How To Test For A Blown Head Gasket (2002-2009 4.2L Chevrolet TrailBlazer, GMC Envoy)

TEST 4: Using A Chemical Block Tester (Combustion Leak Tester)

In some cases, the first three tests cannot exonerate the head gasket, yet a head gasket failure is causing the problem.

In other words, the engine coolant wasn't mixing with the engine oil, the coolant within the radiator wasn't being shot out while cranking the engine (with the radiator open), and engine compression was OK. Yet the engine is overheating for no apparent reason at all.

In these hard to diagnose blown head gasket cases, the only way to find out if the head gasket is blown (or not) is to do a block test with a block tester.

You do have to buy (or borrow) the block tester, but this tool is worth every penny since it's a test that gives you a very accurate test result.

In a nutshell, this is how a block tester works:

• A blue liquid chemical, which is blue in color, is placed in the tester (see photo above).
• The tester assembly is then placed on the open radiator neck (you may have to drain some of the coolant in the radiator since this tool needs to ‘gulp’ some of the air inside the radiator).
• The rubber bellow is then squeezed to suck in the air up through the two fluid-filled chambers. As the air bubbles up through the fluid, it will cause a chemical reaction.
• If the blue chemical turns yellow (for gasoline engines), combustion gases are entering the radiator. This result confirms a head gasket failure, a cracked block, or a cracked cylinder head issue.
• If the blue chemical doesn't change color, you can conclude that you don't have a head gasket failure, a cracked block, or a cracked cylinder head issue.

Where can you get the chemical and block tester? At your local auto parts store or here:

Frequently Asked Questions

1.)  How can I tell if the cylinder head is cracked?

The cylinder head has to be removed and visually inspected. If the crack is wide enough, you'll be able to easily see it.

Sometimes, a visual inspection proves inconclusive, this is why it's important to let a machine shop pressure test it for you.

2.)  Do I need to resurface the cylinder head?

YES, you need to resurface the cylinder head! You should never reinstall the cylinder head without first having an automotive machine shop resurface it. Especially if your 4.2L Chevy TrailBlazer or 4.2L GMC Envoy overheated.

Now, of course, if you (or the machine shop) have checked it with a straight-edge and there's no warpage, then, and only then, is the cylinder head not resurfaced.

More GM 4.2L Test Tutorials

If this tutorial was helpful/informative, you can find a complete list of tutorials here:

• GM 4.2L Index Of Articles.

Here's a sample of the tutorials you'll find there:

• Camshaft Position Actuator Solenoid Test (2002-2009 4.2L Chevrolet TrailBlazer).
• How Often Should I Replace The Spark Plugs? (2002-2009 4.2L Chevrolet TrailBlazer).
• How To Test APP Sensor 1 And 2 (2002-2003 4.2L Chevrolet TrailBlazer).
• Electronic Throttle Body Tests (2002-2007 4.2L Chevrolet TrailBlazer).