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  #26  
Old 08-23-2019, 07:14 AM
Autoputzer Autoputzer is offline
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Quote:
Originally Posted by icesailor View Post
That's $50.00 per year. If you saw a $20.00 bill on the sidewalk, would you walk on by or pick it up. When you keep track pf things and not just do it in an arbitrary and capricious manner, you can discover all kinds of interesting things. Like I drove one of my Ford work vans over 100.000 miles. Then I traded it in on another one, That's when I discovered that I had hundreds of pounds of tools in the truck that I might need occasionally that just went on a 100,000 mile ride. Because it was easier to leave them in the truck rather than take them out and put them in the shop. I noticed an immediate improvement in performance. That included gas mileage.

BMW went to RTF's. They are not going back. They like the weight savings. Something that may be hard to figure out with your New Math. I use the old "Retard Math". How many miles did I travel and how many gallons did I use in doing it. I have that Eco-Boost feature. I used it for a while. One time, it said that I went an extra 15 miles because of it. It sure didn't show up in my use/mileage calculations. When I go for a ride, I reset the odometer. Between 70 MPH and under 80 MPH, the trip Odometer says I get over 33 MPG. But the calculations show slightly less. Less if I left the Econo-Boost on. Even less if I drive over 80 MPH,

Back to basics.
You're right. What you're describing is what engineers are indoctrinated in, called Lean Six Sigma. "L6S" can be summed up as "large improvements can be gained trough the sum of many small improvements."

American Airlines went on an L6S kick years ago. They saved millions of gallons of jet fuel by not painting most of the outside surfaces of their planes, threw away most of the magazines in the cabin, just carrying enough beverages to get them to the next stop, only carrying enough fuel to get to the next stop with a reasonable margin of safety, etc., etc.

I actually shut the car down and roll the windows down when I'm waiting for Frau Putzer to bring me my hot chocolate and tasty breakfast sammich. Frau Putzer, on the other hand, has no problem sitting in a car with the engine and AC running. I haven't done an idle-fuel-consumption measurement on her X3, but since it's a two-liter (compared to my three-liter), it probably burns about 0.25 gallons/hour with the AC running. We met for lunch Tuesday, and she got there a few minutes before I did. There she was sitting in her car/truck with the motor and AC running.

Our BMW's need 91 AKI gas, but they don't sell it here. So, I make my own by blending 1/3 87 AKI and 2/3's 93 AKI. That saves $3 to $4 per tank, and about $800 over the 100k miles we keep our cars. It takes me an extra minute to do that, but that works out to $120 to $160/hour for my time.

I don't use the recommended tire pressures on the door jamb decals. Instead, I measure the tread depths (each circumferential channel on each tire) when I rotate the tires and adjust the tire pressures for even wear. That almost always means higher pressure than what the decal said. My tires last longer (because they wear evenly), and I get better MPG. I give up some ride quality, though. My top five mileages for a set of tires are: 79k, 74k, 70k, 70k, and 68k miles. The 74k mile tires were on a 1/2 pick-up truck. I'll get ~55k miles out of the OE non-run-flats on Frau Putzer's X3. That means I'll only have to buy one set of tires to get to 100k miles.

If I see a line of cars ahead stopped at a red light, I throw my manual transmissions in neutral and start coasting up to them. Later BMW's (starting in 2014) actually build this logic into EcoPro Mode in cars with automatic transmissions. Planning my stops and using engine braking when possible and necessary, my first BMW still had about 1/3 of the front brake pad material left at 115k miles when I sold it.

Actually, BMW's brought back both spare tires and non-RFT's. You can get a spare tire on every platform BMW makes, except for the F2X 2 Series, F3X 4 Series, and the Z4. The F2X and F3X platforms are old and are nearing the end of their production cycle. Their replacement platforms will likely offer spare tires.

BMW's' spare tire rims are now aluminum to save weight. In addition to improving MPG, they're a lot easier to hump in and out of the trunk, and on and off the car when you have a flat tire. At $150, the optional spare tire is the biggest bargain on a BMW option list. On some models, the spare is free if you spec' the optional non-run-flat tires.

Run-flat tires are about four pounds heavier than a comparable non-run-flat. They also have higher rolling resistance, which reduces MPG by about 1%.

Years ago, I had a rental F-150 with a V8. On a 40 mile road trip I achieved 24 MPG. But, on the same road trip in Frau Putzer's 2018 X3 in EcoPro, I can hit 38 MPG if the traffic lights and wind are in my favor.

One thing that really cuts into MPG is sitting at stoplights with the engine running. But, I always disable the Auto Start/Stop (ASS). Yeah, ASS saves gas. But it also puts extra wear on the engine, starter, and turbocharger. Since we keep our cars 100k miles, I don't want to risk having to replace those components.

