How To Do An Engine Compression Test (2.8L S10 Pickup and 2.8L S10 Blazer)

This tutorial will help you test the engine compression on your 2.8L S10 Pick-up or 2.8L S10 Blazer. It'll also help you to further narrow down the problem, if you do find low compression in one or more cylinders, to either the cylinder head valves or the engine piston compression rings with a ‘Wet’ compression test.

Contents of this tutorial at a quick glance:

  1. Important Tips And Suggestions.
  2. Symptoms of Low or No Cylinder Compression.
  3. TEST 1: Dry Compression Test.
  4. TEST 2: Wet Compression Test.
  5. Which Compression Tester Should I Buy?

En Español Puedes encontrar este tutorial en Español aquí: Cómo Probar la Compresión del Motor (2.8L V6 GM) (en: autotecnico-online.com).

Important Tips And Suggestions

TIP 1: If your 2.8L equipped GM S10 Pick Up (Blazer) starts and runs, it's standard procedure to do the compression test with a slightly warmed up engine. The key words here are ‘slightly warmed up engine’ since the engine should not be hot.

To slightly warm up the engine, start her up (from a completely cold condition) and let her run for no more than 10 to 12 minutes.

TIP 2: You'll be working around a cranking engine as you perform the engine compression test so take all necessary safety precautions. Your safety is your responsibility... so use common sense and think safety all of the time.

TIP 3: Never remove the spark plugs with a hot engine. This is important because you could strip the spark plug hole threads as you're removing the spark plugs from the hot cylinder heads. Believe me, you don't want this happening to you!

Symptoms of Low or No Cylinder Compression

Quite a few things can cause a rough idle condition. Among them is low engine compression in one or more cylinders. So there's a good chance that if you are having a hard to diagnose rough idle condition... it could be due to uneven cylinder compression.

Here are some other specific symptoms you may see with low cylinder compression:

  1. Engine cranks but does not start (0 compression in all cylinders).
  2. Blue smoke coming out of the tailpipe.
  3. Rough idle (engine misfires).
  4. Bad gas mileage.
  5. Engine ‘misses’ at idle but ‘miss’ disappears as you accelerate.
  6. Check engine light is illuminated with a MAP sensor trouble code (even tho' the MAP sensor is good).

As you can see, low engine compression doesn't mean the engine is not gonna' start. Let's head down to the next subheading and get testing.

TEST 1: Dry Compression Test

How To Do An Engine Compression Test (2.8L S10 Pickup and 2.8L S10 Blazer)

Testing the engine compression requires that you remove the spark plugs. This means you have to disconnect the spark plug wires from the spark plugs.

Before you unplug the spark plug wires, it's a good idea to label the spark plug wires with the cylinder number they belong to.

One last recommendation: Use a spark plug wire puller tool to unplug the spark plug wire from the spark plug. Why? Cause pulling on the wire boot by hand can cause the wire's metal terminal to pull off and stay stuck on the spark plug. The following tutorial offers you more info: How To Use A Spark Plug Wire Puller And Where To Buy One.

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

  1. 1

    Disable the ignition system by disconnecting the ignition coil from its electrical connectors. This will prevent the ignition coil from sparking during the test.

  2. 2

    Disable the fuel system by disconnecting the two throttle body fuel injectors. Disabling the fuel system will prevent fuel from being injected into each cylinder when the test is performed.

  3. 3

    Remove the spark plugs from a slightly warmed up engine (if it starts and runs). Remember, the engine can not be hot.

    When removing the spark plugs, be careful not to drop any of them on the floor, or you run the risk of having the spark plugs porcelain insulator crack and then you'll have a misfire on your hands.

    If the engine does not start... don't worry about it being warmed up.

  4. 4

    Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder. Hand tighten the compression gauge only! Do not use any type of tool to get it tight.

  5. 5

    Have your helper crank the engine till the needle on the compression gauge stops climbing.

    Now, record on paper the value at which the needle stopped and the number of the engine cylinder on a piece of paper. Release the pressure on the gauge and repeat this step one more time.

    Repeat this test step on the remaining 5 cylinders.

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

CASE 1: If the Engine Does Not Start and the results of the engine compression test are 0 PSI on all 6 cylinders then you have one of the following conditions:

  1. Timing chain problem.
  2. Blown head gasket.
  3. Blown engine.

Any compression value below 100 PSI (even if it does not 0 PSI) means internal mechanical engine trouble.

CASE 2: If the Engine Does Start, the very first thing you'll notice is that the compression values you wrote down for each cylinder are slightly different from one another. This is normal.

What is NOT normal is if the 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?... This is how you do it:

  1. Grab a calculator and multiply the highest compression reading that you recorded by .15. So, let's say that cylinder #4 gave you the highest reading of 170 PSI. Well 170 X .15 gives you 26 (25.5 rounded off).
  2. Now, the next step is to subtract 26 from 170... which gives us 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 make better sense of the above calculation... let's say that my 2.8L S10 Pick Up (2.8L S10 Blazer) produced the following compression test results:

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

The next step is to do the math: 175 x .15= 26, 175-26= 149. So, now I know that cylinder #6 is the one causing the misfire!!