How To Find The Defective Fuel Injector

Diagnosing a defective or clogged fuel injector, to see if it's causing a cylinder misfire, on the 3.8L V6 Ford Mustang engine can seem like a total pain in the neck.

The truth is that there is a ‘method to the madness’ of trying to find the defective or clogged fuel injector. So even though you might have to remove the intake manifold plenum (to check the #4, #5 or #6 fuel injectors), finding out what's behind the misfire is something you can do. And you might not even have to test the fuel injectors under the intake manifold plenum.

In this section I'm gonna' share with you my personal diagnostic strategy. A strategy I've used quite effectively over the years to find the cause of the cylinder misfire (whether the cylinder misfire is due to a fuel injector or not).

Alright, these are the steps that I take when diagnosing a misfire I think may be caused by a fuel injector:

  1. Identify the dead (misfiring) cylinder first.
    1. If you're driving a non OBD2 Ford Mustang, you're going to need to do a manual cylinder balance test. Of course, if your Ford is OBD2 equipped, then all you have to do is read the misfire trouble codes.
  2. Verify that the debt cylinder is getting spark.
    1. It's important that you perform this test with a dedicated spark tester to be able to get a good spark test result.
    2. You also need to make sure that the spark plug isn't broken or damaged in any way shape or form.
    3. This also means checking to make sure that the spark plug isn't covered in carbon tracks. To find out more about carbon carbon tracks, check out this article here: Carbon Tracks Are A Common Cause Of Ignition Misfires.
    4. The key here is to eliminate the distributor cap and spark plug wires and spark plugs as the cause of the cylinder misfire.
  3. Make sure the dead cylinder has good compression.
    1. Checking compression is probably one of the most overlooked tests when trying to diagnose a cylinder misfire. The following tutorial explains the compression test and more importantly how to interpret its test results: How To Do And Interpret An Engine Compression Test (Ford 3.0L, 3.8L) (at:

So just to recap, once I've identified which cylinder is dead (misfiring), I then make sure that it's getting spark. If I have spark going to the cylinder and the spark plug and spark plug wire boot are okay, then I test engine compression.

If the previous test passed with flying colors, then my next step is to look into testing the fuel injector resistance.

Now, if the fuel injector resistance is within specification, and the cylinder is getting spark, and the cylinder has good compression; then I can start worrying about the fuel injector being clogged.

In these cases (where everything passes with flying colors) my next step has been to swap the fuel injector (of the dead cylinder) to the adjacent non-misfiring cylinder. This is to see if the misfire will move to that non-misfiring cylinder. If the misfire moves to that cylinder, then now I know that I've got a clogged fuel injector causing the misfire.

On Removing The Intake Manifold Plenum

Intake Manifold Plenum. How To Test The Fuel Injectors (1994-1995 3.8L Ford Mustang, Ford Thunderbird, Mercury Cougar)

OK, as mentioned at the beginning of this tutorial, fuel injectors for cylinders #4, #5, and #6 are underneath the intake manifold plenum and it has to be removed to test them. Having actually done this several times on the 3.8L V6 Ford Mustangs (Thunderbird and Mercury Cougar), I have 4 important suggestions to share with you that will help you avoid a headache.

Here they are:

  1. Be careful that nothing (like a bolt, nut, etc.) falls into the intake manifold ports once you have removed the plenum.
  2. As you're removing the nuts, bolts, or anything else from the intake manifold plenum, place them far away from the engine compartment (like on a small parts tray on your work table or bench).
  3. Once you have removed the intake manifold plenum, place clean shop rags over the now exposed intake manifold ports.
  4. When you're reinstalling the intake manifold plenum, don't use any type of sealant on the gasket like for example RTV silicone gasket maker.

If anything falls into the exposed intake manifold port after you have removed the plenum, and you start the engine, that metal part (object) will damage the piston and or the valves of the cylinder it falls into.

And the only way to remove whatever fell into the engine, is to remove the cylinder head. This is a nightmare you want to avoid.

Removing the intake manifold without complications is possible -I know since I've done it many many times over. The successful outcome of the job simply depends on taking the above mentioned precautions.

Ford Vehicles:

  • Mustang 3.8L
    • 1994, 1995
  • Thunderbird 3.8L
    • 1994, 1995

Mercury Vehicles:

  • Mustang 3.8L
    • 1994, 1995