Frau Putzer once found a $100 bill on the ground. She damn well sure did pick it up. But, then she said "Look ever' body, I found $100!" I told her to STFU, in case the person who lost it was still around.

A friend if mine found a bail of marijuana floating in the water when he was surfing. He lugged it back to the frat' house and dried it out. Him and his frat' buddies smoked half of it and sold the other half.
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  #27  
Old 08-23-2019, 08:18 AM
RPsX5d RPsX5d is offline
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Quote:
Originally Posted by Autoputzer View Post
. . . The BF thread has a link to a YouTube video, and the video states that the K factor should be adjusted to:

K = (Actual MPG) / (Displayed MPG) x 1000 . . .
Thanks a lot for bringing this up, I had totally forgotten about this one . . . initially (2012-ish), when I did the pump mpg (calculated) vs OBD mpg (indicated), I remember reading about this correction factor . . . when the discrepancy appeared in 2017 I had completely forgotten about the k-factor . . . and not sure why I didn't do the obvious - recheck the pump mpg vs. OBD mpg readings . . . instead, I assumed something had fundamentally changed in the system, most likely a sensor was feeding the system incorrect information . . . and tried to reason out logically what may have happened . . . back then and as it is now, there were no fault codes . . . so after a while, I simply gave up and decided to wait for a fault code to show up, none so far.

That's the best part of this forum . . . post an issue, initial post may not be perfect, but once the more experienced folks start to chime in, more often than not, it gets you an answer or at least point you in the right direction . . . I have been on this forum since 2010, and that has been my experience most of the time.

THANKS to everyone who contributed in a productive manner . . . for now, I will check pump vs OBD mpg for a few more fill-ups . . . I always saw a significant drop in mpg as a sign of something is giving out . . . and if pump mpg is about the same as it was when the vehicle was new, I will call it a day . . . may mess with the K-factor for the fun of it . . .

PS: I found BF a few months after I bought this X5 . . . if I had known about this before my purchase, I would have definitely asked about the adaptive drive, and most likely would have got it . . . was fed some REALLY bad info by three separate sales guys, made me wonder . . . and finally did not get it . . . SORELY regret it!
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  #28  
Old 08-23-2019, 10:58 AM
Autoputzer Autoputzer is offline
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The new definition of K used on Frau Putzer's 2018 X3 makes more sense: Lower the K to lower the displayed MPG, increase the K to increase the displayed MPG.

But, most of the world doesn't think about fuel economy (i.e. distance/volume of fuel, e.g. MPG). The rest of the world thinks about fuel consumption ( i. e. volume of fuel/distance traveled, e.g. liters/100km), and that's what's displayed on the OBC and Trip Computer. For cars that display fuel consumption, the old definition of K used on my 2014 535i makes more sense.

This whole fuel economy vs. fuel consumption is why our analog MPG displays go to infinity when we come to a stop, even though MPG when stopped is zero. The internal logic of the computer is really computing fuel consumption, and then takes the reciprocal of that value to display as fuel economy. At idle, fuel consumption (fuel volume/distance) is infinity, where fuel economy (distance/fuel economy) is zero. But, computers don't like it when you divide any number by zero. So, the needle or analog display of MPG sort of "runs home to Mama" as ST2 Jones said on the USS DALLAS in "The Hunt For Red October."
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  #29  
Old 08-24-2019, 05:20 AM
icesailor icesailor is offline
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In MY experience, that "City Mileage" and "Highway Mileage" is a different animal. Regardless of what the phake ad says on the window at the dealer's lot, my experience with my 2001 325XI wagon throughout its life time was that it got as low as 16 MGH (or lower in city stop ang go traffic, and 28 MPG on an Interstate as long as I kept the speed around 75 MPG. If you want to get worse gas mileage, drive faster. Maybe yo can afford to piss your money away for the Oil Baron Crime Syndicates.

My 2014 X-1 280 X Drive gets 18.0 in the same city stop and go driving but gets 32+ MPG when driving on limited access Interstates when I keep it between 74 MPH and 78 MPG. When you pot your big hoof into that thing on the floor to the right, and those twin turbo's start spooling up and the BOOST goes up, you can almost watch the Computer drop the MPG's. Cruise Control can help. But you still need to give it a little speed when you come to a hill. Like towing heavy horse trailers with almost well enough horse trailers.
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  #30  
Old 09-05-2019, 05:49 AM
Autoputzer Autoputzer is offline
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Here are my results of adjusting the OBC MPG.

I had the dealer do our 2018 (G01) X3, since it's under warranty. Before adjustment, the error was -3.6% (actual MPG was 3.6% less than the OBC MPG). I had the dealer adjust the K Factor from 1000 to 970. After adjustment and three tanks of gas at the same pump, the error now is -0.2%.

