[RPsX5d]

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!

 

[Autoputzer]

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.

 

[RPsX5d]

. . . 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 . . .

 

[Autoputzer]

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.

 

[RPsX5d]

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!

 

[Autoputzer]

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)

 

[RPsX5d]

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?

 

[Autoputzer]

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.

 

[RPsX5d]

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?

 

[Autoputzer]

I've burned four tanks of gas in the 2018 X3 since having the OBC MPG corrected, and two tanks of gas in the 2014 535i since I corrected the OBC MPG myself.

The X3's cumulative correction factor since adjustment is -0.19% (actual MPG is 0.19% lower than the OBC MPG).

The 535i's cumulative correction factor since adjustment is -0.18%.