Car came in with a misfire complaint and check engine light on with code p0300. Looking at misfire history confirms high misfire counts on cylinders 1 and 4 along with lower counts on cylinders 2 & 3.
Fuel trims OK LTFT < 10.
Plugs are fouled.
Compression tests ~200psi on each cylinder.
Using a spark tester indicated good spark on each secondary coil.
Fuel pressure KOER ~55psi.
I forgot to save the capture I did on the fuel injectors but it's what you'd expect. Ramps to 2A with no indication of a shorted primary coil. Secondary looked uniform. So I ruled out injectors.
Now on to the primary ignition side. Fortunately on the 2.2 the ICM sits on top of the coil pack. And the PCM is about 12" away.
Red = CKP Signal
Blue = ICM 1/4 Trigger
Green = Coil Pack Primary
Yellow = CMP Signal
Looks OK for a minute.
But... Why is that coil suddenly drawing fewer amps?
If you compare the ICM signal delta it's a few milliseconds shorter ~5ms compared to ~3ms.
In this one the deltas is down to 2ms
In each case the car has a good cam and crank signal as well as good cam/crank correlation. So you can rule out engine timing as well as ckp/cmp function.
So... Replace the ICM? At this point I wasn't 100% sure and tried a CKP Variation relearn. To no success, all it did was shift the misfire from cylinder 1 to 4. Luckily the ICM had been replaced a little under a year before and thus was under warranty. R&R'd a new ICM and the misfires disappear.
I'm unfamiliar with how the ICM on a GM 2.2 works and am wondering if anyone with greater electrical experience could shed some light. Obviously in this case something within the ICM was commanding a shorter spark duration and didn't seem to be caused by external factors.
I wasn't able to keep the ICM to tear it apart and with my limited electrical knowledge I doubt I would learn something unless it was staring me in the face.
Another regret I have is that I don't have an inline kV probe to look at the secondary side. Maybe it would of shed more light on what was happening.
Why does this happen? Is it poor engineering? This ICM wasn't original and less than a year old. Inspection of the interconnect between the ICM and coil pack didn't reveal anything.
The Whole Capture
A link to the whole capture from idle to 3000 RPM
Requires Shop Stream Connect to view