Blown Head Gasket Test (GM 4.8L, 5.3L, 6.0L)

TEST 2: Coolant Shooting Out Expansion Tank

If you've been following this article from the beginning, you have confirmed that the engine oil is not mixed with coolant (TEST 1).

The next step is to see if coolant will violently shoot out of the radiator when the radiator's coolant expansion tank cap is removed and the engine is cranked.

If it does, then you now know beyond a shadow of a doubt that your GM vehicle did overheat to the point of blowing one or both head gaskets and/or warping the cylinder heads.

IMPORTANT: Before you start this test, the engine in your GM pickup, van or SUV can not be hot (normal operating temperature). If it is, you need to let the engine cool down.

Opening a radiator coolant expansion tank cap with a hot engine will release a burst of steam and hot coolant and may cause severe burns. So, be careful, take all necessary safety precautions and use common sense.

This is what you need to do:

1. 1

   Remove the radiator's coolant expansion tank cap from the radiator.
   
   CAUTION: The engine should be completely cold before you open the radiator cap.

2. 2

   Check the coolant level and if low, top it off before going on to the next step.

3. 3

   Have your helper crank the engine while you observe from a safe distance but within eye-view of th radiator's coolant expansion tank.

4. 4

   You'll see one of two results:
   
   1.) The coolant shoots out violently when the engine was cranked.
   
   2.)The coolant was not disturbed at all.

Let's analyze your test results:

CASE 1: The coolant shot out of the radiator. This test result tells you, beyond a shadow of a doubt, that you do have a blown head gasket and/or warped cylinder heads on your hands.

CASE 2: The coolant DID NOT shoot out of the radiator. So far so good.

If the engine is misfiring (running very rough), your next step is: TEST 3: Checking Engine Compression.

If the engine is overheating within minutes of starting, your next step is: TEST 4: Using A Chemical Block Tester (Combustion Leak Tester).

TEST 3: Checking Engine Compression

There have been several times, over the years, that the vehicle I was diagnosing for a blown head gasket didn't have coolant mixing with the oil and were not shooting the coolant out of the radiator when the engine was cranked.

Yet a blown head gasket they did have and the most important factor that all of these vehicles shared was that they had severely overheated and after the fact, they had a misfire/ rough idle condition.

After doing all of the basic tests, mentioned in TEST 1 and 2 and checking the basics like spark and fuel injector operation (on the misfiring cylinders),, the test that nailed the problem was a compression test.

The compression test confirmed that the cylinder head gasket had burned between to cylinders that were side-by-side and from the same bank of cylinders. The end result of this was (and is) 0 PSI compression on those two cylinders.

How does this happen? This happens because the head gasket burns between two cylinders and thus the compression from one cylinder escapes into the other and vice versa.

OK, to get this show on the road, this is what you'll need to do:

1. 1

   Disable the fuel system and the ignition system.

2. 2

   Remove the spark plugs.
   
   If you already know which cylinders are dead, you won't need to remove all of the spark plugs. It'll suffice to test those two.
   
   If you don't know which ones are the ones you need to test, I suggest doing a cylinder balance test to find out (or you can remove all of the spark plugs and compression test all 8 cylinders).

3. 3

   Install the compression tester (hand tight only) on the first cylinder you're gonna' test.
   
   IMPORTANT: If you have not removed all 8 spark plugs, it is critical that you have the spark plug of the other dead cylinder removed for the test to work.

4. 4

   Have a helper crank the engine while you observe the compression tester.

5. 5

   Write down the compression readings.

6. 6

   Repeat steps 1 through 5 on the next cylinder (cylinders if you removed all 8 spark plugs).

Let's analyze your test results:

CASE 1: You got 2 side by side (adjacent) cylinders with 0 PSI. This confirms beyond a shadow of a doubt that you do have a blown head gasket on your hands.

CASE 2: All cylinders had sufficient compression. This is the correct and expected test result.

If the engine is your GM vehicle is still overheating, take a look at the last test: TEST 4: Using A Chemical Block Tester (Combustion Leak Tester).