My 2014 (F10) 535i took two tries. I found out that the definition of the "K Factor" is the reciprocal of what it is for the 2018 (G01) X3. Before my first adjustment, the recent error was about +2.5% (actual MPG was ~2.5% more than the OBC MPG). I adjusted the K Factor from 1000 to 1025. But, that caused the error to increase (actual MPG being ~5% more than the OBC MPG). So, for the third tank of fuel, I adjusted the K Factor from 1025 to 975. I filled up the car this morning, and the MPG error was -0.23%. Actually, part of that error was round-off of the odometer mileage. The actual trip odometer mileage was 432.6 miles, but 432 is used by the spreadsheet to calculate MPG. Using 432.6 miles, the OBC MPG error would only be -0.1%. Also, my favorite pump was shut down and I had to use another one.

The gas station I use is about six miles from my house, near Hardee's. Gas there is about $0.15/gallon cheaper than near my house. I combine a Top Tier Gas run with a top tier biscuit run to the nearby Hardee's. I leave the house about 6 a.m., so there's no traffic. I take the coastal road which has fewer traffic lights and a 35 MPH speed limit. On the X3, from the house to the gas station, I get about 36 MPG with the AC on. Today, in the 535i, the house to gas station run got me 30.4 MPG.

I blend my gas to get 91 AKI. This morning my actual fuel purchased was only 0.02 gallons off of my prediction (from the OBC MPG and the trip odometer).
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  #31  
Old 09-05-2019, 08:34 AM
RPsX5d RPsX5d is offline
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I wonder what (if anything) would cause the K-factor effect to change? i.e. once set to match manual calculation, why would the two readings drift apart?

For several years, my indicated reading and manually calculated values pretty much matched . . . about 22.5 mpg. This value held for a very long time (several years). Then suddenly the indicated dropped to about 18 mpg, and as I indicated above I didn't repeat the manual calculation . . . I assumed something was going wrong, however, no fault codes.

I am now rechecking indicated vs. manual . . . two tanks done . . . both showing the indicated around 18 mpg . . . manual about 22.5 mpg, same as what it has been for a very long time.

One possibility is after resetting the K-factor your drive profile has changed substantially, re-adjust the K-factor to match the new drive profile. In my case, the drive profile did not change, same fuel station, etc . . .
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  #32  
Old 09-05-2019, 09:39 AM
icesailor icesailor is offline
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I have always used the "Retard" or Short Bus Calculations method. If I drive 10 miles and use 1 gallon of gas, I am getting ten miles per gallon. If I go to a gas station and fill the tank to the very tippy-top (so no more will go in) and drive 10 miles to the next gas station and refill the tank to the very tippy-top and it takes 0ne gallon to fill it. I got 10 MPG. Or so we riders of the short bus might calculate. Fill the tank to the very top and let it sit over night. Gas up. Whatever you have to add is evaporation plus what you consumed getting to the gas station. On multiple attempts. Same as with fuel consumption when idling. Fill up your tank and go directly to the coffee shop. Leave the engine running for as long as possible to keep the car hot or cold. Then go top it off. You might be surprised at how much fuel is consumed. Some of that is consumed by the evaporative fuel recovery systems. Like in the States that made the vas stations install vapor recovery systems on the fuel hoses. They didn't do it to make the dealers spend money. In spite of what the opposition tells you.

Keep it simple,, but don't be stupid. All of those "K" Factors can give you a headache. And the poor technician that doesn't have the time to tweak things like you want them to.
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  #33  
Old 09-05-2019, 10:07 AM
Gary214 Gary214 is offline
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Quote:
Originally Posted by Autoputzer View Post
You're right. What you're describing is what engineers are indoctrinated in, called Lean Six Sigma. "L6S" can be summed up as "large improvements can be gained trough the sum of many small improvements."

American Airlines went on an L6S kick years ago. They saved millions of gallons of jet fuel by not painting most of the outside surfaces of their planes, threw away most of the magazines in the cabin, just carrying enough beverages to get them to the next stop, only carrying enough fuel to get to the next stop with a reasonable margin of safety, etc., etc.

I actually shut the car down and roll the windows down when I'm waiting for Frau Putzer to bring me my hot chocolate and tasty breakfast sammich. Frau Putzer, on the other hand, has no problem sitting in a car with the engine and AC running. I haven't done an idle-fuel-consumption measurement on her X3, but since it's a two-liter (compared to my three-liter), it probably burns about 0.25 gallons/hour with the AC running. We met for lunch Tuesday, and she got there a few minutes before I did. There she was sitting in her car/truck with the motor and AC running.

Our BMW's need 91 AKI gas, but they don't sell it here. So, I make my own by blending 1/3 87 AKI and 2/3's 93 AKI. That saves $3 to $4 per tank, and about $800 over the 100k miles we keep our cars. It takes me an extra minute to do that, but that works out to $120 to $160/hour for my time.

