Interpreting The Compression Test Results

Interpreting the compression test results is easier than you think. It does require doing a little math, but I'll walk you through the entire process.

Before we jump into figuring out if the compression readings are normal or not, I'll tell you that it's normal for the compression value of each cylinder to vary somewhat.

This is due to the fact that the cylinders don't wear out exactly the same. What isn't normal is if the values vary more than 15% of the highest compression reading.

How do you figure this out? You can find out by using my online low compression calculator here: Online Low Engine Compression Calculator or by calculating this 15% difference manually.

To understand how to figure out this 15% thing manually, let's say that my 2.4L Honda CR-V's engine produced the following compression values:

• Cylinder #1 175 PSI.
• Cylinder #2 165 PSI.
• Cylinder #3 160 PSI.
• Cylinder #4 120 PSI.

The next step is to do the following math:

• Multiply .15 (15%) by the highest value: 175 x 0.15. This gives us 26.25, but we'll round it out to 26.
• Next, we subtract 26 from 175: 175 - 26 = 144.
• So now we know that the lowest possible compression value is: 144 PSI.

This means that cylinder #4, which has a compression value of 120 PSI, is the one causing the misfire (P0304) because it's below the 144 PSI minimum.

Once we've found the 'dead' cylinder, the next step is to find out what's causing the low compression value. For this step, go to: TEST 2: Wet Compression Test.

TEST 2: Wet Compression Test

Now that you've found the dead cylinder (or cylinders with low compression), the next step is to find out what's provoking the problem. Low compression in any given cylinder, is usually due to one of two things.

Either the dead cylinder's piston rings are severely worn or its cylinder head valves are severely worn.

This is where a 'wet' compression test comes in handy and it's the test we're about to do in this test section.

OK, this is what you'll need to do:

1. 1

Add a tablespoon (or two) of engine oil in the cylinder you need to retest. I suggest using a small and long funnel so that the oil will reach the inside of the cylinder.

2. 2

Once you've added the oil, install the compression gauge, and as before just hand tighten it.

3. 3

Now, have your helper crank the engine till the needle stops climbing on the compression gauge.

4. 4

As before, your job is to keep an eye on the gauge, and you'll see one of two results:

1.) The needle will climb higher than the previous compression number you recorded for this specific cylinder, or...

2.) The needle will not move at all or stay at the same number you recorded earlier.

5. 5

If you have another cylinder that needs to be tested, repeat steps 1 thru' 4 on it now.

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

CASE 1: The compression value shot up for the low compression cylinder. This test result lets you know that extremely worn piston rings are the ones causing the low compression value in that specific cylinder.

The reason the compression value shot up is due to the fact that the motor oil you just added helped the piston rings to create a tighter seal. This type of test result only happens when the problem is due to worn piston rings.

CASE 2: Your compression value DID NOT shoot up (stayed the same). This test result lets you know that that cylinder's cylinder head valves are worn/damaged and are the ones causing the low compression value in that specific cylinder.

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 2.4L Honda CR-V Tutorials

You can find a complete list of tutorials here: Honda 2.4L Index Of Articles.

Here's a sample of the tutorials you'll find there:

If this info really saved the day, buy me a beer!