How To Check Engine Compression (GM 3.5L, 4.2L)

In this tutorial, I'll show you how to do an engine compression test on your 3.5L (5 cylinder) or 4.2L (6 cylinder) equipped GM pickup or SUV.

Why check the engine compression? Well, because piston rings and cylinder head valves do wear out and when they do, you'll have one of two things on your hands: 1.) a misfire condition (miss at idle or at acceleration) or 2.) a no-start condition.

The only way to find out if the internal engine components are OK or worn out (piston rings and cylinder head valves) and causing any of the two above conditions is to do an engine compression test.

To make the long story short, the engine compression test is a test that you can do (without having to take your pickup or SUV) to the shop. In this tutorial I'll show you how to do both a dry and wet engine compression test and then interpret the results.

Symptoms Of Low Engine Compression

‘Low compression’ in one or several engine cylinders will have a direct impact on the way the engine Idles and the way the engine accelerates. ‘No compression’ on all cylinders and the engine won't start, it'll Crank but Not Start.

Here are other symptoms of low or no compression that you'll see:

1. Rough idle.
2. Check engine light (CEL) on with one or several of the following diagnostic trouble codes (DTCs):
   1. P0300, P0301, P0302, P0303, P0304, P0305, P0306.
3. The spark plug (of the cylinder with Low or no compression) will always get fuel fouled.

No compression in all of the cylinders will result in a cranks but does not start condition. The symptoms you'll see will be:

1. No Start.
2. Everything else works, for example:
   1. The fuel pump will activate, so you'll see fuel pressure at specification (if testing with a fuel pressure gauge).
   2. All of the COP coils will spark.
   3. If the COP coils are sparking, then this indirectly proves that the crank sensor is OK too.
   4. The PCM will still activate all of the fuel injectors.
   5. The spark plugs (of the cylinders with no compression) will always get fuel fouled.

What Tools Do I Need To Test The Engine Compression?

The most important tool that you're gonna' need is a compression tester. If you don't have one, you can rent one from your local Auto Zone or O'Reilly Auto Parts Store.

If you're gonna' buy one and want/need to save some bucks, buy it online and below is where I recommend you buy it.

Since the COP ignition coils and the spark plugs need to be removed, you'll need some of the following basic tools:

1. Ratchet wrench.
2. 5/8'' spark plug socket.
3. Extensions for the ratchet wrench.
4. Motor oil (for the ‘Wet’ compression test part).

In case you're needing to buy a compression tester and want to save money by buying it online, you can shop here:

What Is Engine Compression?

In a nutshell, engine compression refers to the fact that the air that the engine breathes in is compressed (after it's been mixed with fuel).

As you may already be aware, at a certain point in the piston's compression cycle, the compressed air and fuel is ignited by a spark from the spark plug.

Taking a few steps back, and looking at the entire picture, each engine cylinder needs 3 things to run optimally:

1. Air.
2. Fuel.
3. Spark.

If any one of these three is missing from the mix, then that engine cylinder will not produce power (or not enough). This then leads to a misfire condition if one or two cylinders are affected or to a no-start condition if several or all cylinders are affected.

And so the point of all of this is to let you know how important engine compression is, because engine compression belongs to the ‘Air’ part of the above mix of air, fuel, and spark.

If the engine has low to no compression, then you'll have a misfire or a no-start condition.