How To Do A Compression Test (GM 4.8L, 5.3L, 6.0L)

Testing the engine compression on your Chevrolet or GMC 4.8L, 5.0L, or 6.0L engine is not hard to do.

A compression test will help you to determine the health of your engine by measuring the pressure of the air that gets compressed by the piston rings against the cylinder head valves.

A healthy engine will produce similar compression values on your compression tester on all 8 cylinders. An engine with internal problems (worn compression rings or worn cylinder head valves) will produce some or all cylinders with low or no compression.

This is a test that you can do (without having to take your pickup, van or SUV) to the shop and in this article, I'll show you how to do both a dry and wet engine compression test and then interpret the results.

You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (GM 4.8L, 5.3L, 6.0L) (at: autotecnico-online.com).

Symptoms Of Bad Engine Compression

Low compression in one or several engine cylinders will have a direct effect on idle quality. The symptoms you'll see will be:

1. Rough Idle.
2. Misfire condition and misfire codes:
   • P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307,P0308.

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:
   • The fuel pump will activate, so you'll see fuel pressure at specification (if testing with a fuel pressure gauge).
   • All of the COP coils will spark.
   • If the COP ignition coils are sparking, then this indirectly proves that the crank sensor is OK too.
   • The PCM will still activate all of the fuel injectors.

What Tools Do I Need?

The most important tool that you're gonna' need is a compression tester. You can either rent this bad boy from your local auto parts store (Auto Zone, O'Reilly Auto Parts, Advanced Auto Parts, etc.), or you can buy one online.

Since 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).
5. Spark plug wire puller.

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

TEST 1: Dry Compression Test

To get the most accurate result of the health of your engine, you should do the compression test with a slightly warmed up engine (not an engine at normal operating temperature).

Why? Well because both the engine block and the pistons and rings will expand when heated and this will have an effect on your compression tester readings.

This isn't always possible, since your particular vehicle may not start. If your engine doesn't start, you can still do the test and get very useful data that'll help you find out what's going on.

Before you start, take a look at the whole article and familiarize yourself with all of the steps. Please remember to always think safety first, since you'll be working around a cranking engine.

IMPORTANT: The engine should not be hot when removing the spark plugs! If the engine has been running for an extended period of time, let it cool down completely before removing the spark plugs.

OK, let's get this show on the road:

1. 1

   Disable the fuel system. You can easily do this by simply:
   
   Removing the fuse labeled INJ 1, which is a 15 A fuse in the underhood fuse block.
   
   Or disconnecting the fuel injectors from their electrical connectors.
   
   Or, disconnecting the fuel pump relay.
   
   This step is important, so don't skip it.

2. 2

   Disable the ignition system. You can easily do this by simply:
   
   Removing the fuse labeled INJ 1, which is a 15 A fuse in the underhood fuse block (this fuse feeds the fuel injectors and the COP coils with power).
   
   Or disconnect the COP coils from their electrical connectors.
   
   This step is important since it will prevent damage to the ignition coils, so don't skip it.

3. 3

   Remove all 8 spark plugs. As you're taking them out, be careful and don't drop any of them on the floor, or you could cause the spark plug's ceramic insulator to break, and this will cause a misfire!

4. 4

   Install the compression tester. Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder. Hand tighten the compression gauge only! Do not use any type of tool to get it tight.

5. 5

   When ready, crank the engine as you observe the needle on the compression tester's gauge. Once the needle on the gauge stops climbing, have your helper stop cranking the engine.

6. 6

   Write down the compression value. Record this compression reading on a piece of paper. Include the number of the cylinder this reading belongs to. Now repeat steps 1 thru' 6 on the other cylinders.

7. 7

   Interpret the results. After testing all cylinders and having written down all of your compression test readings, now you need to interpret the results.

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

CASE 1: You got a reading of 100 PSI or less (less being 0 PSI) on all of the cylinders. This usually means that you've got serious internal engine mechanical problems.

It's absolutely rare for the engine to get to the point of 0 PSI compression on all cylinders, but if it were to, this would usually indicate a broken timing chain.

What is common, if you have a very high mileage engine, is low compression across the board. Low usually means anything under 120 PSI (although the service manual says 100 PSI is the minimum). If your engine has reached this point, it's also smoking from the quart of oil it's burning every few days. The other symptoms you'll see is that the idle will be very rough.

CASE 2: One or two cylinders gave a low compression value. This might be normal, since each cylinder will not give the exact same pressure value.

To further investigate this, your next step is to go to: Interpreting Your Compression Test Results.