How To Test Engine Compression (1985-1993 2.5L Chevrolet S10, GMC S15, GMC Sonoma)

Interpreting Your Compression Test Results

Up to a point, it's normal for engine cylinders to have different compression levels.

Within a certain range, this variation in compression values does not cause engine performance problems.

But if the compression readings differ by more than 15% from the highest reading obtained from your test, you'll have an engine performance problem (usually an engine cylinder misfire) on your hands.

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

• Do the math with pen and paper.
• Use my low compression calculator.

If you want to use the low compression calculator, go 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 reading you got from your engine compression test is in a good range, you need to do the same calculation. Of course, you must use the highest compression value you got, not the one in the example.

Once you've found the 'dead' cylinder, the next step is to figure out what's causing the low compression reading. For this step, go to: TEST 2: Wet Engine Compression Test.

TEST 2: Wet Engine Compression Test

The two things that cause an engine cylinder to have a low or 0 PSI compression reading are:

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

Thankfully, we don't need to tear the engine apart to find out what's causing the low or 0 PSI engine compression reading you got in TEST 1.

By doing a simple wet compression test, we can determine if the cylinder's low compression reading is due to bad piston rings or bad intake/exhaust valves.

These are the test steps:

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.