How To Test Engine Compression (1994-2003 2.2L Chevrolet S10, GMC Sonoma)

Interpreting Your Compression Test Results

It's not unusual for the compression values to differ, especially on high mileage engines.

A variation in the compression values will not cause any issues up to a certain range. But if the values vary too much (more than 15%), you'll definitely have a rough idle or an engine misfire issue on your hands.

You can figure out the 15% difference in one of two ways:

• Calculate this 15% difference with pen and paper.
• Use my low compression calculator.

You can find the low compression calculator here: Online Low Engine Compression Calculator.

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 I got the following compression readings:

Cylinder	Pressure
#1	165 PSI
#2	95 PSI
#3	155 PSI
#4	175 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.

TEST 2: Wet Engine Compression Test

What usually causes a 0 PSI or a low compression value in a cylinder is one of two things:

• The affected cylinder's piston rings are worn out.
• The affected cylinder's intake/exhaust valves are worn or damaged.

The cool thing is that we can find out what's causing the compression issue in a specific cylinder without having to disassemble the engine.

All we need to do is perform a wet compression test on the cylinder with 0 PSI or low compression.

To be more specific, you'll add about a tablespoon or two of engine oil to the affected cylinder and retest its compression.

If the compression value increases, you can conclude that the compression issue is due to worn-out piston rings.

If the compression value doesn't change, the issue is due to worn intake/exhaust valves.

Let's get started:

1. 1

   Add a small amount of engine oil to the cylinder that reported low compression or no compression in the ‘dry’ compression test.
   
   You don't have to add a lot of oil. The amount should be about 1 to 2 tablespoons of oil.

2. 2

   Install the compression tester onto the cylinder.
   
   Do not use any type of tool to tighten 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:
   
   1.) The compression value will go up (from the one you recorded before).
   
   2.) The compression value will stay the same.

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

CASE 1: The compression value shot up. This test result confirms that the piston compression rings are worn out and causing the low compression value you got for this cylinder in TEST 1.

CASE 2: The compression value stayed the same. This test result confirms that the low compression problem of the affected cylinder is due to worn or damaged cylinder head valves.