How To Test Engine Compression (1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 4.0L Ford Explorer, Aerostar, And Mercury Mountaineer)

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

You can find the 4.0L Ford Ranger (Mazda B4000) engine compression test tutorial here:

APPLIES TO: This tutorial applies to the following vehicles:

  • 4.0L V6 Ford Aerostar: 1991, 1992, 1993, 1994, 1995, 1996, 1997.
  • 4.0L V6 Ford Explorer: 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010.
  • 4.0L V6 Mercury Mountaineer: 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010.

En Español You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (Ford 4.0L) (at: autotecnico-online.com).

Tools You'll Need:

  • Compression Gauge Tester.
  • A Helper.
  • Pen and Paper.

Symptoms Of An Engine Compression Problem

An engine compression problem will usually cause one of two issues:

  • An engine no-start problem (engine cranks but does not start).
  • The engine starts and runs, but overheats within minutes and for no apparent reason.

If your Ford Explorer, Aerostar or Mercury Mountaineer's engine starts and runs but has has an engine compression problem, you'll see one or more of the following symptoms:

  • Bad gas mileage.
  • A heavier exhaust smell coming out of its tailpipe.
  • Engine is not as peppy as it was once.
  • Rough idle that goes away as soon as you accelerate the engine.
  • Misfire trouble codes (on OBD II equipped vehicles):
    • P0300: Random Cylinder Misfire.
    • P0301: Cylinder #1 Misfire.
    • P0302: Cylinder #2 Misfire.
    • P0303: Cylinder #3 Misfire.
    • P0304: Cylinder #4 Misfire.
    • P0305: Cylinder #5 Misfire.
    • P0306: Cylinder #6 Misfire.

Which Compression Tester Should I Buy?

There are lot of engine compression testers to choose from and many places to buy them. I'm gonna' make two recommendations to you:

1) Which one to buy:  The engine compression tester that I have always used is the Actron CP7827 Compression Tester Kit. My only complaint about this engine compression tester is that it does not come with a case to store it in.

Engine Compression Gauge Testers

2) Where to buy:  You can buy an engine compression tester just about anywhere, but you'll end up paying more for it (especially at your local auto parts store). The above links will help you comparison shop. I think you'll agree it's the better way to save money on the compression tester!

TEST 1: ‘Dry’ Engine Compression Test

How To Test Engine Compression (1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 4.0L Ford Explorer, Aerostar, And Mercury Mountaineer)

As you might already know, every engine cylinder in your 4.0L Ford Explorer (Aerostar, Mountaineer) needs spark, fuel, and air to start and maintain the combustion process.

If any one of these is missing, 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.

Unfortunately, the compression test is one of the most overlooked tests when troubleshooting a misfire (rough idle) or an engine no-start.

I've found many misfiring cylinders with 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 behind this test result would be:

  • Broken timing chain.
  • A blown head gasket.
  • The engine threw a rod.

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 bonafide 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
    • 1991, 1992, 1993, 1994, 1995, 1996, 1997
  • Explorer 4.0L
    • 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010

Mercury Vehicles:

  • Mountaineer 4.0L
    • 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010