How To Do And Interpret An Engine Compression Test

Doing a compression test on your Ford or Mercury Taurus, Sable, Windstar equipped 3.0L and 3.8L V6 is an easy test to do. This article will take you thru' the test and interpretation of the cylinder compression results in a step by step fashion.

By the way, this article is geared towards helping you solve a misfire condition or misfire codes (P0300, P0301, P0302, P303, P0304, P0305, P0306), since a low compression reading in only one cylinder, out of the six, will cause a miss, rough idle condition that no matter what gets replaced (like spark plugs, spark plug wires, fuel injector, etc.), the vehicle will continue to miss.

En Español You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (Ford 3.0L, 3.8L) (at: autotecnico-online.com).

Tools You'll Need:

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

Symptoms Of Low Or No Engine Compression

In the majority of cases, engine compression problems will cause one of two problems:

  1. The engine does not start.
  2. The engine starts and runs, but runs with a misfire (rough idle).

On OBD II equipped vehicles, low compression problems will usually set a misfire trouble code:

  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.
  6. P0305: Cylinder #5 Misfire.
  7. P0306: Cylinder #6 Misfire.

You're also gonna' see one or more of the following:

  1. Bad gas mileage.
  2. Engine burns oil when accelerating the vehicle on the road.
  3. Rough idle (or misfire) that goes away once the engine is accelerated.

TEST 1: Engine Compression Test

How To Do And Interpret An Engine Compression Test

You'll be working around a cranking engine, so take all necessary safety precautions and use common sense.

The engine's temperature is a consideration to keep in mind when doing a compression test (if the engine starts and runs).

But keep in mind that the engine in your Ford (or Mercury) can not be hot or you run the risk of damaging the spark plug hole threads.

So, if you have just turned the car or mini-van off from an extended run time, let it cool down for about an hour. If the vehicle is cold, start the car (or mini-van) up and let it warm up for no longer than 10 minutes.

If your vehicle doesn't start, then don't worry, don't worry about the engine being warm -you can still perform the engine compression test.

NOTE: If you don't have a compression tester and need to buy one, take a look at my recommendations here: Which Compression Tester Should I Buy?

OK, let's get testing:

  1. 1

    Disable the fuel system. You can do this by removing the fuel pump relay.

    The reason you need to disable the fuel system is to keep the PCM from injecting fuel into the engine cylinders as you perform the compression test.

  2. 2

    Disable the ignition system. This can be done by simply disconnecting the ignition coil or the ignition coil pack from its electrical connector.

    It's important to disable the ignition system so that you won't have spark firing off at the spark plug wires (while you do the compression test).

  3. 3

    Remove all of the 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.

    Hand tighten the compression gauge only! Do not use any type of tool to get it tight.

  5. 5

    Ask your helper to crank the engine once the compression gauge is set up and hand tight in the spark plug hole.

  6. 6

    Once the needle stops climbing on your compression gauge, have him or her stop cranking the engine.

  7. 7

    Record on paper the value at which the needle stopped and the number of the engine cylinder on a piece of paper.

  8. 8

    Repeat steps 4 thru' 7 on the remaining five cylinders.

OK, let's make sense of all of the numbers you recorded for each cylinder:

CASE 1: You got 0 PSI on all six cylinders. This is not good. The engine won't start if all six cylinders have 0 PSI compression.

0 PSI compression across all six cylinders is usually the result of one the following conditions:

  1. Broken timing chain.
  2. Blown head gasket.
  3. Blown engine.

CASE 2: One or more cylinders had low compression. Up to a certain point this is normal. But if the low compression value or values are lower than 15% of the highest, then those cylinders will misfire.

Thankfully, we can easily figure out if these cylinders (with the low compression values) are causing a problem by doing a little math. Go to: Interpreting The Results Of The Engine Compression Test.

Ford Vehicles:

  • Aerostar 3.0L
    • 1992, 1993, 1994, 1995, 1996, 1997
  • Mustang 3.8L
    • 1997, 1998, 1999, 2000, 2001, 2004
  • Probe 3.0L
    • 1990, 1991, 1992
  • Ranger 3.0L
    • 1991, 1992, 1993, 1994, 1995, 1996, 1997

Ford Vehicles:

  • Taurus 3.0L, 3.8L
    • 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005
  • Tempo 3.0L
    • 1992, 1993, 1994
  • Thunderbird 3.8L
    • 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997

Lincoln Vehicles:

  • Windstar 3.0L, 3.8L
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003

Lincoln Vehicles:

  • Continental 3.8L
    • 1989, 1990, 1991, 1992, 1993, 1994

Mercury Vehicles:

  • Cougar 3.8L
    • 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997
  • Sable 3.0L, 3.8L
    • 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005
  • Topaz 3.0L
    • 1992, 1993, 1994

Mazda Vehicles:

  • B3000 3.0L
    • 1994, 1995, 1996, 1997