TEST 9: Other Things That Can Cause A Misfire

How To Test The Coil Pack (Ford 3.0L, 3.8L, 4.0L, 4.2L)

So you've done all of the spark tests, and still your Ford (or Mercury or Mazda) six pack-er is still running rough (rough idle) and/or the misfire is still present. Well, here are a couple of suggestions that might help...

  1. Inspect the inside of the spark plug wire boots and the ceramic insulator of the spark plug for carbon tracks
    1. The photo above (the orange arrows) show what carbon tracks look like.
    2. Replace the components as affected with carbon tracks.
  2. Engine Compression Test
    1. One of the most overlooked diagnostic tests to find the root cause of misfire is the compression test.
    2. You'll need an engine compression tester of course.
    3. The engine compression readings between cylinders should not vary more 15%.
    4. You can find a great step-by-step engine compression test article by clicking here: How To Do And Interpret An Engine Compression Test (Ford 3.0L, 3.8L) (at: troubleshootmyvehicle.com).
  3. Fuel Injector Test
    1. There are several methods to test them which are beyond the scope of this article but...
    2. The fastest way to test them is to measure them is by checking the resistance (Ohms) value of each one and comparing them to each other.
    3. Any resistance that's not within the average of the others indicates the fuel injector is fried.
    4. You can buy a specific fuel injector tester on line that activates by a predetermined pulse width and measuring the fuel pressure drop on a fuel pressure gauge.
    5. You can find the test here: How To Test The Fuel Injectors (Ford 3.0L, 3.8L) (at: troubleshootmyvehicle.com).
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