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.

  1. 1

    Disable the Fuel System. You can easily do this by simply:

    1. 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:

    1. 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.

  1. 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!

  2. 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.

  3. 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' 6 on the other cylinders.

  4. 6

    Interpret the results. After testing all cylinders and having written down all of your compression test readings, now you need to interpret the results...

Interpreting the Results of the Engine Compression Test

CASE 1: If you got a reading of 100 PSI or less (less being 0 PSI) on all of the cylinders you tested... 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.

What is NOT normal if the pressures vary by 15% or more. That's right, the individual cylinder compression readings of each engine cylinder can not vary more than 15%... and this is how you can find out:

  1. Grab a calculator and multiply the highest compression reading that you recorded by .15.

    Let's use the following compression readings to explain the point:
    1. Cylinder #1 175 PSI.
    2. Cylinder #2 160 PSI.
    3. Cylinder #3 165 PSI .
    4. Cylinder #4 95 PSI .
  2. The next step is to do the math: 175 x .15= 26, 175-26= 149.
  3. So then, 149 PSI is the lowest possible compression reading that any one of the rest of the engine cylinders can have. Any compression reading below this.. and that engine cylinder will misfire.
  4. This means that cylinder #4 is the one causing the misfire

The next step is to do a ‘Wet’ compression test on the dead or low compression cylinder... and this test is explained in the next page...