How To Find The Bad Or Clogged Fuel Injector

Finding the bad fuel injector is not hard to do, if you have a specific diagnostic strategy. In this section, I'm gonna' share with you the way I diagnose a bad fuel injector (and of course, you can modify my testing suggestion to fit your specific needs).

I first start by:

  1. Identifying the misfiring (or ‘dead’ cylinder) first.
    1. If your Jeep is OBD II equipped, this can easily be done by checking for misfire trouble codes with a scan tool.
    2. You won't always have a specific bad fuel injector code, but you'll definitely have a misfire code (around 90% of the time that is).
    3. If no codes are present, then the next best thing to do is a cylinder balance test. This test test is done by simply unplugging one fuel injector at a time, while the engine is running to see if it unplugging it worsens the engines idle.
      1. If the engine idle DOES NOT get worse, then that cylinder is ‘dead’ and is the one causing the misfire.
      2. If the idle DOES get worse, then that cylinder is OK and not the cause of the misfire.
  2. Check the ignition system for spark.
    1. Before I even start thinking that I may have a bad fuel injector, I make sure that every engine cylinder is getting spark.
    2. If the Jeep has an ignition distributor, I check that each spark plug wire is feeding spark to the spark plug. If the Jeep has an individual COP ignition coil, I check them for spark.
    3. I check that the spark plug boot and spark plug are NOT soaked (or swimming) in engine oil.
    4. I 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. Check engine compression.
    1. After making sure that the ignition system and all its components are OK, I check for low engine compression.
    2. This is one of the most overlooked tests when diagnosing a misfire or rough idle condition.
    3. You can find the test here: How To Test Engine Compression (Jeep 4.0L).
  4. Noid Light Test.
    1. If every test above checks out OK, then I do a fuel injector Noid light test.
    2. The Noid light test will help you make sure that the fuel injector is getting activated.
    3. 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).
  5. Swap the fuel injector with its neighbor on the fuel injector rail.
    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 4.) I think the fuel injector is bad, I then swap out that fuel injector with its neighbor. If the misfire now follows that swap, I now know that fuel injector is bad and needs to be replaced.

The above testing strategy may seem like overkill or too difficult but it isn't. Most of the above tests can be done pretty fast and are not hard to do.

I can tell you from experience that the way to save yourself the frustration of replacing good parts, your vehicle doesn't need and that don't solve the problem, is testing everything. Thankfully, there's a test for just about anything on your Jeep!

More 4.0L Jeep Diagnostic Tutorials

To see all of the Jeep 4.0L Wrangler, Cherokee, and Grand Cherokee articles, go to:

  1. Jeep 4.0L Index Of Articles.

Here's a sample of the test articles you'll find in the index:

  1. Jeep PWM Fan Relay Test Troubleshooting An Overheating Condition.
  2. How To Test Engine Compression (Jeep 4.0L).
  3. How To Test For A Blown Head Gasket (Jeep 4.0L).
  4. How To Test The TPS (1993-1995 4.0L Grand Cherokee).
  5. How To Test TPS Codes: P0121, P0122, P0123 (1997-2001 Jeep 4.0L).
  6. How To Test The Ignition Coil (1991-1997 4.0L Jeep).
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Jeep Vehicles:

  • Cherokee 4.0L
    • 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001
  • Comanche 4.0L
    • 1987, 1988, 1989, 1990, 1991, 1992
  • Grand Cherokee 4.0L
    • 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004

Jeep Vehicles:

  • Wagoneer 4.0L
    • 1987, 1988, 1989, 1990
  • Wrangler 4.0L
    • 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004