big drop off in mpg

[diamond777]

91 535i auto 209k runs great -but the mpg went from 21 all the time to 16-18 after a battery change-any ideas????

[632 Regal]

hole in the tank?
91 535i auto 209k runs great -but the mpg went from 21 all the time to 16-18 after a battery change-any ideas????

[Blitzkrieg Bob]

mine dropped after a few tanks of that crap

[SRR2]

The usual culprits are O2 sensor and AFM. The battery change might be a concidence, exacerbated by resetting adaptation.

[califblue]

are you doing mileage by the computer/tripmeter or your actual odometer mileage?

if dojng the first I'll bet you have to recalibrate the puter

[Blitzkrieg Bob]

Since you'v e disconnected the battery recently, the OBC has a shorter duration and mileage data to avergae by.

Just for fun reset AVG 2 and see how quickly your AVG MPG changes with a few stop lights and coasting down a hill.

I know the first few hours after a disconnect, the AVG is quite sensitive to changes in driving conditions.

[Tiger]

Ethanol? Will reduce 10%...

[BillionPa]

actually a 10% ethanol blend reduces mileage by about 20%.

I would think the ecu just needs to relearn its timing adjustments again over a few dozen "warm" miles.

[ThoreauHD]

Who's spiking with 10% ethanol now?

[Bellicose Right Winger]

Not quite. MPG is directly proportional to the heating value of the fuel. Gasoline is ~115,000 btu/gal, ethanol is ~77,000 btu/gal.

(1) gallon E10 is .9*115,000 + .1* 77,000 = 111,200 btu/gal

111,200/115,000 = 96.7%

10% ethanol reduces mpg about 3.3%

Paul Shovestul



......actually a 10% ethanol blend reduces mileage by about 20%

[Hector]

so applying 3.3% to 21 mpg drops fuel economy to 20.3%. If the O2 sensor is bad, that will drop mpg another 5% (at least this was the case in my m30)so you'll be at 19.3%. The 1.3-3.3% difference could be due to one or a combination of the following:

vacuum leak
tire inflation
spark plugs
air filter
AFM
temp sensor





91 535i auto 209k runs great -but the mpg went from 21 all the time to 16-18 after a battery change-any ideas????

[vuyodaddy]

Sorry, really noob question coming up... but what is AFM? My mileage once dropped from 21 to now 17-18.5mpg, using the conventional menthod of dividing miles by gallons of gas filled up at the station. I should really check that and the O2 sensor...

[632 Regal]

air flow meter.

Sorry, really noob question coming up... but what is AFM? My mileage once dropped from 21 to now 17-18.5mpg, using the conventional menthod of dividing miles by gallons of gas filled up at the station. I should really check that and the O2 sensor...

[Bill R.]

been using 10% oxygenated fuels for at least the last 10 years , six months out of the year, is that the typical drop is around 10% in mileage.




Not quite. MPG is directly proportional to the heating value of the fuel. Gasoline is ~115,000 btu/gal, ethanol is ~77,000 btu/gal.

(1) gallon E10 is .9*115,000 + .1* 77,000 = 111,200 btu/gal

111,200/115,000 = 96.7%

10% ethanol reduces mpg about 3.3%

Paul Shovestul

[Bellicose Right Winger]

Any chance this is just the result of resetting the OBC avg mpg calc?
Before the battery disconnect when was the last time the OBC avg mpg calc was reset via S/R button?
If the OBC had thousands of miles of highway fuel consumption data in it's average, then resetting the OBC calc by disconnecting battery and driving in traffic would result in a lower avg.

Paul Shovestul




91 535i auto 209k runs great -but the mpg went from 21 all the time to 16-18 after a battery change-any ideas????

[Bill R.]

imperial gallon for his old average....



Any chance this is just the result of resetting the OBC avg mpg calc?
Before the battery disconnect when was the last time the OBC avg mpg calc was reset via S/R button?
If the OBC had thousands of miles of highway fuel consumption data in it's average, then resetting the OBC calc by disconnecting battery and driving in traffic would result in a lower avg.

Paul Shovestul

[Bellicose Right Winger]

MPG discussions are difficult because 1) it's nearly impossible to keep accurate enough records and 2) no two tanks are consumed under the same conditions. I record every fuel purchase and routinely see tank-to-tank MPG variations of 5-10%. Driving conditions, style and the difficulty in filling tank to same level everytime are the biggest obstacles.

You would have to take all the ethanol out and substitute a completely inert filler to get a 10% reduction in MPG.

Paul Shovestul



In theory that will work out to 3.3 but the reality here where we have been using 10% oxygenated fuels for at least the last 10 years , six months out of the year, is that the typical drop is around 10% in mileage.

[632 Regal]

I average 10% used to get 23mpg now getting 19.1

been using 10% oxygenated fuels for at least the last 10 years , six months out of the year, is that the typical drop is around 10% in mileage.