How To Test Engine Compression (Nissan 3.0L, 3.3L, 3.5L)

Interpreting Your Compression Test Results

When performing a compression test on your Nissan's engine, it's common to find that the compression readings for each cylinder are not exactly the same. This slight variation is normal due to differences in factors like cylinder wear, valve condition, and piston ring sealing.

However, if the compression value variation between cylinders is significant, typically exceeding a 15% variation between cylinders, you may some serious internal engine mechanical issues.

Usually, these variations (in the compression reading) signify problems such as worn piston rings, damaged valves, or cylinder head gasket leaks, which can affect engine performance and reliability.

To determine if the compression readings are problematic, you can do this one of two ways: You can calculate this 15% difference with pen and paper or you can use my low compression calculator. You can find the low compression calculator here: Online Low Engine Compression Calculator (at: easyautodiagnostics.com).

If you want to manually calculate the 15% difference, here's what you'll need to do:

• STEP 1: Multiply the highest compression value by 0.15 (this is the decimal value of 15%).
• STEP 2: Round the result to the nearest one (for example: 25.6 would become 26).
• STEP 3: Subtract the result (the number that was rounded) from the highest compression value.
• ANSWER: The result of this subtraction is the lowest possible compression value any cylinder can have.

Now, let me give you a more specific example: Let's say that the vehicle I'm working on produced the following compression readings:

Cylinder	Pressure
#1	165 PSI
#2	95 PSI
#3	155 PSI
#4	175 PSI
#5	175 PSI
#6	170 PSI

My next step is to do the following calculation:

• STEP 1:  175 x 0.15 = 26.25.
• STEP 2:  26.25 = 26 (rounded to nearest one).
• STEP 3:  175 - 26 = 149.
• ANSWER:  149 PSI. Any cylinder with this compression (or lower) value will misfire.

Since cylinder #2 is only producing 95 PSI, I can now conclude that it's 'dead' and causing a misfire.

To find out if the lowest compression value you got from your engine compression test is within a good range, you'll need to do the same calculation. Of course, you'll need to use the highest compression value you got and not the one in the example.

Once you'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’ Engine Compression Test.

The ‘Wet’ Engine Compression Test

The next step, after finding out that you do have one or more cylinders with low compression, is to add about 2 tablespoons of oil to them and then retest their compression once more.

The 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.

This test, in which you add engine oil to the cylinders, is known as a ‘Wet’ compression test.

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

2. 2

   Install the compression tester onto the cylinder.
   
   IMPORTANT: 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: The compression value will go up (from the one you recorded before) or it will stay the same.

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, 3.3L, or 3.5L Nissan.

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

Why An Engine Compression Test?

An engine compression test is one of the most important tests to perform when trying to solve a hard to diagnose misfire condition / rough idle condition. If only one cylinder has below average compression (compared to the other 5 cylinders), that cylinder will not contribute to engine power and you're gonna' feel it.

Also, no matter what you replace, the misfire condition or misfire codes (P0300, P0301, P0302, P303, P0304, P0305, P0306) will not go away! Unfortunately, the engine compression test is one of the most overlooked misfire troubleshooting tests. Now, testing the compression of each cylinder is usually done after verifying that each cylinder is getting spark (this can be easily accomplished by using a spark tester).

Related Test Articles

You can find more ‘How To Test’ articles for your 3.0L, 3.3L, 3.5L equipped Nissan car, pick up or mini-van at: 3.0L, 3.3L, 3.5L Nissan Index Of Articles.

Here's a small sample of the tutorials you'll find in the index:

1. How To Test The Starter Motor (Nissan 3.0L, 3.3L).
2. Mass Air Flow (MAF) Sensor Test 3.0L Nissan Maxima (1995-1999) (at: easyautodiagnostics.com).
3. Power Transistor Test & Ignition Coil Test 3.3L Nissan (1996-2004) (at: easyautodiagnostics.com).
4. How To Test The Throttle Position Sensor (1996-1999 3.0L Maxima) (at: easyautodiagnostics.com).