TEST 2: Coolant Shooting Out From Open Radiator
For our second test we'll remove the cap from the radiator and crank the engine.
If the coolant is shot out while cranking the engine, then you've confirmed that the head gasket is blown.
If the coolant remains undisturbed as the engine is being cranked, then we're off to the next test.
IMPORTANT: Do not remove the radiator cap from the radiator if the engine is hot or has been running for any length of time. If the engine is hot then let it cool down completely before removing the cap from the radiator.
These are the test steps.
Remove the radiator's cap.
Check the coolant level in the radiator. If the radiator is empty then add some water or coolant.
Have your helper crank the engine while you stand at a safe distance from the open radiator.
You'll see one of two results:
1.) The water or coolant inside the radiator will shoot out of the open radiator.
2.) The coolant remained undisturbed. In other words, cranking the engine will have no effect on the level of the Water or coolant in the radiator.
Let's examine your test result:
CASE 1: The coolant shot out from the radiator as you cranked the engine. This confirms that the head gasket is blown on your 4.0L Jeep Cherokee (Wrangler).
CASE 2: The coolant DID NOT shoot out from the radiator. This is the correct and expected test result.
If the engine does not start or starts but runs with a misfire, go to: TEST 3: Engine Compression Test.
If the engine starts and starts to overheat within minutes, go to: TEST 4: Using A Chemical Block Tester (Combustion Leak Tester).
TEST 3: Engine Compression Test
There are cases where the head gasket burns in a section right between two cylinders (like the head gasket shown at the beginning of this tutorial).
When this happens, those two side by side cylinders will produce no compression and the engine is going to misfire (if it starts).
This type of head gasket failure can by confirmed by doing a cylinder compression test.
NOTE: For a more detailed explaination on how to do and interpret an engine compression test, consult this tutorial: How To Do An Engine Compression Test (1993-2001 4.0L Jeep Cherokee).
These are the test steps:
Disable the ignition system by disconnecting the ignition coil from its connector.
Remove all of the spark plugs.
Install the compression tester by hand in the spark plug hole of cylinder #1.
Do not use any tools to tighten the compression tester. Hand tightening the compression tester is more than enough to get the proper results.
Have your helper crank the engine.
Once it stops climbing have your helper stop cranking the engine.
Write down the reading and what cylinder it belongs to (you can use the illustration above to help you identify the cylinder).
Repeat steps 3 through 6 in the remaining cylinders.
Let's examine your test results:
CASE 1: All cylinder compression readings were above 0 PSI. This is the correct and expected test result.
Your next step is to perform a block test with a block tester. For this test 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 test result confirms that the head gasket is burned at the point between those two cylinders. You'll need to replace the head gasket.