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


Idling Idling

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.

enter image description here

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.

My Question

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

  • 1
    This is a great question! For those of us trying to learn from it, could you define a few acronyms for us: ICM, CKP, PCM, CMP, LTFT, KOER and CKP Variation relearn. Also what is the vertical axis in the plots, DC current? Thanks! – cdunn Feb 19 '16 at 22:32
  • @cdunn PCM Powertrain Control Module, ICM Ignition Control Module, CKP Crankshaft Position Sensor, CMP Camshaft Position Sensor. On GMs the PCM uses the CKP sensor to detect misfires and is learned to the particular part so you can get around manufacturing differences, Usually you only have to do this when certain parts are replaced such as the CKP sensor, flexplate etc.. Sometimes though the PCM can forget the learned values and it's necessary to do a relearn. And the green plot is dc currant, and the rest are dc volts. – Ben Feb 19 '16 at 22:45
  • Just spitballing here, so take it as such: I'm thinking that any electrical device can be damaged by any of several things: heat; vibration; short; physical/impact damage. Any single component can be damaged by by any of these or any combination of these. It can also appear to malfunction due to a bad ground, whether internal or external. I'm thinking since it looked good at first and after a short while started malfunctioning, there is a heat short (continuity issue) or heat short to ground. IOW: after it warms it loses a connection or the connection grounds. Again, just a spitball. – Pᴀᴜʟsᴛᴇʀ2 Feb 19 '16 at 22:55
  • 1
    @ᴘᴀᴜʟsᴛᴇʀ2 This is a pretty regular customer so I guess we'll see how long it takes to malfunction, if it does. And whether the same symptoms appear. Though I'm sure I've seen this before on this engine. I lost quite a bit of saved captures when I had my scope serviced. – Ben Feb 19 '16 at 23:10
  • 1
    The CKP signal and CMP seem to have shifted relative to each other between it the first trace and the following 2, I would have thought they should stay constant. – HandyHowie Feb 20 '16 at 13:20

If I am correct, the CMP signal is generated by the ICM, the engine doesn't actually have a cam position sensor. The ICM monitors the difference in voltages on the HT coils to work out which cylinders are on the compression cycle. This will explain why the CMP and CKP signals are shifting relative to each other. The synthesised CMP signal is sent to the PCM to allow the PCM to generate the ignition timing signals.

Presumably the ICM must have developed a fault in the circuitry that generates the CMP signal which was then confusing the PCM and causing the shortened ignition pulse. This then led to the misfire.

  • I'll accept this, you're correct the ICM generates the CMP signal. Seems the most likely. – Ben Feb 22 '16 at 1:21

Maybe a crack in the ceramic at the top of the coil causing momentary flash over to the engine block. I had that once.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.