TEST 2: Coolant Shooting Out From Open Radiator
The second most common end result of a blown head gasket on your 4.2L Chevrolet TrailBlazer or GMC Envoy is the head gasket allowing compression pressures to escape into the engine cooling system.
We can easily verify if this has happened by cranking the engine with an open radiator.
If the engine's combustion/compression pressures are escaping into its cooling system, you're going to see the coolant (within the radiator) being shot out forcefully as you crank the engine.
This is another very easy test but it requires you perform it with a completely cold engine.
IMPORTANT: Never open a radiator if the engine is hot! If your Chevy TrailBlazer or GMC Envoy's engine has been running for any length of time, let it cool down completely.
Let's get started:
- 1
Remove the radiator's cap.
- 2
Check the coolant level.
If the radiator is empty, add some water or coolant to bring it up to the radiator's neck level. - 3
Crank the engine with the help of helper, while you stand at a safe distance from the open radiator.
- 4
You'll see one of two results:
1.) The water or coolant inside the radiator will shoot up and out of the now open radiator.
2.) The coolant will not be disturbed. In other words, cranking the engine will have no effect on the level of the Water or coolant in the radiator.
Let's examine your test result:
CASE 1: The coolant shot out from the radiator as you cranked the engine. This test result tells you the head gasket is blown on your 4.2L Chevy Trailblazer (GMC Envoy). The normal test result is for the coolant to remain undisturbed as the engine is cranked.
CASE 2: The coolant DID NOT bubble out NOR shoot out from the radiator. This is the correct and expected test result.
You can conclude the head gasket is not letting combustion/compression pressures leak into the cooling system.
Your next step is a compression test. Go to: TEST 3: Engine Compression Test.
TEST 3: Engine Compression Test
A head gasket can burn between two cylinders as a result of severe engine overheating.
The photo at the very beginning of this tutorial shows this type of head gasket failure.
The way to confirm this is to do an engine compression test on all 6 cylinders.
If the head gasket has burned between two cylinders, those two adjacent cylinders will have 0 PSI compression.
NOTE: You can find a complete and detailed explanation of the engine compression test and how to interpret its results here: How To Test Engine Compression (2002-2009 4.2L Chevrolet Trailblazer, GMC Envoy).
These are the test steps:
- 1
Remove all six spark plugs.
- 2
Thread in the compression tester by hand on the first spark plug hole you're gonna' start with.
Do not use any tools to tighten the compression tester. Hand tightening the compression tester is more than enough to get the proper results. - 3
Have a helper crank the engine as you observe the compression tester. The needle will climb, as the engine cranks, till it reaches the maximum cylinder compression.
- 4
Have your helper stop cranking the engine when the needle on the gauge stops climbing.
- 5
Write down the compression reading and what cylinder it belongs to (see the illustration above to help you identify the cylinder).
- 6
Repeat the above steps in the remaining cylinders.
Let's take a look at what your test results mean:
CASE 1: All cylinder compression readings were normal. These compression gauge readings confirm the head gasket is OK and not burned at a point between two cylinders.
If you still suspect a blown head gasket. Go to: TEST 4: Using A Chemical Block Tester (Combustion Leak Tester).
CASE 2: Two side by side cylinders had 0 PSI compression. This engine compression reading confirms the head gasket is burned thru' at the point between those two cylinders. You will need to replace the head gasket.
