Can a low battery charge be related to engine misfires?
 

My 2008 Cooper S is throwing misfire codes P0300, P0301, P0302, and P0303. Also, starts harder and stumbles a bit on cold starts after sitting a few days. The last time the CEL came on was right after such a start. Starts fine after running a while.

I just checked the battery before starting up in the morning and the charge is low, 12.1 V. After driving about 30 min and turning engine off, it's 12.4 V. The battery is five years old. Spark plugs were replaced four years ago; original coils; swapping positions hasn't had an effect on misfires.

According to this website a low battery can affect injectors and cause lean misfires, but I haven't found any additional information:
http://www.autotap.com/techlibrary/d...ion_system.asp

Any thoughts or experience about a relationship between misfires and low battery charge would be much appreciated? Thanks for reading.

Regardless, change the battery. 5 years is pushing it. Why wait till you're stranded? Then troubleshoot further

I agree. If the battery is five years old, it's approaching its end-of-life. Replace it & go from there.

Thanks for replies. Was already planning to replace the battery. Any thoughts on a possible misfire-battery charge relationship?

I have a follow up question:

I called a local Mini dealer about buying a replacement battery. I was planning to install it myself, but was told that it needs computer resets to update battery information. My examination of the negative battery cable indicates no IBS at the negative terminal (cable is a straight connection from the terminal to the ground connection) so no need to register the battery. However, the service rep says that all 2008 R56s have one. Can the IBS be located somewhere other than at the negative terminal?

No, he's pulling your chain, if you have no IBS box on the negative cable, you have no IBS.

The MINI brand battery is pretty wimpy, I've run mine down, when it was new, just listing to the radio while I washed and waxed my car.

The MINI brand battery is made by Exide it is a 570 amp battery and costs about $180.

You an get a Group 47 battery made by another company that has much more capacity for cheaper than the dealer sells their battery, get one with the vent hose connector.

IBS was based upon several factors, see here http://www.realoem.com/bmw/enUS/show...diagId=12_1398

Thanks. No IBS, so no need to register battery. Last questions (I think) follow:
In my Bentley manual is a statement that "If higher rating battery is installed, recode CAS with correct battery specifications using MINI scan tool." Is this independent of the IBS? If not, anything needing to be done apart from making sure door FOB and internal lock switches are working OK? I don't see anyone mentioning this on NAM. Sorry for the almost newbie questions, but this is the first time I am delving into battery issues. Thanks.

I think once the car starts, it runs off of the alternator not the battery. If you have misfire on cold starts, that typical for HPFP dying. just a thought.

You are correct!
 

The alternator provides power to the vehicle when the car is running. But, is it working properly? You should be charging the battery at 13-14 volts when the car is running.


My misfire was caused by a faulty fuel injector.

The link I provided earlier (http://tinyurl.com/yd5qbvk9) states that low battery voltage can cause the fuel pump to run slower; also that it can prevent injectors from opening normally, resulting in misfires and hard starting. So, a possibility of several types of effects.

In my car, the voltage across the battery measured 14.2 V when the car was running, so I don't think the alternator is an issue. This five year old battery doesn't hold the charge well.

I'll plan to report back after I replace the battery, clear the error codes, and see if symptoms return after I run it for a while.

Kinda of. The way it was explained to me, for the most part the car is running off the battery while the alternator is keeping it charged. Makes the battery a bit like a filter, pure DC current.