I don't use the recommended tire pressures on the door jamb decals. Instead, I measure the tread depths (each circumferential channel on each tire) when I rotate the tires and adjust the tire pressures for even wear. That almost always means higher pressure than what the decal said. My tires last longer (because they wear evenly), and I get better MPG. I give up some ride quality, though. My top five mileages for a set of tires are: 79k, 74k, 70k, 70k, and 68k miles. The 74k mile tires were on a 1/2 pick-up truck. I'll get ~55k miles out of the OE non-run-flats on Frau Putzer's X3. That means I'll only have to buy one set of tires to get to 100k miles.

If I see a line of cars ahead stopped at a red light, I throw my manual transmissions in neutral and start coasting up to them. Later BMW's (starting in 2014) actually build this logic into EcoPro Mode in cars with automatic transmissions. Planning my stops and using engine braking when possible and necessary, my first BMW still had about 1/3 of the front brake pad material left at 115k miles when I sold it.

Actually, BMW's brought back both spare tires and non-RFT's. You can get a spare tire on every platform BMW makes, except for the F2X 2 Series, F3X 4 Series, and the Z4. The F2X and F3X platforms are old and are nearing the end of their production cycle. Their replacement platforms will likely offer spare tires.

BMW's' spare tire rims are now aluminum to save weight. In addition to improving MPG, they're a lot easier to hump in and out of the trunk, and on and off the car when you have a flat tire. At $150, the optional spare tire is the biggest bargain on a BMW option list. On some models, the spare is free if you spec' the optional non-run-flat tires.

Run-flat tires are about four pounds heavier than a comparable non-run-flat. They also have higher rolling resistance, which reduces MPG by about 1%.

Years ago, I had a rental F-150 with a V8. On a 40 mile road trip I achieved 24 MPG. But, on the same road trip in Frau Putzer's 2018 X3 in EcoPro, I can hit 38 MPG if the traffic lights and wind are in my favor.

One thing that really cuts into MPG is sitting at stoplights with the engine running. But, I always disable the Auto Start/Stop (ASS). Yeah, ASS saves gas. But it also puts extra wear on the engine, starter, and turbocharger. Since we keep our cars 100k miles, I don't want to risk having to replace those components.

Frau Putzer once found a $100 bill on the ground. She damn well sure did pick it up. But, then she said "Look ever' body, I found $100!" I told her to STFU, in case the person who lost it was still around.

A friend if mine found a bail of marijuana floating in the water when he was surfing. He lugged it back to the frat' house and dried it out. Him and his frat' buddies smoked half of it and sold the other half.





I really enjoyed your post.

Frau Putzer must be one cool lady

Why blend 1/3 and 2/3 ??

I would assume that 50- 50 of 87 and 93 gas would make it 91 gas

Last edited by Gary214; 09-05-2019 at 10:09 AM.
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  #34  
Old 09-05-2019, 10:12 AM
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Doug Huffman Doug Huffman is offline
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  #35  
Old 09-05-2019, 10:58 AM
Autoputzer Autoputzer is offline
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Quote:
Originally Posted by RPsX5d View Post
I wonder what (if anything) would cause the K-factor effect to change? i.e. once set to match manual calculation, why would the two readings drift apart?

For several years, my indicated reading and manually calculated values pretty much matched . . . about 22.5 mpg. This value held for a very long time (several years). Then suddenly the indicated dropped to about 18 mpg, and as I indicated above I didn't repeat the manual calculation . . . I assumed something was going wrong, however, no fault codes.

I am now rechecking indicated vs. manual . . . two tanks done . . . both showing the indicated around 18 mpg . . . manual about 22.5 mpg, same as what it has been for a very long time.

One possibility is after resetting the K-factor your drive profile has changed substantially, re-adjust the K-factor to match the new drive profile. In my case, the drive profile did not change, same fuel station, etc . . .
My guess is that variations in fuel pressure is the biggest factor, followed variations in the injectors and deposits building up in the injectors restricting flow. Our current two BMW's OBC MPG/actual MPG is slowly drifting up, suggesting that fuel flow (and maybe fuel pressure) is going down slightly with time. For diesels, fuel viscosity changes with temperature could be significant.
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  #36  
Old 09-05-2019, 11:09 AM
Autoputzer Autoputzer is offline
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When I was L6S indoctrinated, the boogiemen were: transportation, retooling, inventory, rework, and inspection. The thing is that each one of the boogiemen is a cure for at least one of the other boogiemen. The problem is that L6S-ers get fixated on eliminating just one of the boogiemen, and accidently let the other ones grow into monsters.

