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

The engine compression test is the go-to test to find out the engine's internal health.

Thankfully, testing the engine compression on the 2.2L Chevrolet S10 and GMC Sonoma is pretty easy since the spark plugs are accessible.

In this tutorial, I'll explain the engine compression test step by step and, more importantly, how to interpret its results.

You'll quickly and easily find out if an engine compression problem is causing an engine no-start problem or a misfire.

You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (1994-2003 2.2L Chevrolet S10, GMC Sonoma) (at: autotecnico-online.com).

APPLIES TO: This tutorial applies to the following vehicles:

• 2.2L Chevrolet S10: 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003.
• 2.2L GMC Sonoma: 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003.
• 2.2L Isuzu Hombre: 1996, 1997, 1998, 1999, 2000.

Symptoms Of Low Or No Engine Compression

Depending on the type of engine compression problem your engine is experiencing, you'll see one of the two following issues:

• An engine no-start problem.
• An engine misfire problem.

Here's a basic breakdown of the symptoms you'll see when the engine starts but is suffering an engine compression problem:

• Bad gas mileage.
• A heavier exhaust smell coming out of its tailpipe.
• Engine is not as peppy as it was once.
• Rough idle that goes away as soon as you accelerate the engine.

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 Engine Compression Test

I'm going to recommend that you test the compression of all four cylinders to get the most accurate test result.

If you're wondering why (you need to test all four cylinders), it's to obtain the highest compression value the engine produces to interpret the test results accurately.

Once you have obtained the compression values of all four cylinders, we'll turn the page and interpret what it all means.

If you don't have an engine compression tester, you can borrow one from your local auto parts store (AutoZone, O'Reilly Auto Parts, etc.).

If you'd like to buy one, check out my recommendations here: Which Compression Tester Should I Buy?

IMPORTANT: Do not remove the spark plugs if the engine is hot. If the engine has been running for any amount of time, let it cool down 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!

Okay, to get this show on the road, this is what you need to do:

1. 1

   Disable the fuel system by removing the fuel pump relay.
   
   This will prevent fuel from being injected into the cylinders as you crank the engine.

2. 2

   Disable the ignition system by disconnecting the ignition coil/ignition module assembly from its electrical connector(s).
   
   Don't overlook this step, since disabling the ignition system will prevent the ignition coil pack from firing spark during the test.

3. 3

   Disconnect all 4 spark plug wires from their spark plugs.
   
   I recommend labeling the spark plug wires before removing them so you'll know where they go when you put them back on.

4. 4

   Remove the spark plugs.
   
   As you' re taking them out, be careful and don't drop any of them on the floor, or you could cause the spark plug's ceramic insulator to break, and this will cause a misfire!

5. 5

   Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder (this is the spark plug hole closest to the drive belt).
   
   IMPORTANT: Hand tighten the compression gauge only! Do not use any type of tool to get it tight.

6. 6

   When the tester is set up, ask your helper to crank the engine. Your job is to keep your eye on the compression tester's gauge.

7. 7

   Once the needle on the gauge stops climbing, have your helper stop cranking the engine.

8. 8

   Write down the compression value on a piece of paper.
   
   Include the number of the cylinder this reading belongs to.

9. 9

   Repeat steps 5 thru 8 on the remaining cylinders.

Let's examine your test results:

CASE 1: 0 PSI compression in 2 or all 4 cylinders. This test result indicates a serious internal problem.

The most common issues would be:

• Blown head gasket.
• Broken timing chain or timing gear.
• Engine threw a rod.

CASE 2: Low compression in one or more cylinders. It's not unusual for the compression values to vary between cylinders.

But if these values vary too much, then you're gonna' have a bonafide misfire on your hands.

The next step is to do some math to find out if this low compression value is within a normal parameter or not. Go to: Interpreting Your Compression Test Results.

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.