HEAD GASKET TEST 2: Coolant Shooting Out Of Open Radiator
The second most common test, to see if the head gasket has blown or not, is to remove the radiator cap and then crank and/or start your Jeep. What this test will confirm is if the head gasket is letting engine compression into the cooling system (radiator) or not.
Now, if the engine in your Jeep 4.0L Cherokee (or Grand Cherokee, Wrangler) has been running for a while and it's hot, let it cool down for about an hour, since a radiator cap should never be removed with the engine hot. Please take all necessary safety precautions.
If your Jeep 4.0L Cherokee (or Grand Cherokee, Wrangler) doesn't start, well this will not be an issue since the coolant will be cold.
Alright, this is what you need to do:
- 1
Remove the radiator's cap.
If the radiator does not have any coolant, add some water (for this test, you can add just water and not anti-freeze). - 2
Have a helper crank the engine while observe the radiator's neck from a safe distance.
- 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.
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 not good and indicates beyond a shadow of a doubt that the head gasket is blown on your Jeep 4.0L Cherokee (or Grand Cherokee, Wrangler)).
No further testing is required. Now, in case you're wondering why this is happening: Well, the head gasket's job is to seal both the coolant and engine oil and also the engine compression/combustion process happening inside each engine cylinder. When the head gasket gets blown, one of the most common after-effects is the engine compression/combustion gases escaping into the cooling system, since it's not sealing effectively anymore.
So, by removing the radiator cap and cranking and/or starting your Jeep, you can physically see if this is happening or not.
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.
Most folks think that when the head gasket gets blown, it causes the engine oil to mix with the coolant and vice-versa. Also that engine compression/combustion gases shoot up and out the radiator but not always.
The next test is: HEAD GASKET TEST 3: Compression Test.
HEAD GASKET TEST 3: Compression Test
It does happen 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.
In some occasions, the head gasket will get burned between two cylinders, causing a gap that will let the compression/combustion of one cylinder to leak into the other and vice-versa. If this happens, in a Jeep 4.0L Cherokee (or Grand Cherokee, Wrangler), this will cause a no-start condition.
The way to verify this, is to do a compression test, and 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
Disconnect the ignition coil (if equipped with a distributor) and the fuel injectors. This will disable the ignition system and fuel system. This first step is important.
- 2
Disable the fuel system by removing the ASD relay or disconnecting the fuel injectors.
- 3
Remove the spark plug wires (if equipped with a distributor) or remove all six Coil-On-Plug ignition coils (if equipped with a DIS ignition system).
- 4
Remove all six spark plugs.
If your specific Jeep is equipped with a distributor, label all of the spark plug wires before removing them so that you can easily put them back in their original locations when you're done with the test. - 5
Thread in the compression tester on the first spark plug hole you're gonna' start with.
- 6
Have a helper crank the engine while you observe the compression tester's needle.
- 7
Once the needle on the gauge stops climbing, have your helper stop cranking the engine.
- 8
Write down the reading and what cylinder it applies to.
- 9
Repeat the above steps in the remaining cylinders.
If the head gasket has burned at a location between 2 cylinders, you are going to have 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 my Jeep 4.0L Cherokee (or Grand Cherokee, Wrangler) gave me 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
- Cylinder #6 = 165 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, etc., the main idea is that whatever cylinders are affected, they will always be side by side.
Let's take a look at what your test results mean:
CASE 1: All cylinder compression readings where normal. This tells you that the head gasket is OK.
OK, if the prior 3 tests have confirmed that the head gasket on your Jeep 4.0L Cherokee (or Grand Cherokee, Wrangler) is not blown YET your 4.0L Jeep is still overheating and you still suspect a blown head gasket, take a look at the last test: HEAD GASKET 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.
