TEST 2: ‘Wet’ Compression Test

How To Test Engine Compression (4.9L, 5.0L, 5.8L)

If you've gotten a very low compression reading from any one cylinder on your 5.0L engine (4.9L engine or 5.8L engine)... the next step is to add a little bit of engine oil (to that cylinder) and re-check its compression to see if the compression pressure will go up or not.

Adding engine oil and checking the cylinder's compression is known as a ‘Wet’ compression test. This ‘Wet’ compression test will help you determine if the piston rings or cylinder head valves are the ones causing the low or no compression reading you obtained in TEST 1.

The amount of oil you're gonna' add is about 2 tablespoons. If the piston rings are worn (and the cause of the low compression reading), this oil will cause your compression tester to record a higher compression value.

Now, since oil could never help seal the cylinder head valves.... if the compression value does not go up (from the previous one), then you'll know that the problem lies in the cylinder head valves.

OK, this is what you need to do:

  1. 1

    Add oil to the cylinder.

    1. You should add about 2 tablespoons of oil.
    2. Remember, you're only adding oil to the cylinder or cylinders that reported low or no compression in the ‘Dry’ compression test.
  2. 2

    Install the compression tester.

    1. Once again, hand-tighten the compression tester.
  3. 3

    Crank the engine when all is ready.

    1. Once the needle on your compression tester stops climbing, have your helper stop cranking the engine.
    2. 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 what your test results mean:

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 of your 5.0L engine (4.9L engine or 5.8L engine).

CASE 2: The compression value stayed the same.. This confirms that the problem is in the cylinder head valves.

Why An Engine Compression Test?

One of the things that can cause a misfire condition (also known as a miss or dead cylinder), is an engine cylinder or engine cylinders that produce less than normal compression.

When this happens... not matter what gets replaced (like spark plugs, spark plug wires, fuel injectors), nothing solves the miss. This is when the compression test comes in to save the day.

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!

Related Test Articles

To see all of the Ford 4.9L, 5.0L, 5.8L specific articles, go to: Ford 4.9L, 5.0L, 5.8L Index of Articles

Here's a sample of the articles you'll find in the Index of Articles:

  1. Testing A Blown Head Gasket (Ford 4.9L, 5.0L, 5.8L).
  2. Ignition Coil Test -No Spark No Start Tests (Ford 4.9L, 5.0L, 5.8L).
  3. No Start Troubleshooting Tests.
  4. Distributor Mounted Ignition Control Module Test (at: easyautodiagnostics.com).
  5. Fender Mounted Ignition Control Module Test (at: easyautodiagnostics.com).