How To Find The Bad Or Clogged Fuel Injector

In this section I'll present to you my fuel injector diagnostic strategy. This set of tests will help you to find out if a bad fuel injector is the cause behind your Mazda 626's misfire/rough idle condition.

When I need to find the bad or clogged fuel injector, this is what I do:

  1. Find the ‘dead’ cylinder first.
    1. This is the most important first step. You can do this by connecting a scan tool (or code reader) and identifying the specific misfire code that's registered in your Mazda's PCM's memory.
    2. On the 1993-1995 2.0L Mazdas that are not OBD II equipped (and thus don't have misfire trouble code capability), you'll need to do a manual cylinder balance test to find the ‘dead’.
  2. After identifying the ‘dead’ cylinder, make sure it's getting spark.
    1. It's important that you check for spark with a dedicated spark tester.
    2. It's important that you check that the spark plug boot and spark plug are NOT soaked (or swimming) in engine oil.
    3. You should also remove the spark plugs and check them for cracks or carbon tracks (this is SO important).
      1. Here's a real life case study on carbon tracks and how they can cause a Misfire: Carbon Tracks Are A Common Cause Of Ignition Misfires (at:
  3. If the ‘dead’ cylinder has spark, the next step is to make sure it has good compression.
    1. After making sure that the ‘dead’ cylinder's plug wire is delivering spark, you need to check for low engine compression.
    2. This is one of the most overlooked tests when diagnosing a misfire or rough idle condition. You can find the test here:
      1. How To Test The Engine Compression (2.0L Mazda 626).
  4. If the ‘dead’ cylinder has spark and good compression, the next step is a fuel injector Noid light test.
    1. If every test above checks out OK, then the next step is to make sure that the fuel injector is being activated.
    2. The following Noid light article/tutorial may help you: How To Use A Noid Light And Where To Buy It (I know that this is not the most in-depth article on the subject, but it should give you an idea of what is involved).
  1. If the ‘dead’ cylinder has spark, good compression, and is being activated; the next step is swap the fuel injector.
    1. If I've found out that I have a specific ‘dead’ cylinder and:
      1. The ignition system is not at fault.
      2. That cylinder's compression value is good (compared to the rest of the cylinders).
      3. The fuel injector resistance is good and is being activated by the fuel injection computer...
      4. I think the fuel injector is clogged, I then swap out that fuel injector with its neighbor.
      If the misfire now follows that swap, I now know that fuel injector is clogged (or bad) and needs to be cleaned or replaced.

Finding the bad/clogged fuel injector can be a challenge on your 2.0L Mazda but it's doable. What will help you save a lot of time, money and frustration is to first find the ‘dead’ cylinder. Following the above diagnostic strategy has saved my lunch quite a few times and I think it'll help you too!

Where To Buy The Fuel Injector And Save

Check out the following links and comparison shop the fuel injector on your 2.0L Mazda 626 (MX6):

Not sure if the above fuel injectors fit your particular 2.0L Mazda 626 (MX6)? Don't worry, once you click on the links and arrive on the site, they'll make sure it fits! If it doesn't, they'll find you the right one.

More 2.0L Mazda Tutorials

If this tutorial was helpful, check out the other Mazda 2.0L tutorials that I've written. You can find them all here: Mazda 2.0L Index Of Articles.

Here's a small sample of the tutorials you'll find in the index:

  1. How To Test For A Blown Head Gasket (2.0L Mazda 626).
  2. How To Test The MAF Sensor (1996-1997 2.0L Mazda 626).
  3. How To Test The TPS (1994-2002 2.0L Mazda 626).
  4. How To Test The MAF Sensor (1996-1997 2.0L Mazda 626).
  5. How To Test The Fuel Pump (1994-1999 2.0L Mazda 626).

Mazda Vehicles:

  • 626 2.0L
    • 1993, 1994, 1995, 1996, 1997, 1998, 1999
  • MX6 2.0L
    • 1993, 1994, 1995, 1996, 1997