I managed SONAR system that had portable test sets about the size of a large ice chest. They made enough of them during initial production to last 25 years. Each of the 21 ships with that SONAR got one, and the rest, maybe ten, were stacked in a warehouse in Pennsylvania. The spare parts guy called me up one day, saying that their warehouse management algorithm told him to throw all of them away. He figured he'd better call me first, though. To replace those ten test sets would cost maybe $1M, considering all the necessary design refresh and re-tooling costs. So, the algorithm was going to throw away $1M of stuff to free up literally about 15 square feet of floor space.

Last edited by Autoputzer; 09-05-2019 at 12:22 PM.
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  #37  
Old 09-05-2019, 11:19 AM
Autoputzer Autoputzer is offline
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I really enjoyed your post.

Frau Putzer must be one cool lady

Why blend 1/3 and 2/3 ??

I would assume that 50- 50 of 87 and 93 gas would make it 91 gas
1. Thanks.

2. She has her moments.

3. Octane is roughly a weighted average:

https://www.sunocoracefuels.com/tech...ulating-octane

A 50%/50% blend of 87 AKI and 93 AKI would get you 90 AKI.

A 50%/50% blend of 89 AKI and 93 AKI would get you 91 AKI.

A 33%/67% blend of 87 AKI and 93 AKI would get you 91 AKI, and generally be a little cheaper than the 50%/50% blend of 89 AKI and 93 AKI.

Gas pumps make 89 AKI from a 67%/33% blend of 87 AKI and 93 AKI, or a 50%/50% blend of 87 AKI and 91 AKI. But, they generally charge disproportionally more for 89 AKI.

Last edited by Autoputzer; 09-05-2019 at 11:53 AM.
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  #38  
Old 09-05-2019, 12:34 PM
Gary214 Gary214 is offline
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Originally Posted by Autoputzer View Post
1. Thanks.

2. She has her moments.

3. Octane is roughly a weighted average:

https://www.sunocoracefuels.com/tech...ulating-octane

A 50%/50% blend of 87 AKI and 93 AKI would get you 90 AKI.

A 50%/50% blend of 89 AKI and 93 AKI would get you 91 AKI.

A 33%/67% blend of 87 AKI and 93 AKI would get you 91 AKI, and generally be a little cheaper than the 50%/50% blend of 89 AKI and 93 AKI.

Gas pumps make 89 AKI from a 67%/33% blend of 87 AKI and 93 AKI, or a 50%/50% blend of 87 AKI and 91 AKI. But, they generally charge disproportionally more for 89 AKI
.




Very good info. I am going to use this to fill my x5
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  #39  
Old 09-05-2019, 09:20 PM
RPsX5d RPsX5d is offline
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Quote:
Originally Posted by Autoputzer View Post
My guess is that variations in fuel pressure is the biggest factor, followed variations in the injectors and deposits building up in the injectors restricting flow. Our current two BMW's OBC MPG/actual MPG is slowly drifting up, suggesting that fuel flow (and maybe fuel pressure) is going down slightly with time. For diesels, fuel viscosity changes with temperature could be significant.
Not disagreeing with you . . . only trying to understand what is happening in my case . . .

In my case, initially and for several years (up to about 110,000 miles) both indicated and manual mpg tracked pretty well . . . within a tenth of an mpg.

If, as you suggest, deposits, injector, etc is causing an issue - i.e. what the computer commanded does not match the fuel delivered, then yes, I agree the indicated mpg would be off.

But in my case, the manual mpg continues to remain the same as before . . . indicating no serious deposit buildup or injector issues - i.e. everything is running as before, only indicated mpg changed.

If there were some real issues like deposits and/or injector issues . . . I would have expected the manual mpg to also change as the actual fuel delivered is different. i.e. how can the deposits only impact the indicated mpg and not the manual mpg?
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Old 09-06-2019, 04:37 AM
Autoputzer Autoputzer is offline
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Originally Posted by RPsX5d View Post
Not disagreeing with you . . . only trying to understand what is happening in my case . . .

In my case, initially and for several years (up to about 110,000 miles) both indicated and manual mpg tracked pretty well . . . within a tenth of an mpg.

If, as you suggest, deposits, injector, etc is causing an issue - i.e. what the computer commanded does not match the fuel delivered, then yes, I agree the indicated mpg would be off.

But in my case, the manual mpg continues to remain the same as before . . . indicating no serious deposit buildup or injector issues - i.e. everything is running as before, only indicated mpg changed.

If there were some real issues like deposits and/or injector issues . . . I would have expected the manual mpg to also change as the actual fuel delivered is different. i.e. how can the deposits only impact the indicated mpg and not the manual mpg?
The performance of sensors and electronics "drift" over time. Precision electronic instruments typically have to be periodically calibrated to function properly. I worked on automated test equipment that needed to be calibrated about every six months. We'd spend a day getting everything dialed in perfect and working great. In a few months, the equipment would start acting up again and need calibration again. We'd schedule re-calibration for every six months. But, with heavy use the equipment needed more frequent calibration.

