Puzzled...ideas anyone?

SilverBullet99 · 10-27-2009, 08:59 AM

I was watching that wonderful little gauge at the bottom of our gauge cluster, the one that indicates your fuel consumption, while I was driving last week. Coming down my off-ramp, I put the car in neutral to see if the needle would indeed drop to zero...which it did. But right before I came to a complete stop at the intersection, car is still in neutral, it jumps up to about the half way mark for a split second and then falls back to zero.
Any ideas as to why?
In other words, what is causing fuel consumption when the car is in neutral, and only the brake is applied? Or have I just answered my own question?

Drewfus2101 · 10-27-2009, 09:02 AM

I'm pretty sure that gauge is operated off of engine vacuum. So at very lost speeds or idle it will jump around some. I have noticed that mine will do some weird jumping like you mentioned at very specific times. So I'm pretty sure that it is normal. If you watch it more, you'll notice it doing what you said more often.

BMW#1_323i · 10-27-2009, 09:07 AM

Ummm, it actually should have gone to 50 not 0? 0 would indicate an extremely high rate of acceleration, meaning 0 MPG, whereas, the other end (50) would indicate that you are using very little fuel. Just an observation.

zhp pilot · 10-27-2009, 09:08 AM

Fuel has to be supplied when in neutral or idling to keep the engine running. If you're traveling at high speed and in neutral, the fuel used as you travel 1 mile (coasting) is very low, but as you slow down the amount of fuel needed for that same 1 mile goes up. The actual amount of fuel per unit time doesn't change but it takes longer to go 1 mile. That's why your fuel consumption expressed in MPG goes down the slower you are going while in neutral.

GT172I · 10-27-2009, 09:51 AM

^ This

David F · 10-27-2009, 10:40 AM

Quote:

Originally Posted by Drewfus2101

I'm pretty sure that gauge is operated off of engine vacuum. So at very lost speeds or idle it will jump around some. I have noticed that mine will do some weird jumping like you mentioned at very specific times. So I'm pretty sure that it is normal. If you watch it more, you'll notice it doing what you said more often.

Vacuum is probably the only thing the computer does not measure to calculate MPG. The engine/car does not have a vacuum, manifold absolute pressure, or other similiar type of sensor.

SilverBullet99 · 10-27-2009, 10:45 AM

Quote:

Originally Posted by zhp pilot

Fuel has to be supplied when in neutral or idling to keep the engine running. If you're traveling at high speed and in neutral, the fuel used as you travel 1 mile (coasting) is very low, but as you slow down the amount of fuel needed for that same 1 mile goes up. The actual amount of fuel per unit time doesn't change but it takes longer to go 1 mile. That's why your fuel consumption expressed in MPG goes down the slower you are going while in neutral.

Thanks for the lesson, sounds pretty sound to me.

mick22589 · 10-27-2009, 10:45 AM

the gauge operates off of the vaccum of the engine. ergo more sucky=more gas.

10-27-2009, 09:08 AM	#4
zhp pilot gnillort Join Date: Apr 2008 Location: Cornelius, NC (LKN) Posts: 3,323 My Ride: Is Missing Something View My Garage	Fuel has to be supplied when in neutral or idling to keep the engine running. If you're traveling at high speed and in neutral, the fuel used as you travel 1 mile (coasting) is very low, but as you slow down the amount of fuel needed for that same 1 mile goes up. The actual amount of fuel per unit time doesn't change but it takes longer to go 1 mile. That's why your fuel consumption expressed in MPG goes down the slower you are going while in neutral. __________________ '04 Silbergrau Metallic ZHP 6MT Click sig picture for the most recent photos of das Auto. E46Fanatics Stickers Beemer Evolution Last edited by zhp pilot; 10-27-2009 at 09:52 AM.

10-27-2009, 09:51 AM	#5
GT172I Registered User Join Date: Mar 2009 Location: Atlanta Posts: 57 My Ride: '04 330Ci View My Garage	^ This __________________ GT172I

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10-27-2009, 08:59 AM	#1
SilverBullet99 Registered User Join Date: May 2009 Location: Amstelveen Posts: 148 My Ride: 323ci View My Garage	Puzzled...ideas anyone? I was watching that wonderful little gauge at the bottom of our gauge cluster, the one that indicates your fuel consumption, while I was driving last week. Coming down my off-ramp, I put the car in neutral to see if the needle would indeed drop to zero...which it did. But right before I came to a complete stop at the intersection, car is still in neutral, it jumps up to about the half way mark for a split second and then falls back to zero. Any ideas as to why? In other words, what is causing fuel consumption when the car is in neutral, and only the brake is applied? Or have I just answered my own question? __________________ "Life should not be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways, your body thoroughly used up, totally worn out and screaming, "Woo hoo! What a ride!"

10-27-2009, 09:07 AM	#3
BMW#1_323i Registered User Join Date: Jul 2009 Location: Western NC Posts: 293 My Ride: Slow but satisfying View My Garage	Ummm, it actually should have gone to 50 not 0? 0 would indicate an extremely high rate of acceleration, meaning 0 MPG, whereas, the other end (50) would indicate that you are using very little fuel. Just an observation.

10-27-2009, 10:45 AM	#8
mick22589 Registered User Join Date: Nov 2008 Location: pa Posts: 201 My Ride: 2001 330i View My Garage	the gauge operates off of the vaccum of the engine. ergo more sucky=more gas.