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.

I've found many misfiring cylinders that had both fuel and spark but their mechanical condition was in such a bad state that the cylinder could not produce enough power.

My recommendation to you, if you're diagnosing a misfire issue, is to perform the engine compression test after you have verified that all cylinders are getting spark and fuel.

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

  1. 1

    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.

  2. 2

    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.

  3. 3

    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!

  4. 4

    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.

  5. 5

    Have your helper crank the engine.

  6. 6

    When the tester's needle stops climbing, have your helper stop cranking the engine.

  7. 7

    Now, at whatever number the needle, on the gauge, stops, write this number down along with the number of the cylinder.

    This number will be the maximum compression value for that specific cylinder.

  8. 8

    Repeat steps 4-7 on the other 5 cylinders.

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

CASE 1: No compression in 2 or more cylinders. This test result tells you that the engine has serious internal problems.

The most common issues would be: Broken timing chain. Or a blown head gasket. Or the engine threw a rod. To test for a blown head gasket, see this tutorial: How To Test A Blown Head Gasket (GM 3.1L, 3.4L) (NOTE: this info will apply to your 3.3L V-6 equipped Chrysler mini-van).

CASE 2: Low compression in one or more cylinders. To a certain point, it's normal for the compression to vary a little between cylinders (as the engine accumulates thousands of miles). But if these values vary too much, then you're gonna' have a bona-fide misfire on your hands.

So, your next step is to do the math and find out if these compression values are within normal parameter or not. Go to: Interpreting Your Compression Test Results.

Ford Vehicles:

  • Aerostar 4.0L
    • 1993, 1994, 1995, 1996, 1997
  • Explorer 4.0L
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003
  • Ranger 4.0L
    • 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003

Mercury Vehicles:

  • Mountaineer 4.0L
    • 1997, 1998, 1999, 2000, 2001, 2002, 2003