Both of my direct-injected BMW's have had the actual MPG steadily increase when compared to the indicated MPG. My 535i started out just about perfectly accurate. After 60k miles, the actual MPG was about 2.5% higher than the indicated MPG. The X3 started out with the actual MPG being about 5% less than the indicated MPG. But, it steadily became more accurate. After about 13k miles, the actual MPG was only about 3.5% less than the indicated MPG. In both cars, the trend was that the fuel injection was delivering less and less fuel (better actual MPG) over time than the computer "thought" it was delivering.

I started tracking OBC MPG error in my old port-injected BMW from about 40k miles, and up to 115k miles when I sold the car. The error was more consistent than with my two direct-injected BMW's, with actual MPG being about 5% less than the indicated MPG.

The performance of mechanical systems also change over time. Back in the bad old days of carburetors, mechanical distributors, and ignition "points," cars needed tune-ups to stay running something close to right. A big breakthrough was the invention of the O2 sensor. That "feedback" allows electronic fuel injection systems to sort of continually tune themselves, and correct for the measurement errors and performance variations in the systems.

Last edited by Autoputzer; 09-06-2019 at 05:00 AM.
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  #41  
Old 09-06-2019, 06:03 PM
RPsX5d RPsX5d is offline
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Thanks!

Quite possible I don't fully understand how these things work.

In my case, since the car was new (4 miles on the odometer) up until now (9 years later and 150,000+ miles), the manual mpg pretty much stayed the same - about 22.5 mpg.

To me that means the sensors, the computer control, injectors, etc etc have not shifted/changed much.

For the first 110,000 miles or so, both indicated mpg and manual mpg matched . . . meaning the K-factor setting done at the factory was pretty close. Then the indicated mpg started to drift down, while the manual mpg remained constant.

If the electronics/sensors used to compute the mpg is different from those used to operate the vehicle, then I can understand your explanation - the electronics/sensors used to operate the vehicle remained constant but one or more sensors unique to the indicated mpg electronics started to drift and is now causing this difference.

I find it hard to believe there is a completely different set of electronics/sensors to compute the indicated mpg.

I suppose best to stick with manual mpg . . . no ambiguity about that figure!
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Old 09-06-2019, 06:35 PM
Autoputzer Autoputzer is offline
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The data is just a bonus from the system that actually controls the engine, no second system. Germany required fuel consumption meters in cars way back in the 1980's.

If only the indicated MPG went down, that means the car "thinks" it's using more gas than it really is. That would be consistent with lower fuel flow, and that could be due to lower fuel pressure or some restriction of flow in the injectors. But, the O2 sensors make sure the car doesn't run too lean.

I read somewhere that too low fuel pressure can actually cause excessive fuel flow. The only explanation I can come up with for that is that the fuel pressure is used somehow to shut the fuel flow off in a pulsed injector.

My Cobalt only has one MPG display, not a separate OBC MPG and Trip Computer MPG. I always reset the MPG and trip odometer when I fill up. But, I'd like to know my MPG's in between fill-ups. So, I made a(nother) spreadsheet. I enter the starting and ending MPG and trip odometer mileage to calculate the MPG on an in-between-fill-up trip. Since there's some uncertainty because mileage and MPG is only reported to one decimal place, there is an uncertainty window.

I took a friend to the airport, 44.1 miles by the time I got home. The Cobalt got 30.0 +- 0.4 MPG. Under perfect conditions, I've hit 40 MPG in that car. Amazingly, I've seen 38 MPG in Frau Putzer's G01 X3 xDrive 3.0i. But, not with her driving it.
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  #43  
Old 09-06-2019, 07:46 PM
RPsX5d RPsX5d is offline
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Quote:
Originally Posted by Autoputzer View Post
. . . If only the indicated MPG went down, that means the car "thinks" it's using more gas than it really is. That would be consistent with lower fuel flow, and that could be due to lower fuel pressure or some restriction of flow in the injectors. But, the O2 sensors make sure the car doesn't run too lean. . . .
I understand what you are saying . . . but if the car "thinks" it is consuming more fuel (lower indicated mpg) and the O2 sensor is correcting it . . . won't the indicated mpg then creep back towards the actual/manual mpg?

i.e. if the K-factor is off, there will be a constant difference between indicated and manual mpg. What I don't get is the manual mpg stays the same, K-factor has not changed . . . but the indicated mpg is drifting . . .
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Old 09-06-2019, 11:57 PM
Autoputzer Autoputzer is offline
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Quote:
Originally Posted by RPsX5d View Post
I understand what you are saying . . . but if the car "thinks" it is consuming more fuel (lower indicated mpg) and the O2 sensor is correcting it . . . won't the indicated mpg then creep back towards the actual/manual mpg?

i.e. if the K-factor is off, there will be a constant difference between indicated and manual mpg. What I don't get is the manual mpg stays the same, K-factor has not changed . . . but the indicated mpg is drifting . . .
Because of feedback from the O2 sensors, the fuel injection systems logic is always deciding "I need more fuel" or "I need less fuel." It controls the more or less fuel by adjusting the duration of the fuel injector pulses.

