TEST 2: Coolant Shooting Out From Open Radiator
If the engine oil isn't mixed with coolant, your next test is to open the radiator and then crank the engine to see if the coolant gets shot out (of the radiator).
The correct and expected test result is for the coolant (inside the radiator) to remain undisturbed as you or your helper cranks the engine.
If the coolant gets shot out while cranking the engine, you've got a blown head gasket on your hands.
IMPORTANT: Do not remove the radiator cap from the radiator if the engine is hot or has been running for any length of time. Let the engine cool down completely before removing the cap from the radiator.
Let's get started:
- 1
Remove the radiator cap.
- 2
Check the coolant level in the radiator.
Add water or coolant if necessary. - 3
Have your helper crank the engine while you stand at a safe distance from the open radiator.
- 4
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 one or both head gaskets are blown on your 4.3L V6 Chevy S10 pickup (GMC S15 pickup, GMC Sonoma).
CASE 2: The coolant DID NOT shoot out from the radiator. This is the correct and expected test result.
If your Chevy S10 pickup, GMC S15 pickup or GMC Sonoma's 4.3L V6 engine starts but runs with a misfire, then next step is to test engine compression. For this test go to: TEST 3: Engine Compression Test.
If the engine starts but overheats almost immediately, then your next step is to do a 'block' test. For this test go to: TEST 4: Using A Chemical Block Tester (Combustion Leak Tester).
TEST 3: Engine Compression Test
Head gaskets are known to burn in a spot between two side-by-side cylinders. The photo at the beginning of this tutorial shows such a head gasket failure.
This type of head gasket failure will cause those two side-by-side cylinders to produce 0 PSI compression.
We can check to see if this has happened by performing an engine compression test.
NOTE: You can find a comprehensive explanation of the compression test (and how to interpret its results) here:
These are the test steps:
- 1
Disable the ignition system by disconnecting the ignition coil from its electrical connector.
- 2
Disconnect the spark plug wires from their spark plugs.
IMPORTANT: Label the spark plug wires with the cylinder number they belong to before disconnecting them. This will help you reconnect them to the correct spark plug. - 3
Remove all six spark plugs.
- 4
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. - 5
Have your helper crank the engine.
The needle on the tester will climb as the engine cranks. Once the cylinder reaches its maximum compression the needle will stop climbing. - 6
Once the gauge's needle stops climbing, have your helper stop cranking the engine.
- 7
Write down the reading and what cylinder it belongs to (you can use the illustration above to help you identify the cylinder).
- 8
Repeat steps 4 through 7 in the remaining cylinders.
Let's take a look at what your test results mean:
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 (of that bank of cylinders) is burned at the point between those two cylinders.
Replace both head gaskets.
