How To Test A Blown Head Gasket (GM 3.1L, 3.4L)

TEST 2: Compression Gases Shooting Out From An Open Radiator

The second most common test, to see if the head gasket on your 3.1L, 3.4L GM car or mini-van is blown or not, is to check if engine compression and/or combustion gases are escaping thru' the cooling system. This is another very easy test and does not require any tools whatsoever to do.

Now, before you start, if the engine has been running for an extended amount of time and it's hot, let the engine cool down for at least 1 hour. This is important, or you run the risk of getting scalded with hot coolant. If your 3.1L, 3.4L GM car or mini-van doesn't start, well this is not an issue.

One more very important thing, your specific vehicle may not have a literal radiator cap on the radiator, but a pressurized coolant reservoir and this coolant reservoir will have a screw-on cap. The cap on this coolant reservoir is the ‘radiator cap’ that I'm gonna' be referring to. If your specific 3.1L or 3.4L does have a radiator with a radiator cap on it, you've got nothing to worry about.

OK, here are the test steps:

1. 1

   Remove your mini-van or car's radiator's cap. Check the coolant level, since the radiator has to be full of coolant for this test to work. If empty, add some water or coolant to bring the coolant level to full.

2. 2

   Now, get your helper to crank the engine, while you stand at a safe distance from the open radiator.

3. 3

   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 or pressurized coolant reservoir.
   
   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 bad news and this let's you know beyond a shadow of a doubt that the head gasket on your 3.1L, 3.4L GM car or mini-van is blown. No further testing is required.

CASE 2: The coolant DID NOT bubble out NOR shoot out from the radiator. So far so good, but one more test is needed to make absolutely sure the head gasket is OK, go to: TEST 3: Engine Compression Check.

Now, common sense tells you that if a head gasket is blown, you ARE gonna' have oil mixed with coolant, and engine compression and/or combustion gases are going to be shooting out of the radiator but sometimes this just doesn't happen. So, the next test will further confirm or exonerate the head gasket.

TEST 3: Engine Compression Check

One of the most overlooked tests, to see if the head gasket is bad or not, is an engine compression test. Why? Well because most folks will do the two previous tests (HEAD GASKET TEST 1 and HEAD GASKET TEST 2) and they will not see the engine oil mixing with the coolant nor see the combustion gases jumping out of the open radiator and conclude everything is OK. And whatever information they have available never mentions testing the engine compression.

Well, now you know that it is possible for the head gasket to burn and not cause the oil to mix with coolant nor cause the exhaust gases to escape thru' the cooling system. In this test step, you'll be doing a compression test and more importantly, you'll be able to easily interpret those results (with my help) to further confirm or exonerate a blown head gasket.

This test will see if the head gasket on your 3.1L, 3.4L GM car or mini-van has burned at a point between cylinders.

If this does happen, the compression/combustion of one cylinder to leak into the other and vice-versa and the compression readings will easily let us know this has happened.

OK, here's what you need to do:

1. 1

   Disable the ignition system on your 3.1L, 3.4L GM car or mini-van. It's important that spark is not created and delivered to the spark plug wires.
   
   You can easily accomplish this ‘disabling’ by disconnecting the ignition module's three electrical connectors (the Ignition Control Module sits underneath the three ignition coils).

2. 2

   Disable the fuel system. It's important that fuel not be injected into the engine.
   
   You can do this by disconnecting the fuel pump relay.

3. 3

   Disconnect all spark plug wires (from their spark plugs) and then take out all of the spark plugs. Before you disconnect the spark plug wires from the spark plugs, label them (with tape, etc.) so that you'll know where each one goes back to.

4. 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. 5

   Have a helper crank the engine. Your job is to observe the compression tester.
   
   This is what is gonna' happen: The compression tester's needle will climb, as the engine cranks, till it reaches the maximum cylinder compression. At the point it stops climbing, have your assistant stop cranking the engine.
   
   Now, write down the reading and what cylinder it belongs to (you can use the image in the image viewer to help you identify the cylinder) on a piece of paper. Remove the compression tester and repeat the above steps in the remaining cylinders.

OK, before I jump into the above compression test result interpretations, let me give you some more detailed information as to what you're trying to accomplish with this test, 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 my 3.1L, 3.4L GM car or mini-van and I got the following compression tester readings:

1. Cylinder #1 = 165 PSI
2. Cylinder #2 = 180 PSI
3. Cylinder #3 = 0 PSI
4. Cylinder #4 = 170 PSI
5. Cylinder #5 = 0 PSI
6. Cylinder #6 = 170 PSI

The compression readings for cylinders #3 and #5 would be a dead giveaway that the head gasket got fried between those two cylinders. Now, in your 3.1L, 3.4L GM car or mini-van, you may not see those exact same cylinders with 0 PSI readings. It may be #1 and #3 or it may be #4 and #6, the key here, is that whatever cylinders are affected, two of them will have 0 PSI compression and they will be both be side by side and on the same bank.

OK, let's take a look at what your results mean:

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 3.1L, 3.4L GM car or mini-van is not blown. But if your vehicle is still overheating for no apparent reason and 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 that the head gasket is burned thru' at the point between those two cylinders. You will need to replace the head gasket.