Misfires log - need a guru!

[meddler]

I've been logging misfires over the last few months, out of interest more than anything else, and this is what it's thrown up.

Distance covered = 3,000 miles
Car = BMW 325i Touring (N53 straight-six 3.0 petrol)
Source = Bimmerlink, change in total misfires over time (ie the last 3,000 miles)

Cylinder 1, + 25
Cylinder 2, +76
Cylinder 3, +27
Cylinder 4, +119
Cylinder 5, +424
Cylinder 6, +60

Are there any misfire gurus out there who can tell me whether this looks normal or abnormal? Particularly Cylinder 5. Ta v much!

[Panamax]

Presumably you have no Check Engine Light illuminated but have you checked for codes?

[GreenV8S]

It's perfectly normal to get occasional isolated misfires without any underlying fault. The rate you'rfe seeing is consistent with that.

There are also faults which can provoke misfires, and the ECU can detect some of them. If you see fault codes, it's worth following up what they're warning you about. But a low rate of misfires like you're seeing without any accompanying fault codes is safe to ignore.

[meddler]

Thanks both.

No CEL, no misfire codes.

Do you know of any reason why Cylinder 5 misfires are so much higher than the others?

[GreenV8S]

meddler said:

Do you know of any reason why Cylinder 5 misfires are so much higher than the others?

There are millions of differences between cylinders that usually don't have any significant impact but could be enough to make individual cylinders slightly more susceptible to misfires under specific conditions.

[Panamax]

No codes = no problems.

Routine misfires as per GreenV8S above. I certainly wouldn't start replacing components on the off-chance. If it ain't broke, don't fix it. If you're bothered you could swap some of your existing parts between cylinders to see if it made any difference going forwards.

[jeremyc]

You need to put the data into perspective. If we assume that you've averaged 30mph and 3,000rpm over those 3,000 miles then even your 424 misfires on cylinder 5 represent, on average:

• a single misfire every 7 miles or 14 minutes of driving.
• a misfire every 0.028% of firing opportunities.

Assuming my maths are correct, of course.

[meddler]

Thanks all for the advice. And I’m sure that maths is good

[Krikkit]

If you want to try something swap the coilpack from cylinder 5 to another and see if it moves. But as above I wouldn't worry about that kind of misfire tally at all, only once you get an EML or start to feel it misfiring would I start investigating.

[meddler]

Krikkit said:

If you want to try something swap the coilpack from cylinder 5 to another and see if it moves. But as above I wouldn't worry about that kind of misfire tally at all, only once you get an EML or start to feel it misfiring would I start investigating.

Yeah, that was my thinking. Wouldn’t need to swap plugs because I put in new ones 4,000 miles ago (and the old ones were in remarkably good shape when I took them out). And given I’m not detecting any misfires while driving, I’ll leave it for now.

[meddler]

Update on this…

I replaced all six injectors, ignition coils, and spark plugs about 3,000 miles ago. Engine seems to be running well.

This is the total misfire count since I did the work:

Cylinder 1: 575
Cylinder 2: 121
Cylinder 3: 381
Cylinder 4: 43
Cylinder 5: 237
Cylinder 6: 68

Three questions for the misfire gurus…

1. Do these seem like OK numbers (ie within the acceptable parameters for 3,000 miles)?

2. Why are the counts so much higher for cylinders 1, 3, 5?

3. Why is there so much variation between the counts?

Thanks!

[richhead]

meddler said:

Update on this

I replaced all six injectors, ignition coils, and spark plugs about 3,000 miles ago. Engine seems to be running well.

This is the total misfire count since I did the work:

Cylinder 1: 575
Cylinder 2: 121
Cylinder 3: 381
Cylinder 4: 43
Cylinder 5: 237
Cylinder 6: 68

Three questions for the misfire gurus

1. Do these seem like OK numbers (ie within the acceptable parameters for 3,000 miles)?

2. Why are the counts so much higher for cylinders 1, 3, 5?

3. Why is there so much variation between the counts?

Thanks!

Apt usename.
Why did you change all those parts?

[LunarOne]

I would be tempted to move the spark plug and coil for cyl 5 to another cylinder and see if the stats change over time. If that other cylinder now gets more misfires (even if within the bounds of normality) then I would suspect that either the plug or the coil aren't top-notch. It's been said multiple times, but just because something is new doesn't guarantee that it's good.

[GreenV8S]

meddler said:

1. Do these seem like OK numbers (ie within the acceptable parameters for 3,000 miles)?

2. Why are the counts so much higher for cylinders 1, 3, 5?

3. Why is there so much variation between the counts?

It's normal for engines to misfire occasionally under certain conditions and only a problem if the misfires are frequent or sustained. Those numbers are extremely low for 3000 miles and completely safe to ignore.
Some cylinders may be more prone to misfires due to the physical engine bay layout, how much coke has built up in the induction and cylinder, state of the ignition system components, how damp the engine has got and so on. You would never expect the counts to be exactly equal across all cylinders.

[Krikkit]

To give a rough figure for how many ignitions in total there are compared to a modest number of misifres:

If you put your 6-pot BMW on a dyno, set it to 2000 RPM at a "pretend" 60mph road speed, in the time required for 3000 miles (50 hours) you'd be looking at 18,000,000 cylinder ignitions, or 3,000,000 each. The highest misfire count on there is only 0.019% of the idealised number of ignitions, which in reality will be much much higher.

[meddler]

richhead said:

Apt usename.
Why did you change all those parts?

One injector was leaking badly and causing misfires that ultimately led to cylinder shutdown, and another was leaking a little. They were the factory-fitted originals so I decided to replace the lot, in no small part because I got a great deal on 6 new BMW injectors from a very nice chap at a local dealership (N53/N43 injectors are generally silly expensive). At which point it seemed sensible to also replace the ignition coils (the factory originals) and spark plugs (because I'd already pulled everything else out, and they were nearly due replacing). Also changed the oil in case it was contaminated by fuel from the leaking injectors.

GreenV8S said:

It's normal for engines to misfire occasionally under certain conditions and only a problem if the misfires are frequent or sustained. Those numbers are extremely low for 3000 miles and completely safe to ignore.
Some cylinders may be more prone to misfires due to the physical engine bay layout, how much coke has built up in the induction and cylinder, state of the ignition system components, how damp the engine has got and so on. You would never expect the counts to be exactly equal across all cylinders.

Krikkit said:

To give a rough figure for how many ignitions in total there are compared to a modest number of misifres:

If you put your 6-pot BMW on a dyno, set it to 2000 RPM at a "pretend" 60mph road speed, in the time required for 3000 miles (50 hours) you'd be looking at 18,000,000 cylinder ignitions, or 3,000,000 each. The highest misfire count on there is only 0.019% of the idealised number of ignitions, which in reality will be much much higher.

Thanks both - makes absolute sense. My mind is at rest!