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

How To Test Engine Compression (1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993 2.5L Chevrolet S10 Pickup, GMC S15 Pickup, And 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.

In Spanish 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:

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 two recommendations:

1) Which one to buy:  The engine compression tester that I have always used is the Actron CP7827 Compression Tester Kit. My only complaint about this engine compression tester is that it does not come with a case to store it in.

2) Where to buy:  You can buy an engine compression tester just about anywhere, but you'll end up paying more for it (especially at your local auto parts store). The above links will help you comparison shop. I think you'll agree it's the better way to save money on the compression tester!

TEST 1: Dry Engine Compression Test

How To Test Engine Compression (1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993 2.5L Chevrolet S10 Pickup, GMC S15 Pickup, And GMC Sonoma)

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.

Chevrolet Vehicles:

  • S10 Pickup 2.5L
    • 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993

GMC Vehicles:

  • S15 Pickup 2.5L
    • 1985, 1986, 1987, 1988, 1989, 1990
  • Sonoma 2.5L
    • 1991, 1992, 1993