Misfire and Weak Battery

[DrRobin]

I had a problem last summer and through autumn, my 2001 2.7L Boxster 94k miles developed a misfire as follows:

Under deceleration (engine braking) in 4th or 5th gear, probably for 5 seconds or more (so no fuel going in) then feed the power back in at 1,500 - 2,000 RPM and I got a random misfire, different cylinder every time.

Over the summer I changed:

- All six coil packs
- All six spark plugs
- Cleaned the throttle body
- Changed the air filter
- Changed the MAF sensor
- Changed all four O2 sensors (they were on the way out)

At some point fairly early on in all of this the misfire stopped, but my Boxster ran a bit rough, all of the above work was to get it running sweet again, which it did.

The misfire stopped when I charged the battery. I fitted a new battery in April 2020, but in lock down it had got a bit flat.

Roll on to Jan 2022 and I had a misfire on cylinder 6, just once but under deceleration again, about 1,500 rpm but with headlamps on. I checked my battery it was failry low. I charged it up and the misfire has gone, again.

There has been lots of discussion on here and elsewhere about batteries and misfires, seems to be a emotive subject.

I think one of the following is happening:

A weak battery causes low voltage/low current available and

- this leads to a weak spark?
- one of the sensors or ECU gets an error or signal confused?
- one of the coils produces a weak spark?
- one of the injectors doesn't open correctly?
- the fuel pump doesn't give enough fuel?

Perhaps the weak battery is not the real problem, it might indicate another component is on the way out and with a good charge in the battery that other component is hanging on in there and still working?

Any one had a similar experience?

[Stl-986]

bad battery will cause many issues. Question is, why is your battery not charging, what do you mean it was low? alternator should be charging the battery when the car is running. If you dont drive it much I suggest getting a battery maintainer and having it hooked up when the car is sitting. 3 years is about all batteries are good for anymore, they just dont make them that good anymore and the brand doesn't matter, they all go bad. 5 years seems to be the max on any these days.

[DrRobin]

Thanks for the quick reply.

I have a CTEK Battery Sense fitted, this records battery coltage and temperature every couple of minutes and works out charge level. The battery was showing as 10% charged at the time.

When the misfire occurred the voltage shown on my in car voltmeter was 14.1V, but then that averages the voltage, so there might have been short term dips, I.E. when the coil or injector fires, especially if the battery was low, the battery internal series resitance could be high and this would limited the size of current pulse. The alternator might have too much inductance to supply short term high current peaks?

I had a new alternator on about 5 or 6 years ago, this one looks like it is charging, but I don't use the car much and it's only 10 miles to work, so the battery doesn't really get much put back in.

I have a battery/alternator tester, I will put this on at the weekend and see if the alternator is charging correctly.

I have a CTEK maintainer, but the car lives outside and it's not practical to leave it plugged in all of the time.

I just wondered if others had seen something similar or if it was a known problem?

[Stl-986]

Sounds like a normal issue with an older battery and car that isnt used much. Best solution...get out and drive the car every week or two for a few hours