Ignition Coil Diagnostics

[Tony B]

I have a 2012 E92, which has had a problem with a resonance at constant speed. It is like a 2Hz "beat" currently evident at 3700 rpm in 6th gear. This first appeared at about 4000 miles and BMW changed the transmission and clutch in an attempt to fix it.

This seemed to fix it, but the symptoms have gradually returned until now, when it is worse than ever (at 19000 mls). The only difference is that the engine speed is a little higher than before.

I am wondering if there is a slight uneveness to the engine, which is causing a resonance within the transmission.

First culprit could be a coil operating at one end of its tolerance band, perhaps?

I have an OBD reader, and Torque Pro, so can read/record huge numbers of parameters. Can anyone suggest which I should be looking at?

[Time Lord]

Drive shaft maybe? Do you have the bmwhat app?

[kenwelch]

You can monitor the cylinder misfires using the data screens with you “code reader” (scantool). If you look though the list of PID’s (live data types) you should find each cylinder with a misfire counter listed. The cylinder that has the higher number of misfires is the one to zero in on. The misfire could be caused by a number of things, but the easy test is to swap that coil with another cylinder. Remember to mark the “bad” coil. Now recheck the misfire counter to see if the misfire moved with the coil change. Sometimes the primary wire connector at the coil can be the problem. So with the coil out inspect the terminals carefully.

If the misfire did not move then you need to look next at the spark plug. It might be worth installing a new one in that cylinder. Next on the list: fuel injector, then cylinder leakage test which you may not be able to do at home in the garage.


If there are no misfire indicated.
You can locate a technician with a vibration analyzer to assist with the diagnosis process. This is a little more involved but might get you to the cause of the problem faster than changing expensive parts.

The Vibrate Software site has some good reference material that I have used in my automotive power train class and pictures of what the diagnostic tool looks like.

http://www.vibratesoftware.com/

If you click in the menu on the right side of the page you will find “Diagnostics”. This section will show you the tool and the Vibrate Software screens.

Further down on the home page you will find the “Vehicle Support” link. There is lots of information on tires and drivelines, but you should look at the article on “Engine Speed Related Vibrations” under the “Diagnostic Procedures” heading. The article will help with an explanation and diagnostic chart.

[Tony B]

Originally Posted by Giorgia

Drive shaft maybe? Do you have the bmwhat app?

That would make it purely road speed dependant though, wouldn't it? This resonance happens in 2nd, 3rd, 4th, 6th gears at the same revs (but not 5th gear).

Found it!

[Tony B]

Originally Posted by kenwelch

You can monitor the cylinder misfires using the data screens with you “code reader” (scantool). If you look though the list of PID’s (live data types) you should find each cylinder with a misfire counter listed. The cylinder that has the higher number of misfires is the one to zero in on. The misfire could be caused by a number of things, but the easy test is to swap that coil with another cylinder. Remember to mark the “bad” coil. Now recheck the misfire counter to see if the misfire moved with the coil change. Sometimes the primary wire connector at the coil can be the problem. So with the coil out inspect the terminals carefully.

If the misfire did not move then you need to look next at the spark plug. It might be worth installing a new one in that cylinder. Next on the list: fuel injector, then cylinder leakage test which you may not be able to do at home in the garage.


If there are no misfire indicated.
You can locate a technician with a vibration analyzer to assist with the diagnosis process. This is a little more involved but might get you to the cause of the problem faster than changing expensive parts.

The Vibrate Software site has some good reference material that I have used in my automotive power train class and pictures of what the diagnostic tool looks like.

http://www.vibratesoftware.com/

If you click in the menu on the right side of the page you will find “Diagnostics”. This section will show you the tool and the Vibrate Software screens.

Further down on the home page you will find the “Vehicle Support” link. There is lots of information on tires and drivelines, but you should look at the article on “Engine Speed Related Vibrations” under the “Diagnostic Procedures” heading. The article will help with an explanation and diagnostic chart.

Very interesting post, and website!

When you refere to "vibration RPM" do you mean - in my case - 120 (I should have said 2hz, not 0.5, now corrected, as it is approx 2 beats/second. Rushed my original post)

[kenwelch]

Originally Posted by Tony B

Very interesting post, and website!

When you refere to "vibration RPM" do you mean - in my case - 120 (I should have said 2hz, not 0.5,

I did not refer to "vibration RPM" and still am confused about your question?

[Tony B]

Originally Posted by kenwelch

I did not refer to "vibration RPM" and still am confused about your question?

I must have got that from your website?

[Tony B]

Originally Posted by kenwelch

You can monitor the cylinder misfires using the data screens with you “code reader” (scantool). If you look though the list of PID’s (live data types) you should find each cylinder with a misfire counter listed. The cylinder that has the higher number of misfires is the one to zero in on.
If there are no misfire indicated.
You can locate a technician with a vibration analyzer to assist with the diagnosis process. This is a little more involved but might get you to the cause of the problem faster than changing expensive parts.

Well....over a 110 mile journey, with the resonance clearly audible to me at the usual speed, no misfires were recorded by the TorquePro software. Not sure whether that means that there were No Misfires, or that TorquePro did not capture the data.

I still have a feeling that 1 or more of the coils is operating at the extreme of its tolerance band (so not actually causing misfiring) whilst the others are maybe at the middle, or opposite end of the tolerance.

I think it might take a very sophisticated vibration analyser to capture the signal, as I can feel nothing (e.g. by resting my fingertips on the gearlever), only hear it.

[coloradoe92m3]

Why are you convinced a vibration is a coil? Could be a engine mount or any number of things. Stressed coil really wouldn't cause vibration

[kenwelch]

Originally Posted by Tony B

I still have a feeling that 1 or more of the coils is operating at the extreme of its tolerance band (so not actually causing misfiring) whilst the others are maybe at the middle, or opposite end of the tolerance.

If you have not measured the running secondary output voltage for each of the coils you cannot tell if it at it's "tolerance".
There are also oscilloscope methods for viewing the primary voltage which can show abnormal operation.

The coils either create enough voltage to jump the spark plug gap (normal ignition) or you have a misfire (no spark). The coils can also create enough secondary voltage, but arc somewhere other than the spark plug which will still be a misfire.

If you want to learn more about coil construction and operation look here:
http://www.picoauto.com/tutorials/pr...secondary.html

[Tony B]

Originally Posted by coloradoe92m3

Why are you convinced a vibration is a coil? Could be a engine mount or any number of things. Stressed coil really wouldn't cause vibration

Good question.

My reasoning was that something in the driveline is forcing the resonance, and this might be an easily identifiable cause and an easy fix.

In previous visits to the dealer, the engine mounts were checked, released and re-torqued to spec as were the gearbox mounts, exhaust. This made absolutely no difference.

The replacement of the gearbox and clutch (at 7500 miles) resulted in no audible resonance for several thousand miles, but it has come back over the last 2 or 3000. I believe this is due to a slight amount of backlash developing in the gearbox after it has broken in which then provides a sound path into the car.

I am going to look at more parameters via TorquePro (there are plenty) to see if something else stands out.

It only happens at constant speed, light throttle. It goes away if I declutch, but hold the engine revs at the critical number.

Maybe one cylinder is slightly down on compression, maybe one plug is not performing as efficiently as the others, maybe an injector.....

All I know is - it is now back to the very annoying level that had reached pre-gearbox swap.

Maybe a remap will fix it....but that screws my warranty, so I will leave that for a while.

A vibration analyser is on my horizon, if I can hire not buy.