How To Test The Fuel Injectors (Dodge 3.9L, 5.2L, 5.9L)

TEST 2: Swapping Fuel Injectors

Every now and then, you run into a situation where you have a clogged fuel injector that's causing the cylinder to misfire but you're not sure it's being caused by a clogged fuel injector.

In a case such as this (where you suspect a clogged fuel injector), I do the following leg work:

One: Make sure that all six (or eight) fuel injectors have passed the resistance test.

Two: Make sure that all six (or eight) spark plug wires are sparking and checked/verified the spark plugs are OK too.

Three: Checked and confirmed that the cylinders' compression values are normal.

Once I have eliminated all of the above, then I swap the fuel injector that belongs to the ‘dead’ cylinder with the one right next to it and see if the miss follows the injector. If the miss follows the injector to its new cylinder, then I can confidently conclude the fuel injector is clogged (and I replace it).

Here are the specific test steps:

1. 1

   Verify the cylinder is ‘dead’.
   
   You can accomplish this by simply disconnecting the fuel injector's connector (while the engine is running) and making sure that unplugging it DOES NOT make the RPMs drop.

2. 2

   Find a cylinder that IS NOT ‘dead’.
   
   You can accomplish this by simply disconnecting the fuel injector's connector (while the engine is running) and making sure that unplugging it DOES make the RPMs drop.

3. 3

   Let the engine cool down and then swap the fuel injectors.
   
   Be careful, since swapping the fuel injectors will cause fuel to spill from the fuel rail.

4. 4

   Start the engine and check t see if the misfire has moved to the other fuel injector's location.
   
   You can accomplish this by simply disconnecting the fuel injector's connector (while the engine is running) and seeing if the good cylinder is now ‘dead’.

Let's take a look at your test results:

CASE 1: The miss moved to the good cylinder. If the miss follows the injector, then you can replace it knowing you've ‘hit the nail on the head’.

This is only true if you have verified that the ‘dead’ cylinder has spark and good compression.

CASE 2: The miss DID NOT move to the good cylinder. This test result tells you that the fuel injector itself is not the cause of the misfire for that specific cylinder.

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

I would recommend to test them all since they are easy to get to. But the main reason is that by testing them all, you can catch the one that's fried.

Here's why:

1. All fuel injectors have a specific internal resistance that can be measured in Ohms mode on a multimeter.
2. When a fuel injector goes bad, it'll normally have a completely different resistance value than the other ones.

Therefore, the purpose of the test is to find the bad fuel injector by reading it's resistance and comparing this reading to two other fuel injectors.

Let me give you some examples of what you're looking for when I say “a completely different resistance value” :

Let's say that I tested all six fuel injectors on 1997 Dodge Dakota with a 3.9L V6, and I got the following resistance values (these are just to illustrate my point, they are not what you'll see on your specific Dodge vehicle):

• Cylinder #1: 11.4 Ohms
• Cylinder #2: 11.8 Ohms
• Cylinder #3:  4.4 Ohms
• Cylinder #4: 11.6 Ohms
• Cylinder #5: 11.2 Ohms
• Cylinder #6: 11.4 Ohms

As you can see, the fuel injector for Cylinder #3 is the one that's fried because the resistance value is drastically different than others. Although the other 5 do show some difference, they are all still similar and let me know they are OK.

Of course, a resistance test only diagnoses a completely fried fuel injector. It doesn't test for clogged fuel injectors that may be behind the engine misfire on your Dodge/Jeep vehicle. To test for a clogged fuel injector, you'll need to follow the suggested test steps here: TEST 2: Swapping Fuel Injectors.