How to Test Engine Compression
(Jeep 4.0L)

Engine Compression Test 2

Now, if you're wondering if the low compression reading you just obtained is coming from worn out piston rings or from worn out cylinder head valves... there's a test for that too.

The next step, is to add about 2 tablespoons of engine oil to the cylinder that gave you the low compression reading in the previous page. This is known as a ‘Wet’ compression test.

This is what you'll need to do:

  1. 1

    Grab some motor oil and add about 3 tablespoons of oil into the engine cylinder that gave you the low compression reading.

  2. 2

    Install the compression tester onto this cylinder and hand-tighten it once again.

  3. 3

    When ready, have your helper crank the engine once again and observe if the needle on the tester will climb further than before. You'll see one of two results:

    1.) The needle will climb to a reading much higher than before, or

    2.) The compression tester's will read the exact same reading as before.

Let's take a look at what your test results mean:

CASE 1: If the compression tester register a higher reading: This result indicates that the piston rings are the ones that are worn and are the ones causing the low compression reading.

CASE 2: If the compression tester DID NOT register a higher reading: This result indicates that the cylinder head valves are the ones that are worn and are the ones causing the low compression reading.

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 in any auto parts store in any neighborhood, in any city... but you'll be paying at least twice as much. Go to the above compression tester links, browse and compare, you'll see a big price difference!

Related Test Articles

To see all of the Jeep 4.0L Wrangler, Cherokee, and Grand Cherokee articles, go to Jeep 4.0L Index of Articles.

Here's a sample of the test articles you'll find in the index:

  1. Jeep PWM Fan Relay Test Troubleshooting an Overheating Condition.
  2. How to Troubleshoot a BAD Fuel Injector (Jeep 4.0L).
  3. How to Test for a blown head gasket (Jeep 4.0L).

Jeep Vehicles:

  • Cherokee 4.0L
    • 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001
  • Comanche 4.0L
    • 1987, 1988, 1989, 1990, 1991, 1992
  • Grand Cherokee 4.0L
    • 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004

Jeep Vehicles:

  • Wagoneer 4.0L
    • 1987, 1988, 1989, 1990
  • Wrangler 4.0L
    • 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006

“The secret to success is to go from mistake to mistake without losing your enthusiasm.”
Anonymous

MORE TUTORIALS: