How To Test Engine Compression (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma)

This tutorial will help you test the engine compression on your 2.8L V6 S10 Pickup (GMC S15 pickup, GMC Sonoma) 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.

You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (2.8L V6 GM) (at: autotecnico-online.com).

APPLIES TO: This tutorial applies to the following vehicles:

• 2.8L Chevrolet S10 Pickup: 1988, 1989, 1990, 1991, 1992, 1993.
• 2.8L GMC S15 Pickup: 1988, 1989, 1990.
• 2.8L GMC Sonoma: 1991, 1992, 1993.

This tutorial also applies to the 2.8L Chevy S10 Blazer, 2.8L GMC S15 Jimmy. See the 'Applies To' box on the left column (desktop) or at the bottom of the page (mobile device) for more info.

Important Tips And Suggestions

TIP 1: If your 2.8L equipped GM S10 pickup (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 near a cranking engine when performing the engine compression test, so take all necessary safety precautions. Be careful and always keep safety in mind.

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. This includes low engine compression in one or more cylinders.

If you've got a hard-to-diagnose rough idle condition on your hands, there's a good chance it could be due to uneven cylinder compression.

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

• Engine cranks but does not start (0 compression in all cylinders).
• Blue smoke coming out of the tailpipe.
• Rough idle (engine misfires).
• Bad gas mileage.
• Engine 'misses' at idle but 'miss' disappears as you accelerate.
• 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.

Which Compression Tester Should I Buy?

There are lot of engine compression testers to choose from and many places to buy them. I'm gonna' make some recommendations to you:

TEST 1: Dry Compression Test

To test engine compression, you must remove the spark plugs. This means you need to disconnect the spark plug wires from the spark plugs.

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

One last recommendation: Use a spark plug wire puller to disconnect the spark plug wire from the spark plug. Why? Because pulling the spark plug wire boot by hand may cause the wire's metal terminal to come loose and stay stuck on the spark plug. The following tutorial provides you with more information: How To Use A Spark Plug Wire Puller And Where To Buy One.

IMPORTANT: Do not remove the spark plugs when the engine is hot. If the engine has been running for any length of time, allow it to cool completely before removing the spark plugs.

CAUTION: Take all necessary safety precautions. The engine has to be cranked to perform the engine compression test. Be careful and think safety all the time!

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.
   
   Disconnecting the ignition coil will prevent it 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

   Disconnect all six spark plug wires from their spark plugs.
   
   IMPORTANT: Label the spark plug wires with the cylinder number they belong to before removing them. This will help you to reconnect them in the correct firing order when you're done with the compression test.

4. 4

   Remove the spark plugs.
   
   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.

5. 5

   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.

6. 6

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

7. 7

   Record on paper the value at which the needle stopped and the number of the engine cylinder on a piece of paper.

8. 8

   Release the pressure on the gauge and repeat step 6 and 7 one more time.

9. 9

   Check the compression of the remaining cylinders.

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

CASE 1: The engine does not start and the results of the engine compression test are 0 PSI on all 6 cylinders. This test result usually indicates you have one of the following problems:

• A timing chain problem.
• A blown head gasket.
• A blown engine.

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

CASE 2: The engine does start but the compression values you wrote down for each cylinder are slightly different from one another. Up to a certain point this could be normal, but we need to investigate a little further.

Your next step is to go to: Interpreting Your Compression Test Results.

Interpreting Your Compression Test Results

Within a certain range, a difference in compression between cylinders does not affect engine performance.

Your 2.8L V6 Chevy S10 pickup or GMC S15 pickup or GMC Sonoma's engine performance will only be affected if the compression values vary too much.

The cool thing is that we can find out whether or not the differences in the values you noted in TEST 1 indicate a problem (with that cylinder).

The rule of thumb is that the compression values 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).

The rule of thumb is that the compression readings can't differ by more than 15%, and if they do, you'll have a cylinder misfire problem or possibly an engine no-start issue (if more than one cylinder is affected).

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

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

You can find my 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
#3	165 PSI
#4	160 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

If in TEST 1 you found that you do have one or more cylinders with low (or no) compression, then the next step is to do a 'wet' compression test.

The 'wet' compression test will let us pinpoint the problem to worn cylinder head valves or worn piston rings (as the cause of the low or no compression test result).

At the bottom, I'll show you how to interpret your test results.

OK, let's get testing:

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

   Have your helper crank the engine.

4. 4

   Stop cranking the engine once the needle stops climbing on the compression gauge.

5. 5

   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.

6. 6

   Write the compression value down on a piece of paper.

7. 7

   Repeat steps 1 through 6 on any other cylinder you need to test.

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

CASE 1: The compression value shot up. This compression test result confirms that the low compression reading recorded on this cylinder in the dry test is due to worn piston rings.

The reason the compression reading went up is because the engine oil you just added helped the piston rings create a tighter seal. This type of test result only occurs if the problem is due to worn piston rings.

CASE 2: The compression value DID NOT shoot up (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 2.8L Chevy S10 Pickup, GMC S15 Pickup, And GMC Sonoma Tutorials

You can find a complete list of tutorials for the 2.8L V6 Chevrolet S10 pickup, GMC S15 pickup, and GMC Sonoma in this index:

• 2.8L Chevrolet S10, GMC S15, And GMC Sonoma Index of Articles.

Here's a sample of the articles, you'll find in the Index of Articles:

• How To Test The Fuel Pump (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
• How To Diagnose An Engine No-Start Problem (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
• How To Test The EGR Valve (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
• How To Test The Starter Motor (1988-1993 2.8L V6 Chevrolet S10 Pickup, GMC S15 Pickup).