How To Test Compression (Ford 4.0L)

A compression test will let you know if the hard to diagnose misfire (or rough idle) condition or no start condition is due to something wrong inside the engine (like worn piston rings or worn cylinder head valves).

This article will help you to do a compression test and more importantly, how to interpret your compression test results in a detailed step-by-step way on your 4.0L Ford Explorer (or 4.0L Aerostar, or 4.0L Ranger or 4.0L Mercury Mountaineer).

En Español You can find this tutorial in Spanish here: Cómo Probar la Compresión del Motor (Ford 4.0L) (at:

Tools You'll Need:

  1. Compression Gauge Tester.
  2. A Helper.
  3. Pen and Paper.

TEST 1: ‘Dry’ Engine Compression Test

How To Test Compression (Ford 4.0L)

As you might already know every cylinder, in the engine, of your Ford Explorer (or Aerostar, Ranger, Mountaineer) needs Spark, Fuel and Air to start and maintain the combustion process, and yadda yadda yadda. If any one of these is missing (Spark, Fuel or Air), that cylinder will not produce power and you are gonna' feel it as you drive down the road or when you come to an idle.

One of the most overlooked things, when troubleshooting a misfire (rough idle) or a no start, is the compression test since that specific cylinder that is misfiring could have both fuel and spark but its mechanical condition could be in such a BAD state that that cylinder still could not produce enough power. And so, my recommendation is that the engine compression test should be done after you have verified both spark and fuel.

OK, to get this show on the road, this is what you'll need to do:

  1. 1

    The engine should be warm (that is if your Ford Explorer, Aerostar, etc. cranks and starts), but not hot. So, if your Ford pick up, or SUV, or mini-van's engine is completely cold, start the vehicle up and let the engine run for about 20 minutes. If the engine is hot, let it cool down for about one hour.

    What if you vehicle doesn't start?? Well, in that case... don't worry about it and continue with the test.

  2. 2

    Disable the fuel system. You can do this by simply removing the fuel pump relay. This will prevent fuel from being injected into the cylinders as you perform the test.

  3. 3

    Disable the ignition system. You can do this by simply disconnecting the electrical connector from the ignition coil pack. This will prevent the ignition coil pack from creating and delivering spark to the spark plug wires.

  4. 4

    Now, disconnect the spark plug wires (label them before you disconnect them) and the remove all 6 Spark Plugs. 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!

  5. 5

    Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder (this is the spark plug hole closest to the drive belt). Hand tighten the compression gauge only! Do not use any type of tool to get it tight.

  6. 6

    OK, when the compression tester is set up in the spark plug hole... have you helper crank the engine.

    What you'll see is the needle on the gauge climb, as the engine cranks, and then stop. When the needle stops climbing, have your helper stop cranking the engine.

    Now, at whatever number the needle, on the gauge, stops, this will be the maximum compression value for that specific cylinder. Write this number down along with the number of the cylinder. Repeat steps 1-6 on the other 5 cylinders.

  7. 7

    When you're done testing, turn to the next page to find out how to interpret your test results...

Let's take a look at how to interpret your test results:

The idea behind the engine compression test (in the case of a misfire condition or rough idle condition) is to find out if any one engine cylinder is not contributing 100% of its power to the overall engine output. If only one cylinder has lower than normal compression, your Ford 4.0L pick up, or SUV, or mini-van will run rough or cause a misfire code to set and turn on the check engine light.

So then, to wrap things up, 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.

    So, let's say that cylinder #4 gave you the highest reading of 170 PSI. Well 170 X .15 gives you 26 (25.5 rounded off).
  2. Now, the next step is to subtract 26 from 170... which gives us 144 PSI.
  3. So then, 144 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.

Now, so that this calculation can make more sense to you... let's say that my 4.0L Ford Aerostar (or Explorer, or Ranger, or Mercury Mountaineer) gave me the following compression readings:

  1. Cylinder #1 175 PSI.
  2. Cylinder #2 160 PSI.
  3. Cylinder #3 165 PSI.
  4. Cylinder #4 125 PSI.
  5. Cylinder #5 160 PSI.
  6. Cylinder #6 165 PSI.

The next step is to do the math: 175 x .15= 26, 175-26= 149. So, now I know that cylinder #4 is the one causing the misfire!!