How To Test Engine Compression (1996-2004 3.3L Frontier, Pathfinder, Xterra)

Interpreting The Compression Test Results

In this section, I'm gonna' explain how to interpret the compression test results you got from TEST 1.

Depending on the mileage and age of the engine it's normal for the engine compression results to vary somewhat between each other.

This is due to the fact that engine cylinders don't wear out at the exact same rate over the course of thousands of miles.

But if the variation in the values is more than 15% then you're gonna' have problems. And for the most part you're gonna' see a rough idle condition or a cylinder misfire problem.

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, I'll use the following compression test results:

• Cylinder #1 175 PSI.
• Cylinder #2 165 PSI.
• Cylinder #3 160 PSI.
• Cylinder #4 120 PSI.
• Cylinder #5 150 PSI.
• Cylinder #6 150 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 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 Engine Compression Test

In this section, I'm gonna' explain how to do a wet compression test.

A wet compression test will help you pinpoint the source of the low compression value you got from a specific cylinder in TEST 1.

Usually two types of problems cause a low compression test result. Either the low compression problem will be caused by worn or damaged cylinder head valves. Or the compression problem will be caused by worn piston rings.

You and I can find out without the need to disassemble the engine. And it's done by adding a small amount of engine oil to the cylinder with the low compression value.

Then that cylinder's compression is tested again. If the compression value increases then you can conclude that the low compression problem is due to worn piston rings.

If the compression value does not increase then you can conclude that that cylinder's head valves are worn out or damaged.

These are the test steps:

1. 1

   Add 1 or 2 tablespoon 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

   Install the compression gauge on the cylinder you just added oil to.

3. 3

   Have your helper crank the engine till the needle stops climbing on the compression gauge.

4. 4

   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.
   
   What ever value your compression tester reads, write it down again.

5. 5

   Repeat steps 1 thru' 4 on any other cylinder you need to check.

Let's examine your test results:

CASE 1: The compression value of the cylinder you added oil to increased. This test result tells you that the low or near 0 PSI compression value is due to worn out piston rings of that specific cylinder.

CASE 2: The compression value of the cylinder you added oil to DID NOT increase. This test result confirms that the cylinder head valves of that cylinder are worn-out or damaged.

More Nissan 3.3L V6 Tutorials

If this tutorial was helpful, check out the others I've written for the 3.3L V6 equipped Nissan vehicles. You can find a complete list of tutorials here: Nissan 3.3L Index Of Articles.

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

• Mass Air Flow (MAF) Sensor Test 3.3L Frontier, Quest, Pathfinder, XTerra.
• How To Test The Camshaft Position Sensor (3.3L V6 Frontier, Quest, Pathfinder, Xterra).
• Power Transistor Test And Ignition Coil Test 3.3L Nissan (1996-2004).
• Ignition System Wiring Diagram (1999-2004 3.3L Frontier And Xterra).