There's a function (equation or graph) that describes how much fuel is delivered given particular duration of the pulse on a perfectly operating injector.

If fuel pressure is low or an injector is restricted, the injectors will deliver less fuel that would be predicted by the function (equation or graph). The feedback from the O2 sensors will correct the lean condition (not enough fuel) by saying "I need me more fuel," and by increasing the duration of the injector pulses. This then provides enough fuel, and the engine runs like it's supposed to (power and drivability, low emissions, and high fuel economy) .... the beauty of feedback!

The MPG displays (on-board and trip) don't really know how much fuel is really being delivered. They only know how long the injector pulses are. They assume the injectors are delivering the amount of fuel as described by the function (equation or graph), and then calculate MPG according to the function (equation or graph).

The K Factor is the ratio of how much fuel was actually delivered (converted to MPG) and how much fuel was theoretically delivered according to the pulse durations and the function (equation or graph) (and converted to MPG). The K factor is used to manually adjust the display, once the technician or car owner knows the ratio of actual MPG/displayed MPG (or displayed MPG/actual MPG).

The fuel injection system never sees the K factor. It just decides "I need more fuel" or "I need less fuel," and adjusts the injector pulse durations accordingly.

If your actual MPG is different than your displayed MPG, that only means that the injectors are not behaving according to that function (equation or graph). But, because of feedback from the O2 sensors, they are still providing the correct amount of fuel.

Last edited by Autoputzer; 09-07-2019 at 12:05 AM.
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  #45  
Old 09-07-2019, 05:04 PM
RPsX5d RPsX5d is offline
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Thanks again . . . now it all makes sense . . .
  • Assuming the K-factor is set correctly at the factory, initially both indicated mpg and manual mpg match
  • Over time injectors start to clog up, rail pressure decreases, etc . . . causing less fuel to flow into the chamber for a given number of pulses/time.
  • O2 sensor senses this reduced fuel condition, based on O2 sensor feedback, computer commands a few more pulses . . . until O2 feedback say it is fine.
  • From a manual calculation perspective, mpg will not change as the same amount of fuel is consumed for a given distance driven . . . except now it takes a few more pulses to get the same amount of fuel into the chamber.
  • Manual mpg calculation does not involve pulses or time, so it remains the same.
  • I am assuming there is no flow meter in this vehicle . . . so fuel volume is computed indirectly using rail pressure and the number of pulses or time.
  • Over time fuel flow gets restricted due to reduced rail pressure, carbon build-up, etc . . . so for the same number of pulses, less fuel is consumed (lean situation).
  • Countering this lean effect could be an injector leak (rich) . . . but this won't result in improved manual (or indicated) mpg . . . because fuel is simply getting dumped, not delivered at a very precise moment.
All of this makes perfect logical sense . . . since indicated mpg uses pulses to determine fuel consumed, more pulses mean more fuel, and reduced indicated mpg . . . but in reality, due to lower rail pressure and/or carbon build-up, the same amount of fuel is consumed for a mile driven . . . which keeps the manual mpg figure the same.

Second, given more pulses are needed to deliver the same amount of fuel, the indicated mpg will always trail (less than) the manual mpg . . . and that is indeed what I am seeing.

Finally, if I had tracked indicated mpg vs manual mpg from when the vehicle was brand new . . . I could have used that chart as a VERY crude way to check for carbon build-up, injector, rail pressure issues . . . it would have started off as a 45 line and eventually the slope will start to drop below 45 (plotting manual mpg as the independent variable along the x-axis).

If, all of this is correct, indicated mpg has to trail manual mpg with time . . . i.e. it can never be the other way around.

Once again a sincere thank you for all your time to get this straight in my mind!
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  #46  
Old 09-07-2019, 05:56 PM
Autoputzer Autoputzer is offline
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There's no adjustment of the K factor at the factory. They assume that the calculated fuel flow is the same as the actual fuel flow. The factory default K factor is 1000. It's probably random which MPG is higher when the car is new. Other than that, you pretty much got it!

It's been my experience with two direct-injected BMW's is that the indicated MPG increases over time when compared to the actual MPG. My 2014 535i started out just about perfectly accurate, and over 60k miles the indicated MPG drifted to being about 2.5% more than the actual MPG, meaning the fuel flow became less and less than the computer thought it was. Frau Putzer's 2018 X3 indicated MPG started out about 4% below the actual MPG and drifted to about 3.5% below actual MPG over about 13k miles. Initially, the fuel flow was more than the computer thought it was, but the trend was that the fuel flow was going toward being less than the computer thought.

