How To Test Engine Compression (3.0L Nissan)

Interpreting The Results Of The Engine Compression Test

Up to a certain point, having the cylinder compression values differ doesn't affect engine performance or cause any driveability issues.

It's when these values differ by more than 15% of the highest compression value that you're gonna see/feel some engine performance issues.

How do you figure out if the values you got from your test are causing an issue? You can find out by using my online low compression calculator here: Online Low Engine Compression Calculator or manually this way:

• 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.

To make better sense of the above calculation, let's say that my compression test produced the following results:

Cylinder	Pressure
#1	175 PSI
#2	170 PSI
#3	165 PSI
#4	120 PSI
#5	165 PSI
#6	170 PSI

The next step is to do the math:

• STEP 1: 175 x 0.15 = 26.25.
• STEP 2: 26.25 = 26.
• STEP 2: 175 - 26= 149.
• ANSWER: 149 PSI is the lowest possible compression value.

Since cylinder #4 is producing 120 PSI, I can conclude that it's the one causing the misfire. The next step is to find out what's causing the low compression value. Go to: TEST 2: Wet Compression Test.

TEST 2: Wet Compression Test

What causes one (or more cylinders) to have low compression? It's usually due to one of two things: Worn/damaged cylinder head valves or worn/damaged piston compression rings.

You and I can find out which of the two it is without having to tear into the engine by doing a 'Wet' compression test.

At the bottom, I'll show you how to interpret your test results.

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

1. 1

   Add a 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, and as before just hand tighten it.

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.
   
   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

   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: Your compression value shot up. This compression test result confirms that the low compression value registered in this cylinder in the dry test is due to worn piston rings.

CASE 2: Your compression value DID NOT shoot up (stayed the same). This result tells you that the low compression value registered in this cylinder (in the dry test) is due to worn/damaged cylinder head valves.