TEST 2: Combustion Gases Shooting Out From An Open Radiator
The second most common test, to check to see if the head gasket is blown or not, is checking to see if engine compression and/or exhaust gas is shooting out of an open radiator.
A word of caution: If your vehicle starts and runs, then you need to do this test with a completely cold engine. Why? Because this test step requires that you remove the radiator cap and then crank the engine. Be careful and remember that a radiator cap should never be removed from a hot radiator.
There's a good chance that your 3.5L/4.2L equipped pickup or SUV isn't starting, in this case you don't have to worry about a hot engine.
- 1
Remove the radiator's cap.
- 2
Check to see 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. - 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. If cranking the engine had no visible effect on the level of the coolant in the open radiator, this is normal.
If you were to ask 10 persons, what is the most common symptom of a blown head gasket, 9 out 10 would say, without hesitation, engine oil mixing with coolant and engine compression/combustion gases shooting out of an open radiator neck and yes they would be right but not in all of the cases.
So the next step is to either do a compression test or a block test with a block tester. If the engine doesn't start or starts and runs but runs with a misfire, go to: TEST 3: Doing A Compression Test To Check For A Blown Head Gasket.
If the engine starts and runs but overheats within minutes, go to: 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, 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
Remove all 5 (3.5L) or 6 (4.2L) ignition coils from the top of the engine (they must remain disconnected from their electrical connector for the remainder of the compression test).
- 2
Remove all 5 or 6 spark plugs.
Take care not to drop any of them or you might have a misfire down the road from a broken one. - 3
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. - 4
Have a helper crank the engine while you keep your eyes on the compression tester.
- 5
At the point it stops climbing, have your helper stop cranking the engine.
- 6
On a piece of paper, write down the reading and what cylinder it belongs to (you can use the illustration above to help you identify the cylinder).
- 7
Repeat the above steps in the remaining cylinders.
If the head gasket is burned at a location between 2 cylinders, your compression test readings will give you 2 good compression readings and 2 compression readings that will be 0 PSI. Let me give you a more specific example:
Let's say that I tested 2004 Chevy Colorado and I got the following cylinder compression readings:
- Cylinder #1 = 175 PSI
- Cylinder #2 = 165 PSI
- Cylinder #3 = 0 PSI
- Cylinder #4 = 0 PSI
- Cylinder #5 = 170 PSI
As you can see from the above compression readings, cylinders #3 and #4 have 0 PSI readings. And this is a clear indication that the head gasket has burned at the point between them both. Now, in your specific case, you may not see those exact same cylinders with 0 PSI readings. It may be #1 and #2 or it may be #2 and #3, the main idea is that whatever cylinders are affected, they will always be side by side.
Let's take a closer look at the result of your compression test:
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 3.5L/4.2L equipped pickup or SUV is not blown.
CASE 2: Two side by side cylinders had 0 PSI compression. These engine compression reading confirm that the head gasket is burned thru' at the point between those two cylinders. You will need to replace the head gasket.