My port injected 2007 Chevy Cobalt's actual MPG started out about 2% over the indicated MPG. But, after 12 years and 123k miles, it's just about spot on now.

The precise way to measure indicated MPG over several tanks of fuel is to calculate the "on-board computer (OBC) fuel used for each tank, being the miles driven on the tank of fuel divided by the OBC MPG. You then sum up the OBC fuel used for each tank to get the multiple tanks OBC fuel used. With the multiple tanks OBC fuel used, you can calculate the multiple tanks OBC MPG. Then you compare the multiple tanks actual MPG and OBC MPG to determine what to change the K factor to. Using the data for multiple tanks reduces the randomness in actual MPG for a single tank of fuel. That's how I (eventually) determined my new K factors, and got my indicated MPG so close to the actual MPG.

Depending on the particular BMW, the new K factor can be:

K(new) = (OBC MPG / actual MPG) x K(current) (as for Frau Putzer's G01 X3)

or:

K(new) = (actual MPG / OBC MPG) x K(current) (as for my F10 535i)
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  #47  
Old 09-07-2019, 06:16 PM
RPsX5d RPsX5d is offline
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Yes, I understand what you are saying about using "multiple tank OBC" numbers when determining the K-factor . . .

For me, indicated mpg is of little value . . . I almost never look at it, most of the time that display is set to "range" . . . and yes, I understand if mpg is incorrect, range will also be incorrect . . . BUT if indicated mpg is below actual/manual mpg, then I have a built-in margin of error in my favor. In any event, I almost never let the fuel tank that close to empty . . . just don't want to run out of fuel and then have to deal with air pockets, etc.

It was by accident I saw the indicated mpg had dropped . . . when I was clicking through it I saw 18.5 mpg and I knew that number was closer to 23 mpg for a very long time . . . that lead me to believe - incorrectly - that something was wrong, and as a result, the mpg had dropped. I should have just checked the manual mpg then . . . and I would have realized nothing was wrong, and that also would have explained why I was not getting any fault codes!

Curious - how do you explain your indicated mpg being higher than the actual mpg? Rail pressure sensor going bad and is now reading a lower pressure than it actually is?
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  #48  
Old 09-07-2019, 06:47 PM
Autoputzer Autoputzer is offline
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The range display is based on the recent MPG and on the measured fuel tank level. The fuel tank level accuracy drifts a lot, far more than the indicated MPG drifts, and the definition of "empty" varies BMW to BMW. When my 535i gets to zero range, I have about 1.5 gallons left. If Frau Putzer's X3 would get to zero range, it would be pretty close to being really empty.

When I'm stretching out a tank of fuel as much as I can, as I'm wont to do, I calculate fuel burned (trip odometer/indicated MPG). This works since I always reset the OBC MPG and trip odometer when I fill up the cars. I also factor in the OBC MPG error when doing my fuel-burned calculations. For 2019 my average fill up of my 535i's 18.5 gallon tank has been 17.0 gallons, and the average fill-up of Frau Putzer's X3's 17.2 gallon tank ahs been 14.6 gallons. I like filling up in the early morning (to avoid the dregs who hang around convenience stores, and lines at the pumps), and she only lets me do that on Thursdays since she leaves the house later on Thursdays. So, those X3 fill-ups are sometime premature by my standards. We're eventually moving to the foothills of the Smoky Mountains from flat Florida. So, I'll have to fill up my tanks sooner to avoid fuel starving.

I've asked BMW to add "Fuel Consumed" to the OBC and Trip Computer displays.

Yes, there are fuel pressure sensors, and those values are used to adjust the output of the fuel pumps (low pressure and high pressure) This is another example of "feedback." I suspect (but can't be sure) that the variation in indicated MPG is primarily based on drift of those pressure sensors.
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  #49  
Old 09-08-2019, 09:28 AM
RPsX5d RPsX5d is offline
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Thanks again . . . now everything is clicking into place nicely . . . at least for now I am comfortable to be in a watch-and-see mode . . .

When I first noticed this drop in indicated mpg, none of my searches yielded anything definitive, I did post this question here . . . thinking one or more sensors may have gone bad and as a result, the computer has shifted to a different part of the air/fuel ratio map . . . with no fault codes, I didn't want to do anything . . .

As always, never skip the obvious and simple checks . . .

BTW O2 sensor . . . what's the service life of these sensors? Mine is a bit over 100,000 miles now. I view these O2 sensors, like the NOx sensors, as maintenance items.

On our M57, there is only one O2 sensor - correct?
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