How To Test For A Blown Head Gasket (4.7L Chrysler, Dodge, Jeep)

TEST 2: Compression/Exhaust Gases Shooting Out Of The Radiator

The second most common test, to see if the head gasket 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.

IMPORTANT: This test has to be done with a completely cold engine or you run the risk of getting scalded by hot coolant. If the engine in your 4.7L Dodge or Jeep vehicle starts and runs and has been running for any length of time, let the engine cool down completely before doing this test.

OK, here are the test steps:

1. 1

   Remove the radiator's cap from the radiator.

   1. Check the coolant level, since the radiator has to be full of coolant for this test to work.
   2. If empty, add some water or coolant to bring the coolant level to full.
2. 2

   Crank the engine.

   1. Have a helper crank the engine while you stand at a safe distance from the open radiator.
3. 3

   You'll see one of two results:

   1. Either the water or coolant inside the radiator will shoot up and out of the now open radiator.
   2. Or 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 shot out from the radiator as you cranked the engine. This is bad news and this let's you know beyond a shadow of a doubt that the head gasket or head gaskets are blown. No further testing is required.

CASE 2: The coolant DID NOT shoot out from the radiator as you cranked the engine. So far so good, but one more test is needed to make absolutely sure the head gasket is OK, go to: TEST 3: Compression Test

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: Compression Test

It doesn't happen very often, but every now and then, the head gasket from one cylinder head burns in a spot between 2 adjacent cylinders.

When this happens in your 4.7L V8 engine, you'll see one of these two symptoms:

1. Cranks but does not start.
2. The engine runs with a misfire and you have two misfire codes registering in the PCM's memory.
   • These two misfire codes accuse side-by-side cylinders from the same bank.

If this condition exists on your vehicle's engine, the compression/combustion of one cylinder will leak into the other and vice-versa as the engine cranks and/or runs (and the compression pressure will be 0 PSI for both of the affected cylinders). The best way to confirm this is with a compression test.

OK, here's what you need to do:

1. 1

   Remove the spark plugs.

   1. You don't have to test all 8 cylinders.
   2. If you know which 2 cylinders are the dead ones, you can test the compression of just those 2.
2. 2

   Disable the fuel system.

   1. You can accomplish this by unplugging the fuel injectors' connectors.
   2. Or by removing the ASD (Auto Shut Down) Relay.
3. 3

   Install the compression tester.

   1. Thread in the compression tester by hand, on the first spark plug hole you're gonna' start with.
   2. 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. 4

   Have a helper crank the engine.

   1. Your job is to observe the compression tester.
   2. Crank the engine till the needle on the compression tester stops climbing.
   3. NOTE: Be careful when working around a cranking engine.
5. 5

   Test the next cylinder.

   1. Crank the engine till the needle on the compression tester stops climbing.

If the head gasket is burned at a location between 2 cylinders, your compression tester will read 0 PSI for those two adjacent cylinders (on the same bank of cylinders) and the other 6 will have good compression readings. Let me give you a more specific example:

Let's say that I tested my 4.7L Dodge and I got the following compression tester readings:

1. Cylinder #1 = 165 PSI
2. Cylinder #2 = 180 PSI
3. Cylinder #3 = 0 PSI (bad)
4. Cylinder #4 = 170 PSI
5. Cylinder #5 = 0 PSI (bad)
6. Cylinder #6 = 170 PSI
7. Cylinder #7 = 175 PSI
8. Cylinder #8 = 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 because both of these cylinders belong to the same bank of cylinders.

Now, 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.

Let's take a look at your test results:

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.

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.

In case you do have a blown head gasket on your hands, you may be wondering:

1. How can I tell if the cylinder head is cracked?
2. How can I find out if the cylinder head is warped?
3. Do I need to resurface the cylinder head?
4. Do I need to remove both cylinder heads?

You can find the answers to the above questions here: Frequently Asked Questions.