How To Test The Fuel Injectors (2.5L Nissan Altima, Sentra)

How To Find The Bad Or Clogged Fuel Injector

There is a logical way to finding out if a bad fuel injector is behind the misfire (rough idle) behind your 2.5L Altima (or 2.5L Sentra). The most important first step is to find out which of the 4 cylinders is misfiring.

Once you've identified the 'dead' cylinder (misfiring cylinder), the next important step is eliminating the ignition coils as bad. Then, there's making sure that the engine cylinders' compression is not the cause of the problem.

So, as you can already probably deduce, this means that for the most part you should leave testing the fuel injector's internal resistance as a last step.

OK, below are the diagnostic steps I take when trying to diagnose a bad fuel injector:

1. Find the 'dead' cylinder first.
   • This means connecting a scan tool (or a code reader) and checking for misfire trouble codes.
   • If no codes are registered, the next step is doing a cylinder balance test to find the 'dead' cylinder.
2. Check all 4 ignition coils for spark.
   • It's important to make sure that each cylinder is getting spark with a dedicated spark tester.
     • This tutorial will help you test the ignition coils: Coil-On-Plug (COP) Coil Test 2.5L Nissan Altima, Sentra (2002-2006).
   • It's important that you check that the spark plug boot and spark plug are NOT soaked (or swimming) in engine oil.
   • You should also 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.
3. Check engine compression.
   • After making sure that all 4 ignition coils are creating and delivering spark, you need to 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 (2.5L Nissan Altima, Sentra).
4. Noid light test.
   • If every test above checks out OK, then the next step is to do a fuel injector Noid Light test.
   • The Noid light test will help you make sure that the fuel injector is being activated.
   • 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.
   • 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 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 2.5 Nissan Sentra (2.5L Altima) 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!

Related Test Articles

To see all of the 2.5L Nissan specific articles, go to: Nissan 2.5L Index Of Articles.

Here's a sample of the articles, you'll find in the Index of Articles:

• Coil-On-Plug (COP) Coil Test 2.5L Nissan Altima, Sentra (2002-2006).
• How To Test Engine Compression (2.5L Nissan Altima, Sentra).
• Testing A Bad Alternator: Symptoms And Diagnosis.
• How To Bench Test The Starter Motor.
• How To Troubleshoot a Blown Head Gasket (2.5L) (at: troubleshootmyvehicle.com).