Fuel mileage conflicted

erhayes

Contributor
:
2022CX5 PP
I frequently checked my fuel consumption mathematically and via dash readout during the first years of ownership. The two methods were consistent to + or - 0.5 mpg. I gave up the math idea since the dash readout was so reliable. I decided to recheck again recently and the 0.5 mpg has increased to 3 mpg. The hand math calculates to the same old 28 to 28.5 mpg but, the dash readout shows ~25 mpg. I suspect that the dash readout is the problem. Where is the sensor located that samples the fuel flow? Any ideas as to the cause? Thank you Ed
 
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Have you replace your tires recently? I found that replacing with General Altimax RT43 7k miles ago immediately dropped the dash reading about 1.5 mpg. Lately it's been creeping back to where it was before the switch. Go figure!
 
I did replace my tires with AltaMax RT43 but, that doesn't enplane the discrepancy between calculated and dash readout. I also noticed the loss of mileage with the new tires and found the increase in circumference was about 0.69 % if I remember correctly. Ed
 
I did replace my tires with AltaMax RT43 but, that doesn't enplane the discrepancy between calculated and dash readout. I also noticed the loss of mileage with the new tires and found the increase in circumference was about 0.69 % if I remember correctly. Ed
That could be it. I never bothered to investigate the cause as one mpg more or less isn't that important to me.
 
I frequently checked my fuel consumption mathematically and via dash readout during the first years of ownership. The two methods were consistent to + or - 0.5 mpg. I gave up the math idea since the dash readout was so reliable. I decided to recheck again recently and the 0.5 mpg has increased to 3 mpg. The hand math calculates to the same old 28 to 28.5 mpg but, the dash readout shows ~25 mpg. I suspect that the dash readout is the problem. Where is the sensor located that samples the fuel flow? Any ideas as to the cause? Thank you Ed
Not sure where the flow meter is located for dash MPG calculation, but I'd try to pour a couple bottles of Techron to clean up the fuel path through the flow meter which may restore the accuracy of the flow meter.
 
Is this consistent across several fill ups from different fueling stations?
 
Dash readout should be straight off what the ECU is doing with the fuel. A big discrepancy is odd; I'd tend to think the fuel station meter would be less accurate.
 
I frequently checked my fuel consumption mathematically and via dash readout during the first years of ownership. The two methods were consistent to + or - 0.5 mpg. I gave up the math idea since the dash readout was so reliable. I decided to recheck again recently and the 0.5 mpg has increased to 3 mpg. The hand math calculates to the same old 28 to 28.5 mpg but, the dash readout shows ~25 mpg. I suspect that the dash readout is the problem. Where is the sensor located that samples the fuel flow? Any ideas as to the cause? Thank you Ed

On most cars the Mass Air Flow sensor (MAF) is used to estimate the fuel consumption.
The MAF measures how much air the engine is using and it knows that for every 14.7 grams of air, one gram of fuel is used.

If your MAF is flaky, you'll see a discrepancy between pump and dash readouts. The car will still be able to run OK because the actual amount if fuel is adjusted in real-time based on the oxygen sensors in the exhaust.

Another factor to consider is fuel quality.
If the fuel has high energy content (ethanol free gas for example) the engine will use less fuel per unit air and the dash readout will be lower than the pump calculation by ~3%.
 
The confusing thing to me is that the calculated value is larger than the dash read out and as mentioned before the values were +/-0.5 dash to ~ 3.0 calculated. I expect any malfunction of the MAF would result in a CEL. Ed
 
Not sure where the flow meter is located for dash MPG calculation, but I'd try to pour a couple bottles of Techron to clean up the fuel path through the flow meter which may restore the accuracy of the flow meter.

There is no "fuel flow meter" in this modern computer controlled system. At least not in the sense of the in-fuel line paddle wheel type sensors used in years gone by.

The fuel flow will be a value calculated by the ecu based injector duty cycles. In simple terms, the ecu knows how much fuel it is directing the injectors to inject which of course is influenced by signals from the myriad sensors in the system. This allows total fuel flow to be derived. Mpg is then calculated based on flow and distance/speed.

Things like dirty injectors therefore would be potential causes of inaccuracies. That, along with new tyres with a slightly different overall diameter and even tyre pressures that havent been checked for a while, dirty engine air filter or even dirty fuel filters, are likely to be contributing factors.

If it was me, I would run a few shots of a good injector/fuel system cleaner thru, check the tyre pressures, and drop a new air filter in. If that didnt improve things, I would think about having the injectors professionally cleaned.

There is of course another variable in this whole deal, and that is the gas station at which you refuel the car. The gas station variables include:
- pump accuracy
- pump flow speed
- the angle that the car is sitting - if the tarmac is flat or at a slight uphill or downhill angle, which will contribute to how full your tank is filled
- whether you fill to the same "fullness" each time.
- how much ethanol they have quietly added to their fuel...

So if you have changed gas stations, or your existing station has recalibrated their pumps recently, or put in new higher flow pumps ....
 
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The confusing thing to me is that the calculated value is larger than the dash read out and as mentioned before the values were +/-0.5 dash to ~ 3.0 calculated. I expect any malfunction of the MAF would result in a CEL. Ed

Not necessarily. Small MAF reading variations get compensated automatically by the fuel trims.
You can hook up a reader and read your long term fuel trim. It'll probably be negative (meaning the engine is injecting less fuel than the MAF calculation tells it to).

Did the discrepancy between pump and dash occur around the time Irma hit? Maybe you're just getting some really good gas now?
 
I don't really know when the discrepancy occurred because for the past several years or so, I only used the dash value since every time I check it with the calculated value it was good for many years. I started comparison checking about a month before our hurricane. Tomorrow I will clean the MAF sensor and report back after I fill up and check this tank. Thanks for all the information and if anyone has another idea, please post it. Thanks so much. Ed Hayes
 
I don't really know when the discrepancy occurred because for the past several years or so, I only used the dash value since every time I check it with the calculated value it was good for many years. I started comparison checking about a month before our hurricane. Tomorrow I will clean the MAF sensor and report back after I fill up and check this tank. Thanks for all the information and if anyone has another idea, please post it. Thanks so much. Ed Hayes

I wouldn't bother cleaning it. I'd say this falls firmly in the if it ain't broke don't fix it category.
Your gas pump measured fuel mileage is still good, and as long as you don't have any other concerns I really would advise against cleaning the MAF.
 
I wouldn't bother cleaning it. I'd say this falls firmly in the if it ain't broke don't fix it category.
Your gas pump measured fuel mileage is still good, and as long as you don't have any other concerns I really would advise against cleaning the MAF.
Amen!
 
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