Ignition Coil-On-Plug (COP) Test (GM 3.5L, 4.2L)

TEST 3: Swap The ‘No Spark’ COP Coil

Ignition Coil-On-Plug (COP) Test (GM 3.5L, 4.2L)

OK, in TEST 2 you confirmed that you do have one COP coil that's not sparking.

In this test step, you're gonna' place one of the other COP coils (that is sparking) in the place of the one that isn't.

If the good COP ignition coil sparks in its new place, then you now know beyond a shadow of a doubt that the COP coil is getting all 3 signals from its connector.

This is what you'll need to do:

  1. Disconnect and remove the COP ignition coil that did not spark.
  2. Choose one of the other COP coils that is sparking and remove it from its place.
    • If you need to make sure that this COP ignition coil is sparking then you can test it with your spark tester.
  3. Once the good COP coil is removed, place it in the location of the bad COP ignition coil.
    • Next, connect the HEI spark tester to this COP coil.
    • Ground the HEI spark tester with a jump start cable directly on the battery negative (-) terminal.
  4. Now, place the bad COP ignition coil in the location of the good one you just removed and bolt it down.
  5. Once everything is ready, have your helper crank the engine.
  6. What you need to see is:
    • That the good COP coil is still sparking
      • This would confirm that the 3 signals are present in the COP coil connector.

Let's take a look at your test results:

CASE 1: The COP ignition coil sparked. This tells you that the COP coil that did not spark in TEST 2 is bad and needs to be replaced.

Here's why: By placing a good and sparking ignition coil in place of the bad one and having spark come out of the good one then this proves that:

  • Circuit A wire is providing power (10 to 12 Volts).
  • Circuit B wire is providing the PCM's Triggering Signal (known as the IC (Ignition Control) Signal).
  • Circuit C wire is providing Ground.

If you need/want to save a few bucks and buy a good quality COP coil for your vehicle, look at this section: ‘Where To Buy A COP Ignition Coil’ for my suggestion.

CASE 2: The COP ignition coil DID NOT spark. This tells you that the reason why the COP coil did not spark is because it's lacking one of the 3 signals it needs to spark.

The next step for you is to:

  • Check that Circuit A wire is providing power (10 to 12 Volts).
  • Check that Circuit B wire is providing the PCM's Triggering Signal (known as the IC (Ignition Control) Signal).
  • Check that Circuit C wire is providing Ground.

TEST 4: Cylinder Balance Test

A cylinder balance test will help you determine which engine cylinder is the one missing (misfiring).

In a nutshell, the cylinder balance test involves starting the engine, letting it idle, and then unplugging one COP ignition coil at a time to see which cylinder is ‘dead’.

By ‘dead’ I mean not contributing to engine power. Now, let me go into more specifics: If an engine cylinder is getting both spark, fuel, and has good compression and you unplug the COP coil connector, you're gonna' visibly see the engine shake more.

In other words, disconnecting the COP coil (while the engine is running) will cut power from that cylinder and you'll see/feel a difference.

NOTE: There's one very important thing to keep in mind when doing a cylinder balance test that involves disconnecting the COP coil connector and this is that you can't spend too much time with the engine idling with the COP coil disconnected.

Why? Because this will let too much unburned fuel overload the catalytic converter, and this is never a good idea.

OK, this is what you need to do:

  1. Start the engine and let it idle.
  2. Once the idle has stabilized, briefly unplug COP ignition coil for cylinder #1.
  3. If the cylinder is ‘dead’
    • You won't see/feel a difference in the engine's idle quality.
  4. If the cylinder is NOT ‘dead’
    • You will see/feel a difference in the engine's idle quality.

Let's take a look at your test results:

CASE 1: The cylinder balance test identified a ‘dead’ cylinder. Now that you have identified the cylinder with the misfire, the next step is to check and confirm that it's not being caused by a bad COP (Coil-On-Plug) ignition coil.

This involves checking the COP ignition coil for spark. You can now start with: TEST 2.

CASE 2: The cylinder balance test DID NOT identify a ‘dead’ cylinder. This tells you that you don't have a specific ‘dead’ cylinder.

Now, if your vehicle is experiencing a rough idle condition then it's something that's affecting all of the cylinders and not just one. My suggestion in this case is to look at: TEST 5.

TEST 5: Most Common Cause Of A P0300

One of the most common causes of a P0300: Random Cylinder Misfire (at the auto repair shop where I work), has been leaking intake manifold gaskets.

The intake manifold gaskets are made out of rubber and over time, this rubber gets compressed to the point that it looses its sealing ability.

What really sucks about this condition, is that you really can't test it. Spraying carburetor cleaner around the base of the intake manifold and checking to see if the Fuel Trim value is going lean doesn't help very much either.

So then, how do I check/confirm this? By first testing all COP coils for spark. Visually inspecting all of the spark plugs. Making sure that the valve cover gasket is not leaking engine oil onto the spark plugs and COP coil boot.

If all these things check out, and the engine has over 70,000 miles then I can safely assume that the intake gaskets are the cause of the problem (and need to be replaced to solve the P0300 code). So far, this has proven effective around 90% of the time.

I know that this is not the usual step-by-step in-depth testing instructions most of you are accustomed to from me but it works.


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Buick Vehicles:

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Hummer Vehicles:

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Oldsmobile Vehicles:

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    • 2004