How To Troubleshoot A Bad Fuel Injector (1987-2006 4.0L Jeep Cherokee, Grand Cherokee, Wrangler)

Fuel injectors rarely go bad, but when they do, they will cause the engine in your Jeep 4.0L Grand Cherokee (or Cherokee or Wrangler) to miss and/or idle rough.

The really cool thing is that they can be easily tested with a multimeter and this article will show you how to do a simple fuel injector resistance test.

Depending on the year of your specific Jeep (1995 and newer), a bad fuel injector may or may not set a specific fuel injector diagnostic trouble code (DTC) like: P0201, P0202, P0203, P0204, P0205, P0206.

In most cases, all you'll see code-wise are misfire codes: P0300, P0301, P0303, P0304, P0305, P0306.

ES You can find this tutorial in Spanish here: Cómo Probar Los Inyectores De Combustible (4.0L Jeep) (at: autotecnico-online.com).

APPLIES TO: This tutorial applies to the following vehicles:

• 4.0L Jeep Cherokee: 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001.
• 4.0L Jeep Comanche: 1987, 1988, 1989, 1990, 1991, 1992.
• 4.0L Jeep Grand Cherokee: 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004.
• 4.0L Jeep Wagoneer: 1987, 1988, 1989, 1990.
• 4.0L Jeep Wrangler: 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006.

Important Tips

To successfully accomplish the tests in this article, here are some important tips:

TIP 1: The engine in your 4.0L Jeep Cherokee (Grand Cherokee, Wrangler) needs to be at operating temperature to get an accurate snapshot of the condition of the fuel injectors.

TIP 2: If your Jeep doesn't start, you don't have to worry about performing the test with a warmed up engine.

Where To Buy The Fuel Injector And Save

The best place to comparison shop and get an idea of how much your Grand Cherokee's fuel injectors cost (and how much you can save) is here:

Not sure if the above fuel injector fits your particular Jeep Grand Cherokee? Don't worry, once you get to the site, they'll make sure it fits by asking you the particulars of your vehicle. If it doesn't fit, they'll find you the right one.

The Fuel Injector Resistance Test

What makes testing the fuel injectors on your Jeep's 4.0L 6 cylinder engine is that they are in plain view and easy to get to.

This makes it possible to check for a bad fuel injector in about 10 to 20 minutes.

Now, to give you an idea of what's involved in testing for a bad fuel injector, we're going to simply check the internal resistance of each fuel injector and compare it to the factory spec.

You can either use a digital multimeter or an analog multimeter for this test.

Alright, here are the steps:

1. 1

   Disconnect the fuel injectors from their electrical connectors.

2. 2

   Place your multimeter in Ohms mode.

3. 3

   Probe the cylinder #1 fuel injector's two male spade terminals with the multimeter test leads.

4. 4

   Write it down the resistance value on a piece of paper.
   
   The fuel injector resistance should be within 13.3 to 15.7 Ohms (1991-95) or 10.8 to 13.2 Ohms (1996-2001).
   
   If your reading is different, don't panic. Write it down and continue testing the other 5 fuel injectors (you'll see why when you interpret the resistance value of all six).

5. 5

   Repeat steps 3 through 5 on the remaining fuel injectors.

Let's find out what your specific multimeter test results mean for your 4.0L Jeep Wrangler (or 4.0L Cherokee/Grand Cherokee):

CASE 1: All fuel injectors registered the same resistance values. This confirms that the fuel injector your are testing is OK.

Here's why: If any one of the fuel injectors were bad, your multimeter would've registered a completely and totally different resistance value (for that fuel injector). Since your resistance (Ohms) value were the same for all, this is an indication that the fuel injectors do not have an internal electrical fault.

CASE 2: One of the fuel injectors registered a completely different resistance value. This indicates that the fuel injector is bad. Replace the fuel injector.

Which Fuel Injector Do I Test First? Do I Test All Of Them?

You don't have to test them all, if you don't have to. This is especially true if you have a repair manual for your specific Jeep SUV and that repair manual has the fuel injector resistance specification.

The main reason I suggest you test them all is to avoid having to search for the resistance value, either online or in a book. Now, don't get me wrong, the more info you have at your disposal, the better. But, if you test them all, you'll find out what the average Ohms value is without having to look it up.

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.
   • If your Jeep is OBD II equipped, this can easily be done by checking for misfire trouble codes with a scan tool.
   • 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).
   • 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.
     • If the engine idle DOES NOT get worse, then that cylinder is 'dead' and is the one causing the misfire.
     • 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.
   • Before I even start thinking that I may have a bad fuel injector, I make sure that every engine cylinder is getting spark.
   • 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.
   • I check that the spark plug boot and spark plug are NOT soaked (or swimming) in engine oil.
   • I remove the spark plugs and check them for cracks or carbon tracks (this is SO important).
     • 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: easyautodiagnostics.com).
3. Check engine compression.
   • After making sure that the ignition system and all its components are OK, I check for low engine compression.
   • This is one of the most overlooked tests when diagnosing a misfire or rough idle condition.
   • You can find the test here: How To Test Engine Compression (1987-2006 4.0L Jeep Cherokee, Grand Cherokee, Wrangler).
4. Noid Light Test.
   • If every test above checks out OK, I do a fuel injector Noid light test.
   • The Noid light test will help you make sure that the fuel injector is getting activated.
   • The following Noid light article/tutorial may help you: How To Use A Noid Light And Where To Buy It (at: easyautodiagnostics.com -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.
   • 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:

• Jeep 4.0L Index Of Articles.

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

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