 10 MPG which is why the gauge moves when you about to slow down.
He is right. The gauge should not be anywhere but 0 when he is at idle. Although the engine is using fuel, the gauge isnt calibrated/designed to reflect that at idle. It's meant to measure fuel consumption while driving based on throttle input, load and other factors. Someone already explained this. It's not a debate.

OP, there is obviously something wrong. I don't know what. I would suspect vacuum leaks for starters. I've seen lots of BMWs do this after mileage and age and I'm sure theres TONS of explanations if you search.
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Old 03-08-2012, 10:58 AM   #17
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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.
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Old 03-08-2012, 11:07 AM   #18
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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.
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Old 03-08-2012, 11:10 AM   #19
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well when the car is stopped, you cant really calculate MPG anymore because you cant divide by 0. thats why BMW decided just put the needle at far left.
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Old 03-08-2012, 11:11 AM   #20
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And to correct my earlier post, the needle should be at 50mpg not 0 while at idle. (too early in the morning)

either way, not in the middle (or at 12)
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