MPG significantly drops after replacing battery

will007 · Post by **will007** » 24 Sep 2013, 17:19

My car is 2006 XC90.
I took my car to walmart last Saturday to replace the 7 years old battery.
I oberserved the MPG calculated by the MCU gradually drops from 21.8 to 19.8 now in less than 50 miles.
I read some articles saying it is caused by the learning mode. When the battery is removed, the MCU is reset. Some people says it requires go to a specefic process to get a better habit. Unfortunately I went to free way right after I replaced the battery.

Any idea?

Thanks.

difflock54 · Post by **difflock54** » 24 Sep 2013, 18:25

Given that freeway driving normally will give you better MPG, surely that would be a plus if your theory is correct.
Had you done some slow city driving / stop and start running that would reset the ECM to achieve poorer resulting mileages per gallon.

Post by **MoVolvos** » 24 Sep 2013, 18:41

**

I think I read somewhere you need to remove the + Cable and ground it with the - Cable in order to reset the TCM?

Blessings,

BKM

**

difflock54 · Post by **difflock54** » 24 Sep 2013, 19:35

Yes, I have read that from a number of posts also. several claim this procedure works in lieu of taking the vehicle to a dealer to reprogram.
Completely disconnect the battery then place the cars + and - cables together in contact for about 15-20 seconds.
After that re-secure each terminal back to the battery posts in their correct polarity naturally.
Radio head unit security code will need to be reset also.

will007 · Post by **will007** » 24 Sep 2013, 21:12

I do understand how to reset the mcu.
The probelm is I don't know how to generate a best new driving mode.
Some peoples says after replacing the battery the car should be put to idle for 5min and slow driving for some, and then...
I didn't remember exactly.

My previous car 99 camry doesn't have this kind of problem at all after changing the battery.
Therefore I believe it depends.
Thanks.

will007 · Post by **will007** » 24 Sep 2013, 23:29

for example: http://repairpal.com/engine-stalling-af ... eplacement
this is somewhat extreme example.

will007 · Post by **will007** » 24 Sep 2013, 23:32

this is another extreme example: http://www.bmwmoa.org/forum/showthread. ... el-mileage
MPG drops from 42-45 to 30-33 after replacing battery on a BMW. I didn't get a clear idea on how to solve the problem.