How To Test For A Blown Head Gasket (GM 3.5L, 4.2L)

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

In some cases, the head gasket fails but it doesn't let coolant into the crankcase and it doesn't let compression/exhaust gases into the cooling system. So the previous tests don't confirm or exonerate the head gasket as bad.

Also, the engine starts and runs but it overheats within minutes (and for no apparent reason).

In these cases only a block test with a block tester will confirm that the head gasket is blown and behind the problem.

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 engine is started.
• 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.

You can buy one here:

Related Test Articles

There are several more ‘how to’ tutorials that I've written that are 3.5L, 4.2L Buick, Chevy and GMC specific, that may be of further help. You can find the ones that are located here, at this site, by going to the GM 3.5L Index Of Articles.

Here's a sample of the articles you'll find in the index:

• How To Test The Mass Air Flow (MAF) Sensor (GM 3.5L)
• How To Diagnose The MAP Sensor Testing P0106, P0107, P0108