How To Test The Engine Compression (2.3L Ranger, Mustang, B2300)

This tutorial will help you do a ‘dry’ and ‘wet’ engine compression test on your 2.3L Ford Ranger or Ford Mustang (or 2.3L Mazda B2300).

This tutorial will help you troubleshoot 2 different conditions:

  1. A rough idle condition (misfire condition) due to low compression in one or two cylinders.
  2. A no start due to low or no compression on all 4 cylinders.

There are two different types of compression tests that I'm gonna' show you, one's called a ‘dry’ compression test and the other is called a ‘wet’ compression test.

Both of these tests will help you determine the health of the engine. The ‘wet’ compression test will help you further pinpoint a low compression test result to either worn cylinder head valves or worn piston rings.

You can find this tutorial in Spanish here: Cómo Verificar La Compresión Del Motor (2.3L Ford) (en: autotecnico-online.com).

Symptoms Of Bad Engine Compression

The most obvious symptom, that you're gonna' feel, when one or several cylinders have low compression is a rough idle condition.

Although it doesn't happen all of the time, but if your 2.3L Ford Vehicle is OBD II equipped (1996+), you're gonna' see misfire codes stored in the PCM's memory:

  1. Misfire Diagnostic Trouble Codes:
    1. P0300: Random Cylinder Misfire.
    2. P0301: Cylinder #1 Misfire.
    3. P0302: Cylinder #2 Misfire.
    4. P0303: Cylinder #3 Misfire.
    5. P0304: Cylinder #4 Misfire.

No compression in all of the cylinders will result in a 'cranks but does not start' condition. The symptoms you'll see will be:

  1. No start.
  2. Everything else works, for example:
    1. The fuel pump will activate, so you'll see fuel pressure at specification (if testing with a fuel pressure gauge).
    2. All of the spark plug wires have spark.
    3. If the coil packs are sparking, then this indirectly proves that the crankshaft position sensor is OK too.
    4. The PCM will still activate all of the fuel injectors.

What Tools Do I Need?

The most important tool that you're gonna' need is a compression tester. You can either rent this bad boy from your local auto parts store (Auto Zone, O'reilly, Advanced Auto Parts, etc.), or you can buy one online.

Since the spark plugs need to be removed, you'll need some of the following basic tools:

  1. Ratchet wrench.
  2. 5/8" spark plug socket.
  3. Extensions for the ratchet wrench.
  4. Motor oil (for the ‘wet’ compression test part).
  5. Spark plug wire puller.

In case you're needing to buy a compression tester and want to save money by buying it online, you can shop here:

TEST 1: Dry Compression Test

How To Test The Engine Compression (2.3L Ranger, Mustang, B2300)

If your 2.3L Ford Ranger or 2.3L Ford Mustang (or 2.3L Mazda B2300) starts and runs then I suggest you do the engine compression test with a slightly warmed up engine.

If you're wondering ‘why?’, it's because you'll get a more accurate compression tester reading (of the state of the engine piston rings and cylinder head valves). This is due to the fact that heat expands metal and the engine creates heat when running, so by testing the engine with some temperature will factor in this expansion.

Now, for those of you who have an engine that ‘cranks but does not start’, you can still do the test and get very useful data that'll help you find out what's going on.

Let's get testing:

  1. 1

    Disable the fuel system. You can easily do this by simply disconnecting the fuel pump relay.

    This step is important, so don't skip it. Why? Because it will prevent the PCM from injecting fuel into the cylinders during the compression test.

  2. 2

    Disable the ignition system. You can easily do this by simply disconnecting both coil packs from their electrical connectors.

    This step is important since it will prevent damage to the ignition coil packs, so don't skip it.

  3. 3

    Remove ONLY the exhaust manifold side spark plugs. Leave the 4 intake manifold spark plugs in their place.

    As your taking them out, be careful and don't drop any of them on the floor, or you could cause the spark plug's ceramic insulator to break, and this will cause a misfire!

  4. 4

    Install the compression tester. Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder. Hand tighten the compression gauge only! Do not use any type of tool to get it tight.

  5. 5

    When ready, crank the engine as you observe the needle on the compression tester's gauge. Once the needle on the gauge stops climbing, have your helper stop cranking the engine.

    Write down the compression value. Record this compression reading on a piece of paper. Include the number of the cylinder this reading belongs to. Now repeat steps 1 thru' 5 on the other cylinders.

Let's take a look at what your test results mean:

CASE 1: You got a reading of 100 PSI or less (less being 0 PSI) on all of the cylinders. This test result tells you that you've got serious engine mechanical problems.

If you got 0 PSI compression on all 4 cylinders, it sounds like you've got a broken timing belt on your hands. The next step is to remove the timing cover and check the actual condition of the timing belt.

What is common, if you have a very high mileage engine, is low compression across the board. Low usually means anything under 120 PSI (although the service manual says 100 PSI is the minimum). If your engine has reached this point, it's also smoking from the quart of oil it's burning every few days. The other symptoms you'll see is that the idle will be very rough.

CASE 2: One or two cylinders gave a low compression value. This might be normal, since each cylinder will not give the exact same pressure value. Or these low compression values could be causing a problem.

To find out, go to: How To Interpret The Engine Compression Test Results.

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