TEST 2: Combustion Gases Expelled From An Open Radiator
The next test, to see if the head gasket is burned or not on your FWD 3.5L equipped GM vehicle is to see if combustion pressures are being expelled thru the cooling system.
Specifically, you'll remove the radiator cap (or the coolant reservoir cap) and then see if the coolant will be forcefully shot out while cranking the engine.
IMPORTANT: This test has to be done with a cold engine. If your vehicle still starts and runs, let the engine cool down for at least an hour. Never remove the radiator cap (or coolant reservoir cap) from a hot engine or you risk serious burns. Be careful and think safety all of the time.
- 1
Remove the radiator's cap.
- 2
Check if there is coolant in the radiator. If the radiator is empty, add some water or coolant to bring it up to the radiator's neck level.
More likely than not, your vehicle does not have a radiator cap. In this case, you'll need to check the coolant level on the overflow tank (see photo above). - 3
Have a helper crank the engine, 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.
OK, now that the testing part is done, let's take a look at what your results mean:
CASE 1: The coolant bubbled out or shot out from the radiator. This is a clear indication that the head gasket is blown.
This test result only happens when the head gasket has blown and/or the cylinder head has warped due to the engine overheating. No further testing is required.
CASE 2: The coolant DID NOT bubble out NOR shoot out from the radiator. This is the normal and expected test result.
Your next step is to either do a compression test or a block test. If your vehicle is experiencing a misfire, your next step is a compression test. If it's overheating, your next step is a block test:
- TEST 3: Doing A Compression Test To Check For A Blown Head Gasket.
- TEST 4: Using A Chemical Block Tester (Combustion Leak Tester).
TEST 3: Doing A Compression Test To Check For A Blown Head Gasket
What leads a lot of folks to misdiagnose a blown head gasket is that, in their particular case, they checked the engine oil and it was not mixed with coolant. They also did the radiator with the cap off test and the engine's compression or combustion gases were not escaping from there.
They never realized that a head gasket can get blown without causing the coolant to mix with the engine oil and without causing combustion gases from entering the cooling system. I have seen this enough to know that one more test has to be done.
How is this possible? Well, this happens because the head gasket has burned at a point between two cylinders, the resulting gap in the head gasket will let only the compression/combustion of one cylinder to leak into the other and vice-versa, but nothing else (like coolant). If this happens, in a FWD 3.5L equipped GM vehicle, this will cause a no-start condition.
This condition can be very easily verified by doing a compression test. In this test step I'll show you how and more importantly, how to interpret the compression test results to see if the head gasket is burned or not. This is what you'll do:
- 1
Disable the ignition system.
You can do this by disconnecting the ignition coil pack's connector. - 2
Remove all 6 spark plug wires.
- 3
Remove all of the spark plugs.
- 4
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. - 5
Have a helper crank the engine while you keep your eyes on the compression tester.
- 6
When the gauge's needle stops climbing, have your helper stop cranking the engine.
- 7
Write down the reading and what cylinder it belongs to (you can use the illustration above to help you identify the cylinder).
- 8
Repeat the above steps in the remaining cylinders.
Let's interpret your test results:
CASE 1: All cylinder compression readings where normal. These compression gauge readings confirm that the head gasket is OK and not burned at a point between two cylinders.
OK, 3 out of 3 tests have confirmed that the head gasket on your FWD 3.5L GM equipped vehicle is not blown.
CASE 2: Two side by side cylinders had 0 PSI compression. This engine compression reading confirms that the head gasket is burned thru' at the point between those two cylinders. You will need to replace the head gasket.
