How To Test Engine Compression (2002-2009 4.2L Chevrolet TrailBlazer, GMC Envoy)

How To Test The Engine Compression (2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 4.2L Chevrolet Trailblazer And GMC Envoy)

In this tutorial, I'm going to show you how to test the engine compression on the 4.2L equipped 2002-2009 Chevrolet TrailBlazer or GMC Envoy.

The engine compression test will help you troubleshoot a hard to diagnose misfire condition and its accompanying misfire trouble code.

The compression test will also help you troubleshoot a no-start condition if its due to internal engine problems.

In Spanish You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (2002-2009 4.2L Chevrolet TrailBlazer) (at:

Symptoms Of Low Or No Compression

Engine compression issues can manifest in various ways and can significantly impact the performance and reliability of a vehicle. Let's break down the details:

Low Compression or Zero PSI Compression:

  • Low Compression in Some but Not All Cylinders: Low compression in one or more cylinders means that the cylinders are not generating sufficient pressure during the compression stroke. This can be caused by worn piston rings, damaged cylinder walls, or leaking valves.
  • Zero PSI Compression in Some or All Cylinders: Zero PSI compression indicates a complete failure of compression in one or more cylinders, which can be caused by severe engine damage such as a blown head gasket or catastrophic piston failure.

Regardless of whether the compression is low or zero, you'll notice several symptoms that indicate there's an issue with the engine's compression:

  • Hard Starting or No Start: Reduced compression can make it difficult for the engine to start, or it may not start at all.
  • Poor Engine Performance: Decreased compression leads to weakened power output and overall poor engine performance.
  • Cylinder Misfire Issues: Compression issues can cause misfires in one or more cylinders, resulting in rough idling and poor acceleration. You'll see one or more of the following misfire codes stored in the PCM's memory:
    • P0301: Random Cylinder Misfire.
    • P0302: Cylinder Number 2 Misfire.
    • P0303: Cylinder Number 3 Misfire.
    • P0304: Cylinder Number 4 Misfire.
    • P0305: Cylinder Number 5 Misfire.
    • P0306: Cylinder Number 6 Misfire.
  • Reduced Power and Performance: The engine may lack the power it needs for proper acceleration and performance due to inadequate compression.
  • Excessive Oil Consumption: Compression problems can lead to oil leaks into the combustion chamber, resulting in increased oil consumption.
  • Blue Smoke Coming From The Tailpipe: This can be a sign of piston ring wear, valve seal failure, or other internal engine issues related to compression.
  • Audible Engine Noise: Issues like piston slap or valve noise may become more pronounced due to reduced compression.

Important Tips And Suggestions

TIP 1: The spark plugs have to be removed from the engine to test its compression. It's important you do not remove the spark plugs from a hot engine.

If the engine has been running for any length of time, on your 4.2L Chevrolet TrailBlazer or GMC Envoy, you need to let it cool down for at least an hour before attempting to remove the spark plugs.

Attempting to remove the spark plugs from a hot engine can damage the threads in the spark plug holes. This is a nightmare you want to avoid.

TIP 2: If you find that the spark plugs are swimming in engine oil, as you are removing them from their spark plug tubes, you'll need to replace the valve cover gasket.

If the spark plugs are soaked in engine oil, it's important to replace them with new ones.

If the spark plug boots on the ignition coils, are soaked in engine oil, it's important to replace them too.

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 two recommendations to you:

1) Which one to buy:  The engine compression tester I have always used is the Actron CP7827 Compression Tester Kit. My only complaint about this engine compression tester is 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 Compression Test

How To Test The Engine Compression (2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 4.2L Chevrolet Trailblazer And GMC Envoy)

I have divided the compression test into two parts. In this first part we're going to test the compression of all 6 cylinders.

In the second part, which is in the next page, we're going to test only the cylinders that have low engine compression.

Depending on the amount of wear and tear and mileage on your 4.2L engine, you'll see some of the cylinders with compression values that are lower than the majority.

Up to a certain point this is normal, since engine cylinders do not wear out at the exact same rate. Figuring out if these compression values are causing a problem is not hard and I'll explain how to find out.

Let's get testing:

  1. 1

    Remove the ignition coils.

  2. 2

    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.

  3. 3

    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.

  4. 4

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

  5. 5

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

  6. 6

    Repeat steps 3-5 on the remaining cylinders.

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

CASE 1: You got 0 PSI in the majority of the cylinders. This tells you your Chevy TrailBlazer or GMC Envoy's engine has serious internal problems. This is usually due to:

  • Busted timing chain.
  • Engine threw a rod.

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

CASE 2: All cylinders have compression, but their values are not the same. It's normal for each cylinder's compression value to vary slightly from one another. But if they vary too much, you'll have a bonafide misfire or rough idle condition on your hands.

To find out, the next step is to go to: Calculating The Compression Values To See If They Are Within Range.

Chevrolet Vehicles:

  • Trailblazer 4.2L
    • 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009

GMC Vehicles:

  • Envoy 4.2L
    • 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009