Calculating The Compression Values To See If They Are Within Range

It's not unusual to see compression values that are not similar to each other when doing an engine compression test (particularly in high-mileage engines).

This is usually not a cause for concern since engine cylinders do not wear out at the exact same rate as time and miles accumulate on your Chevrolet TrailBlazer or GMC Envoy's engine.

When a low engine compression value does cause a problem, it's when this compression value varies more than 15% of the highest engine compression value.

So in this test section I'm going to explain how to figure out if the low engine compression value or values you got in TEST 1 are causing a problem.

Let's say that the highest compression reading you got was 170 PSI. Here's how you can figure this ‘15%’ out:

  1. Grab a calculator and multiply 170 X 0.15. The result is: 25.5 (rounded out to 26).
  2. Now, the next step is to subtract 26 from 170. The result is: 144. Which for the purpose of our discussion means 144 PSI.
  3. So then, 144 PSI is the lowest possible compression reading that any one of the rest of the engine cylinders can have. Any compression reading below this and that engine cylinder will misfire.

To better explain the math, let's say that I got the following compression test results:

  1. Cylinder #1 175 PSI.
  2. Cylinder #2 165 PSI.
  3. Cylinder #3 160 PSI.
  4. Cylinder #4 110 PSI.
  5. Cylinder #5 165 PSI.
  6. Cylinder #6 165 PSI.

The next step is to do the math: 175 x 0.15= 26, 175-26= 149. So, now I know that cylinder #4 is the one causing the misfire!!

TEST 2: Wet Compression Test

A cylinder will produce a low engine compression value if it has a problem with its piston rings or its cylinder head valves.

The cool thing is is that we don't have to remove and disassemble the engine to find out where the problem lies.

How? You might be wondering. Well, we can do this by adding a small amount of engine oil into the cylinder with low compression and retesting it's compression.

If the problem of low compression is due to worn-out piston rings, then the oil that we added to the cylinder will cause the compression reading to shoot up.

If the compression value does not increase, after adding oil to the low compression cylinder, then we can conclude that the problem is due to worn-out or damaged cylinder head valves.

These are the test steps:

  1. 1

    Add a tablespoon (or two) of engine oil in the cylinder you need to retest.

    Use a funnel to make sure that the oil reaches the inside of the cylinder.

  2. 2

    Install the compression gauge on the cylinder and 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 to test, repeat steps 1 thru' 4 on it now.

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

CASE 1: The compression value increased. This tells you that the low compression problem is due to worn piston compression rings.

CASE 2: The compression value DID NOT increase (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.

More GM 4.2L Test Tutorials

If this tutorial was helpful/informative, you can find a complete list of tutorials here: GM 4.2L Index Of Articles.

Here's a sample of the tutorials you'll find there:

  1. How To Test For A Blown Head Gasket (2002-2009 4.2L Chevrolet TrailBlazer).
  2. How Often Should I Replace The Spark Plugs? (2002-2009 4.2L Chevrolet TrailBlazer).
  3. How To Test APP Sensor 1 And 2 (2002-2003 4.2L Chevrolet TrailBlazer).
  4. Electronic Throttle Body Tests (2002-2007 4.2L Chevrolet TrailBlazer).
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Chevrolet Vehicles:

  • Trailblazer 4.2L
    • 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009

GMC Vehicles:

  • Envoy 4.2L
    • 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009