‘Wet’ Engine Compression Test

How To Do And Interpret an Engine Compression Test

You might be asking yourself, “What's a Wet Compression Test?”... It's the same compression test you did before with the difference being that you're gonna' add about two tablespoons of oil to the engine cylinder that showed a low compression reading in the ‘Dry’ compression test.

The engine oil that you're gonna' add to the cylinder will help determine if the low cylinder pressure or pressures you recorded in the ‘Dry’ compression test are caused by worn piston rings or worn cylinder head valves.

Depending on whether the compression pressure rises (on your compression tester) or not, you'll be able to say that the problem lies in the piston's rings or in the cylinder head valves.

OK, this is what you need to do:

  1. 1

    Add a small amount of engine oil to the cylinder that reported low compression or no compression in the ‘Dry’ compression test.

    1. The amount should be about 1 to 2 tablespoons of oil.
  2. 2

    Install the compression tester onto the cylinder.

    1. Do not use any type of tool to tightened the compression tester... hand tight is fine.
  3. 3

    When all is set up, have your helper crank the engine.

  4. 4

    You'll get one of two results, either the compression value will go up (from the one you recorded before) or it will stay the same.

Let's take a look at your test results:

CASE 1: The compression value shot up.. This tells you that the piston compression rings are worn out and thus the problem is in the bottom end (block) of the engine in your 3.0L/3.8L Ford vehicle.

CASE 2: The compression value stayed the same.. This test result tells you that the low compression in this specific cylinder is due to bad cylinder head valves.

Why an Engine Compression Test?

This is one of the most overlooked tests to find the root cause of a misfire code, rough idle or an engine miss or a blown head gasket.

Over the years, I have solved many unsolvable misfire codes, rough idle, lack of power issues by doing a simple engine compression test and if you're faced with something similar... I highly recommend doing an engine compression test.

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!

More 3.0L, 3.8L Diagnostic Tutorials

You can find a complete list of 3.0L, 3.8L Ford diagnostic tutorials in this index: Ford 3.0L, 3.8L Index of Articles.

If this tutorial did not help you solve your misfire condition, misfire code, or rough idle condition, I recommend taking a look at the following specific Ford V6 article(s):

At easyautodiagnostics.com, you'll find the following articles:

  1. How To Test The Ford V6 Ignition Coil Pack.
  2. How To Test The Ignition Control Module (older 3.0L and 3.8L with a Distributor).
  3. How To Test The MAF Sensor.
  4. How To Test The Throttle Position Sensor (TPS).