Do you know if there is a component that measures the "amount" of fuel being fed into the engine?

 

The accelerator pedal is drive by wire and sends input to the ECU computer to allow for more fuel to be mixed with air in the engine so that the car can accelerate or maintain speed.

So the ECU or computer ultimately monitors and measures the fuel to air mixture.

Why do you ask?

 

So does it moniter the Air fuel mixture by results of its combustion or by actually measuring the amount of air passing in and measuring the amount of fuel being fed in?

I ask because I'm trying to help my dealership troubleshoot a problem with the reported fuel mileage being much too high as well as an overestimation of miles left on the tank of fuel. My suspicians are that there's a sensor of some sort that reports to the computer how much fuel is being fed into the engine. If you take the miles travel over t amount of time and divide into that the amount of fuel consumed over t amount of time, then you can get MPG. So if the amount of fuel is underestimated, it can lead to the bloated MPG and miles left display that I'm experiencing.

 

If by amount you mean vol. (gallons, pints, etc) the answer is no. minihume is correct, The engine is controlled by the ecu. The ecu measures various inputs, mostly, your right foot, emmissions, O2, etc. The system is more concerned with air/fuel ratios than amount of fuel.

Motor on

 

So where does the computer get the amount of fuel consumed to be able to calculate MPG?

 

>>So where does the computer get the amount of fuel consumed to be able to calculate MPG?


I'm sure there's a flow meter in there somewhere............

 

>>I'm sure there's a flow meter in there somewhere............



I'm not sure there is. :smile:

Many ECUs, I don't know if the MINI is one of them,
calculate MPG using vehicle speed, engine RPM and
fuel injector duration, assuming the fuel-rail pressure
is some known constant.

That's why they are not superbly accurate.

 

>>fuel injector duration, assuming the fuel-rail pressure
>>is some known constant.

Couldn't this be considered a "flow meter"? It must be able to determine the amount of fuel being passed thru the fuel injectors assuming a constant pressue. What happens is the fuel injector becomes slightly clogged? Then the MPG would be stated much too low. What happens when the fuel-rail pressure is too high? Then the MPG would be reading much too high since more fuel is being pushed thru than the computer thinks, right?

 

>>What happens if the fuel injector becomes slightly clogged?

This is actually pretty cool.

The engine management computer (EMC) watches the output
from the exhaust-gas oxygen sensors, and if the value is not in
the range it wants for "perfect" combustion, it runs the fuel injectors
a little longer or a little shorter than the normal setting.

So, as the fuel injectors get clogged, the O2 sensor starts to tell
the EMC that the exhaust gasses were the result of a too-lean
combustion, and the EMC lets the injectors stay open a little bit longer.

You can actually read out the values for "long-term fuel-trim" using
the OBD connector. I've done it.

>> What happens when the fuel-rail pressure is too high?

Well, they could monitor the fuel-rain pressure, or they could have a
fuel-rail pressure regulator.

But if it's too high or low, your OBC display reads wrong.

That's why I calculate the fuel-economy using the actual
odometer reading and the actual fuel-pump reading at the
gas station.

 

>>That's why I calculate the fuel-economy using the actual
>>odometer reading and the actual fuel-pump reading at the
>>gas station.

So do I and I keep great notes, that's the only reason they've gone as far as replacing the head unit in the dash. They thought that it was what was causing the incorrect readings. Now with nearly three tanks of fuel since that replacement, it still reads high by the same 15%-20% factor on the MPG calculations. The estimate on miles left in the tank are much worse than they were with the old unit. When I fill up it says I have 500-600 miles on the tank. The old on used to say 410-430 on the tank. But I still only can squeeze out 350 on a tank under normal driving conditions. This MPG computation abnormality has been going on since I bought the car new. But it is trending worse. When I plot the 15% difference on a graph you can see it steadily rising from the initial ~12% difference to where it is now ~18% difference. So what ever if the problem, it is getting worse and not better.

 

Since the day I got my first car out of college, I've always tracked mileage in my cars, so yes, I'm a bit of a nudge about this stuff as well. I've got a 97 Explorer with the onboard displays. From the day I bought it, the computed MPG has always been high by an average 15% (13-17% range). My gut feel has always been that Ford has fudged on the high side when making assumptions about some variable in the MPG equation (either fuel delivery rate or raw amount). They probably fudge high to make the numbers look better. With a V8 and AWD, I get a screaming 15-18 MPG in commute driving.

On my computer, it also states the gallons used and gallons remaining. The gallons used is also always low by an average of 15%, so the only thing I can think of is that whatever sensor they use to determine fuel flow (and thus the fuel usage), is also used for the MPG calculation. I don't know if this is a technical issue, or a marketing issue (being a marketing guy, I tend to go with the latter explanation )

I had really hoped the computer in the MINI would be more accurate than the one in my Explorer, but after reading the many postings about this subject, I've concluded that it's an issue I'll have to live with, which is OK as long as it doesn't vary too much.

Personally, I think they should be able to accurately measure the fuel usage (flow meter or the like). Why can't they, or don't they ??? Your guess is as good as mine.

Just my .02

BB

 

Hi Blue, the issue here is there's people out there with MINIs that are getting within .5 MPG between hand calculated and computer indicated. I've also heard from people that their's reads much too low. So I know this issue is one of adjustment... I just have to walk my service department guys thru it step by step and was hoping with all the technical people around here on NAM, that there'd be one who knew for sure how the fuel consumption is actually registered.

On a related note, the indication for how long until next service doesn't decrease by miles driven even though it indicates miles left until service. I didn't get my first service until 13,300. When it was reset it said 15,000 miles until the next one. Now I have 20,500 miles on the car and it says I have another 10500 to go. So by motoring 7,200 miles it only decrease my indicator by 4,500 miles!!! So when the service department told me that the miles left are based upon how much fuel is consumed, that got me thinking that the sensor itself was miscalibrated causing too low of indications to both the MPG computer and the service computer.