How To Test For A Blown Head Gasket (1987-2006 4.0L Jeep Cherokee, Grand Cherokee, Wrangler)

This article will present you with the four most common ways to test for a blown head gasket on a Jeep 4.0L inline 6 cylinder engine.

All four are very simple and easy tests that will help you confirm (or disprove) a blown head gasket.

Two of the tests you can do in about 5 minutes or less and don't require any tools. The third blown head gasket test requires doing a compression test, for which you'll need a compression tester. The fourth is done with a combustion leak detector (block tester). Before you start any of them, take a look at the entire article first.

ES You can find this tutorial in Spanish here: Cómo Probar Un Empaque De Cabeza Quemado (4.0L Jeep) (at: autotecnico-online.com).

APPLIES TO: This tutorial applies to the following vehicles:

• 4.0L Jeep Cherokee: 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001.
• 4.0L Jeep Comanche: 1987, 1988, 1989, 1990, 1991, 1992.
• 4.0L Jeep Grand Cherokee: 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004.
• 4.0L Jeep Wagoneer: 1987, 1988, 1989, 1990.
• 4.0L Jeep Wrangler: 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006.

Symptoms Of A Blown Head Gasket

The most common cause of a blown head gasket is that the engine overheated due to: 1) A bad radiator fan or fan clutch 2) No coolant in the engine due to a leak somewhere in the coolant system 3) Bad thermostat. The most common symptoms a blown head gasket are:

1. Your Jeep is overheating. You know it's not the fan or thermostat.
2. White smoke is coming out of the tail-pipe and it smells like anti-freeze being cooked.
3. The Jeep won't start:
   • You have already verified it's not an ignition system problem because you have spark coming out at all of the spark plug wires.
   • You know it's not a lack of fuel, because you have verified that the fuel pump is delivering fuel to the fuel injectors.
4. The engine oil is thick and tan to off-white color, like coffee with too much creamer.

HEAD GASKET TEST 1: Is Engine Oil Mixed With Coolant?

One of the most common symptoms of a blown head gasket is coolant mixing with the engine oil and this first test will check for that.

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

1. 1

   Open the hood on your Jeep 4.0L Wrangler (or Cherokee, Grand Cherokee).

2. 2

   Pull the engine oil dipstick.

3. 3

   You'll see one of two things:
   
   1.) The color of the oil, on the dipstick, is the color of coffee with too much cream (a milky white/tan color).
   
   2.) The color of the oil is its normal color.

OK, let's take a look at what the color of the engine oil means:

CASE 1: The color of the oil is a light tan, like coffee with too much creamer. This result verifies that the head gasket is blown on your Jeep 4.0L Grand Cherokee (or Wrangler or Cherokee). You can stop here, since no further testing is required.

As mentioned earlier, the reason the oil looks this way is:

1.) The engine in your Jeep severely overheated, resulting in the cylinder head warping. This in turn causes the head gasket to burn.

2.) Since the job of the head gasket is to seal compression/exhaust gases, engine oil and coolant, a burned head gasket won't be able to keep any of them separated.

3.) This leads to the coolant entering the engine oil pan and mixing with the oil. As both of these mix, the resulting oil gets thick and turns to an off-white color.

CASE 2: The color of the oil is its normal color. So far so good, but it doesn't exonerate the head gasket yet, go to: HEAD GASKET TEST 2: Coolant Shooting Out Of Open Radiator.

Here's why: Although coolant mixing with the engine oil is a common result of a blown head gasket, this doesn't happen in all of the cases. Your Jeep 4.0L Cherokee (or Grand Cherokee, Wrangler) could have a blown head gasket and yet the engine oil doesn't show a trace of having mixed with coolant. Thankfully, there are several more tests you can do to further verify a blown head gasket.

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. 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. 2

   Have a helper crank the engine while observe the radiator's neck from a safe distance.

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.
   
   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. This is the normal and expected test result.

Your next head gasket test depends on one of two things:

• The engine runs with a cylinder misfire. Go to: HEAD GASKET TEST 3: Compression Test.
• The engine starts and runs, but overheats within minutes (and for no apparent reason). Go to: HEAD GASKET TEST 4: Using A Chemical Block Tester (Combustion Leak Tester).

HEAD GASKET TEST 3: Compression Test

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.

When this happens, your Jeep may not start and if it does, it'll run on only four cylinders (causing a very noticeable misfire 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.

NOTE: You can find a more detailed explanation of how to do and interpret the results of the compression test in this tutorial: How To Test Engine Compression (1991-2006 4.0L Jeep Cherokee, Grand Cherokee, Wrangler).

This is what you'll do:

1. 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. 2

   Disable the fuel system by removing the ASD relay or disconnecting the fuel injectors.

3. 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. 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. 5

   Thread in the compression tester on the first spark plug hole you're gonna' start with.

6. 6

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

7. 7

   Once the needle on the gauge stops climbing, have your helper stop cranking the engine.

8. 8

   Write down the reading and what cylinder it applies to.

9. 9

   Repeat the above steps in the remaining cylinders.

If the head gasket has burned at a location between 2 cylinders, you're 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.

HEAD GASKET TEST 4: Using A Chemical Block Tester (Combustion Leak Tester)

Every now and then a blown head gasket is tough to diagnose. If in your case, the 3 prior tests cannot confirm (or disprove) a blown head gasket, then you need to perform a combustion leak test with a block tester.

This test is accurate and the only reason I don't include it at the beginning of the tutorial is that the first 3 tests correctly identify a blown head gasket 95% of the time (not to mention you don't have to buy a block tester). In that other hard to diagnose 5% of the cases a combustion leak test is THE test.

This is how it works:

• A blue liquid chemical, which is blue in color, is placed in the tester (see photo above).
• The tester assembly is then placed on the open radiator neck (you may have to drain some of the coolant in the radiator since this tool needs to 'gulp' some of the air inside the radiator).
• The rubber bellow is then squeezed to suck in the air up through the two fluid-filled chambers. As the air bubbles up through the fluid, it will cause a chemical reaction.
• If the blue chemical turns yellow (for gasoline engines), combustion gases are entering the radiator. This result confirms a head gasket failure, a cracked block, or a cracked cylinder head issue.
• If the blue chemical doesn't change color, you can conclude that you don't have a head gasket failure, a cracked block, or a cracked cylinder head issue.

You can shop for one here:

More 4.0L Jeep Diagnostic Tutorials

I've written several more tutorials that may be of interest to you. These are specific Jeep 4.0L Cherokee (or Grand Cherokee, Wrangler) that you can find here:

• Jeep 4.0L Index Of Articles.

Here's a sample of the Jeep 4.0L articles you'll find:

• How To Test Engine Compression (Jeep 4.0L).
• How To Troubleshoot A Bad Fuel Injector (Jeep 4.0L).
• How To Test The Ignition Coil (1991-1997 4.0L Jeep).
• How To Test The TPS (1993-1995 4.0L Grand Cherokee).
• How To Test The Crankshaft Position Sensor (1994-1996 4.0L Jeep).