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

If you suspect that an engine compression issue is causing an engine performance or an engine no-start problem, this is the tutorial that will help you find out.

In this tutorial, you'll find step-by-step instructions on how to perform and interpret an engine compression test.

You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (1985-1993 2.5L Chevrolet S10, GMC S15, GMC Sonoma) (at: autotecnico-online.com).

APPLIES TO: This tutorial applies to the following vehicles:

• 2.5L Chevrolet S10 Pickup: 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993.
• 2.5L GMC S15 Pickup: 1985, 1986, 1987, 1988, 1989, 1990.
• 2.5L GMC Sonoma: 1991, 1992, 1993.

Symptoms Of Low Or No Engine Compression

Generally, an engine compression problem will cause one of two 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 many engine compression testers to choose from and many places to buy them. I'm gonna make these recommendations:

TEST 1: Dry Engine Compression Test

In order to accurately interpret your compression test results, it's important that you test all four cylinders. The reason for testing all four cylinders is to get the highest cylinder compression reading your Chevrolet S10 pickup or GMC S15 pickup will produce.

Once you get the highest compression reading, you can calculate exactly what the lowest compression reading of all the other cylinders can be before they start misfiring.

If you don't have an engine compression tester, you can borrow one from your local AutoZone or O'Reilly Auto Parts.

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

NOTE: 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!

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

1. 1

   Disable the fuel system by disconnecting the fuel injectors from their electrical connectors.
   
   Disabling the fuel system will prevent fuel from being injected into the cylinders while cranking the engine.

2. 2

   Disable the ignition system by disconnecting the ignition coil or ignition module from their electrical connectors.
   
   Don't overlook this step, since disabling the ignition system will prevent the ignition coil from firing spark during the test.

3. 3

   Disconnect all four 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.
   
   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 while you observe the compression tester's needle.

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

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.