Testing the engine compression on your 4.3L or 5.0L or 5.7L GM equipped vehicle is not hard. In this article, I'll guide you every step of the way and I'll also show you how to interpret the results of the compression test.
Why do an engine compression test? Well, if you're wanting to know the mechanical condition and/or sealing effectiveness of the piston rings or cylinder head valves, an engine compression test is the only way to ascertain this info.
Not only that, but your 4.3L (or 5.0L, 5.7) car, pick up, mini-van, van or SUV may be suffering a hard to diagnose misfire code that has you pulling your hair out (and maybe even spending money on parts your vehicle doesn't need) and lighting up the check engine light (MIL) with the following misfire diagnostics trouble codes: P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308. If this is happening to you, then an engine compression test is in order for your vehicle.
Contents of this tutorial:
You can find this tutorial in Spanish here: Prueba: Compresión del Motor (GM 4.3L, 5.0L, 5.7L) (at: autotecnico-online.com).
Tools You'll Need:
- Compression Gauge Tester.
- A Helper
- Pen and Paper
TEST 1: The ‘Dry’ Engine Compression Test
The image viewer on the left, has two images that will help you to identify the individual engine cylinders. This will come in super handy in the following test steps.
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.
OK, to get this show on the road, I'll first explain the test steps. At the end of the test steps, you'll find two possible test results that will help you to interpret your specific test results.
NOTE: If possible, the engine in your vehicle must not be HOT and yet not completely cold. So, if your car or pick up (or mini-van, van, SUV) has bee running for an extended amount of time, let it cool down for about an hour. Now, if your car doesn't start and you're testing a no start condition, don't worry about having a warm engine for the compression test.
Let's get started:
Disable the fuel system. You can easily do this by simply disconnecting the fuel injector ‘Spider’ assembly. This step is important, so don't skip it.
Disable the ignition system. You can accomplish this by disconnecting the ignition coil or the ignition module. This step is also very important.
Now, remove all six (or eight spark plugs) spark plugs.
As your 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!
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.
Have a helper crank engine as you observe the needle on the compression tester's gauge.
Once the needle on the gauge stops climbing, have him or her stop cranking the engine.
Write down this compression reading on a piece of paper. Include the number of the cylinder this reading belongs to.
Now repeat steps 4 thru' 7 on the other cylinders.
Let's examine your test results:
CASE 1: All eight cylinders had 0 PSI. This test result tells you that the engine has an internal mechanical problem.
The most common cause of this condition is a broken timing chain or a blown head gasket.
Your next step should be to check the condition of the timing chain and perform a blown head gasket test.
CASE 2: One or more cylinders had a low compression value compared to the others. This could be normal or it could be causing a problem.
To find out if the compression values are normal or not, go to: How To Interpret The Engine Compression Test Results.
CASE 3: All eight compression values were similar and above 120 PSI. This lets you know that a compression problem is not behind the no start or misfire problem you're trying to troubleshoot.