How To Test The Engine Compression (2.5L Suzuki Verona)

Interpreting The Results Of The Engine Compression Test

There's a good chance that the compression values you wrote down for each cylinder are slightly different from one another. This could normal and nothing to worry about yet or it could be a problem.

What could cause a problem is if the cylinder compression values vary too much. The cool thing is that we can find out if the variations in the values, you wrote down, indicate a problem (with that cylinder) or not.

The rule of thumb is that they can not vary more than 15% from each other and if they do, you're gonna' have a genuine misfire condition on your hands or possibly a no-start condition (if more than one cylinder is affected).

How do you figure this out? 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 I got the following compression test 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

Low cylinder compression is usually due to 1 of 2 things. Either the cylinder head valves (of the affected cylinder) are severely worn/damaged or the piston rings are worn/damaged.

You and I can find out by adding a small amount of motor oil to the low compression cylinder. This is what is known as a 'Wet' compression test.

If the problem is bad piston rings, the motor oil that we're gonna' add will help bring the compression value up when we check that cylinder's compression again.

If the problem is bad cylinder head valves, the motor oil WILL NOT help raise the compression value of the cylinder. It's as simple as that!

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.
   
   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: The compression gauge registered a higher compression value after adding motor oil. This tells you that the low compression problem is due to worn piston compression rings.

Here's why: 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: The compression gauge DID NOT register a higher compression value after adding motor oil (in other words, it 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.