What Causes A Misfire Condition? (Continued)

Fuel System: The fuel system component that will usually cause a Miss (misfire, ‘dead’ cylinder) is the fuel injector.

Here are some more specifics:

  1. Bad fuel injector(s):
    1. Injector is fried internally and does not spray any fuel.
    2. Injector is clogged and doesn't inject enough fuel.
    3. Injector is clogged/dirty and it's spray pattern of fuel doesn't atomize the gasoline correctly.
    4. Fuel injector is not being activated by the PCM.
    5. Fuel injector is not getting power (12 Volts).

Engine Mechanical Condition: The engine's internal condition is one of the most overlooked problem areas when diagnosing a misfire (rough idle, ‘dead’ cylinder).

An engine cylinder that is not producing enough compression (either because of worn valves or piston rings), even if it has spark and fuel, will cause a misfire condition.

Another thing that will cause a misfire (normally at idle) is vacuum leaks coming from a large vacuum hose or the intake manifold gasket.

  1. Low or No engine compression:
    1. One or two cylinders have low or no compression.
    2. Low or No engine compression will be the result of:
      1. Worn cylinder head valves.
      2. Worn piston rings.
      3. Blown head gasket.
  2. Bad intake manifold gasket:
    1. Leaking intake gaskets will not set a specific misfire code, if OBD II equipped, but will set a P0300 Random misfire code and a P0174 Lean condition trouble code.

What Tests Can I Perform To Find The Cause Of The Misfire Condition?

It is possible to find out exactly what's causing the misfire condition and/or misfire code on your 2.3L equipped Ford Mustang or Ranger (Mazda 2300).

The very first thing that has to be done, is to identify the cylinder that is misfiring (or dead), then you can go about doing specific tests to find out if it's being caused by a lack of spark or fuel or enough cylinder compression.

Finding out which cylinder is the one that's misfiring is pretty easy on the OBD II equipped vehicles, since the PCM will usually set a misfire code (P0301, P0302, P0303, P0304). On OBD I equipped 2.3L's, it's a little bit more challenging.

The following suggestions take into account if the vehicle is OBD I or OBD II equipped:

  1. OBD II: Read the diagnostic trouble codes with a scan tool. You should see a specific cylinder misfire code that will tell you what engine cylinder is the one being affected.
    1. There's a good chance, that even tho' your vehicle is OBD II equipped, the PCM will not set a specific misfire trouble code, when this happens, you'll need to do a cylinder balance test.
  2. OBD I: On older vehicles (that are not OBD II equipped), you'll need to do a cylinder balance test to find out the missing (dead) cylinder.
    1. The cylinder balance test involves disconnecting and connecting one fuel injector at a time to see if this has any effect on the engine's idle.
    2. If the cylinder is ‘dead’ to begin with (for any reason, like a lack of fuel or spark or low engine compression), when you unplug the fuel injector from its connector, you won't see any difference in the engine's idle.
    3. If the cylinder is OK, unplugging the fuel injector will ‘cut out’ the cylinder and you'll see/feel a difference in the engine's idle.
    4. NOTE: This test is done with the engine running and so you need to take all safety precautions and think and act safety all of the time.
  1. Test the ignition system first, this is whether you have a misfire diagnostic trouble code or not.
    1. Troubleshooting the spark plug wires.
    2. Troubleshooting the 2 ignition coil packs that feed the 8 spark plugs.
    3. Troubleshooting the spark plugs.
    4. Troubleshooting the ignition control module (ICM), if equipped.
    Why test the ignition system first? Because the majority of misfires are due to an Ignition System component having failed.
  2. Test the compression of all engine cylinders.
    1. Testing the engine compression is a pretty fast test, since you only have to worry about 4 cylinders.
    2. Low engine compression in one or several cylinders is a common issue on high mileage engines.
  3. Testing the fuel injectors. This involves:
    1. Checking the internal resistance of the fuel injector.
    2. Doing a Noid light test.
    3. Swapping fuel injectors to see if misfire moves.
  4. Check for vacuum leaks.

It seems like a long and complicated list, doesn't it? Well, don't worry, one of the most important things you need to successfully diagnose the misfire or rough idle on your 2.3L Ranger or Mustang is a diagnostic plan. The above diagnostic strategy is the one that I have used with a lot of success and it should also help you too.

In the next page, I'm going to show you how to test all of these systems and/or components so that you can find out what is the component that has gone bad and is causing the misfire code(s) (P0300, P0301, P0302, P0303, P0304) or the misfire condition.

Ford Vehicles:

  • Mustang 2.3L
    • 1991, 1992, 1993
  • Ranger 2.3L
    • 1989, 1990, 1991, 1992, 1993,1994, 1995, 1996, 1997, 1998

Mazda Vehicles:

  • B2300 2.3L
    • 1994, 1995