How to test a blown head gasket on GM 3.5L/4.2L

There are 3 very common tests that are done to find out if the head gasket is blown on your GM 3.5L or 4.2L equipped pickup or SUV.

These three tests are:

  1. Checking that the engine oil isn't mixed with coolant.
  2. Checking that there isn't exhaust gases and/or engine compression pressure exiting from an open radiator.
  3. Checking engine compression.

All 3 tests are very easy to do and in this turorial I'll show you how to perform them in a step-by-step manner.

Here are the contents of this tutorial:

  1. Symptoms of a Blown Head Gasket.
  2. HEAD GASKET TEST 1.
  3. HEAD GASKET TEST 2.
  4. HEAD GASKET TEST 3.
  5. Related Test Articles.

Symptoms of a Blown Head Gasket

A blown head gasket is the direct result of engine overheating. The root causes of an engine overheating are pretty diverse.... here are some of the most common causes:

  1. The radiator fan not working (if equipped with an electric fan motor).
  2. Bad fan clutch (if equipped).
  3. Engine lost all, or most, of the coolant because:
    1. Water pump is bad and leaking coolant.
    2. Bad and/or ruptured radiator hose.
    3. Leaking radiator.
    4. Leaking heater core.
  4. Thermostat went BAD and is stuck closed.

The most common symptoms a blown head gasket are:

  1. Your pickup or SUV 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 pickup or SUV won't start.
    1. You have already verified it's not an ignition system problem because you have spark coming out at all of the spark plug wires.
    2. 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.

TEST 1: Engine Oil the Color
of ‘Coffee With Too Much Cream’

Checking the engine oil to see if it's mixed with coolant

OK, to get this kettle boiling... the very first test I'm gonna' ask you to do is to check to see if the engine oil is mixed with coolant.

This is what you need to do:

  1. 1

    Open your pickup's hood and pull the engine oil dipstick out of the engine.

  2. 2

    Now, visually check the color of the oil and its viscosity (that's on the dipstick). You're looking for one of two things:

    1. Is the oil on the dipstick a creamy tan and is the oil thick? Or...
    2. Is the oil on the dipstick its normal color and viscosity

Let's take a look at what your test results mean:

CASE 1: The color of the oil is a light tan, like ‘coffee with too much cream’, this confirms that the Head Gasket is Blown and that it's letting coolant mix with the engine oil.

If you're wondering why the oil looks like this? The principal reason is that your 3.5L/4.2L GM equipped vehicle over-heated and:

  1. The cylinder head warped... causing the head gasket to burn.
  2. Since a burned/blown head gasket can not seal and keep the engine oil and coolant separated in both the engine block and the cylinder head... both oil and coolant mix... the resulting combination gets thick and becomes an off-white color.

CASE 2: The color of the engine oil is normal, Although this is a good result, it does not confirm the condition of the head gasket yet, go to TEST 2.

Here's why: In the majority of the cases, a blown head gasket will cause the engine's coolant to mix with the oil, but not always. Therefore one or maybe two more tests are needed to make sure that the Head Gasket is really blown or not. The next test is to see if the engine's compression/ combustion gases are escaping thru' the radiator.