In this tutorial I'll explain what is involved in doing a compression test and more importantly how to interpret the results of your test.
You'll be able to find out if you have one or more dead cylinders causing a misfire condition. Or, if the engine is not starting, you'll be able to find out if engine compression problems are behind it.
NOTE: This tutorial applies to all 3.5L V6 equipped GM vehicles. These are the: 2006-2010 Chevrolet Impala, 2005-2009 Chevrolet Malibu, 2005-2006 Chevrolet Uplander, 2005-2010 Pontiac G6, 2005-2006 Pontiac Montana.
Contents of this tutorial at a glance:
You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (2004-2008 3.5L Malibu) (at: autotecnico-online.com).
Symptoms Of Low Or No Engine Cylinder Compression
Engine compression problems usually fall into one of two categories: either the compression problem causes a misfire and/or rough idle condition or it keeps the engine from starting.
If the engine compression problem is causing a misfire condition, you're going to see one or more of the following misfire trouble codes lighting up the check engine light of your 3.5 V6 Chevrolet Malibu:
- P0300: Random Cylinder Misfire.
- P0301: Cylinder #1 Misfire.
- P0302: Cylinder #2 Misfire.
- P0303: Cylinder #3 Misfire.
- P0304: Cylinder #4 Misfire.
- P0305: Cylinder #5 Misfire.
- P0306: Cylinder #6 Misfire.
Besides a cylinder misfire condition (or a rough idle condition), your 3.5L V6 engine is going to suffer from bad gas mileage. You're also going to notice a heavier exhaust smell coming out of its tailpipe. Also, since the engine is not running optimally, it's not going to be as peppy as it was once.
If the engine compression problem is causing your engine to not start, well it's not going to start.
With the help of the test instructions in this engine compression test tutorial, you'll be able to find out if a compression problem is behind the misfire condition or no start condition of your 3.5L V6 engine.
If the engine compression problem is causing your vehicle's 3.5L V6 engine to not start, it's usually because most of the cylinders have 0 PSI compression. A problem like this is usually caused when the engine has overheated and has blown head gaskets or it has thrown a rod.
TEST 1: Finding The Dead Cylinders
To test the engine compression, you'll need to remove all of the spark plugs from your vehicle's 3.5L V6 engine.
Before you remove them, it's important that you label the spark plug wires before unplugging them from their respective spark plugs. This way, you won't connect them to the wrong spark plug and lose the ignition systems firing order.
If you don't have a compression tester, you can borrow one from your local auto parts store or you can buy one Online. Check out my recommendations here: Which Compression Tester Should I Buy?
IMPORTANT: Do not remove the spark plugs from a hot engine or you run the risk of stripping the threads of the spark plug holes in the cylinder heads. If the engine has been running for any length of time, let it cool down completely before removing them.
OK, these are the test steps:
Disable the ignition system by disconnecting the ignition coil pack from its electrical connectors. This will prevent the ignition coil from sparking during the test.
Remove the spark plugs. Remember, the engine can not be hot!
When removing the spark plugs, be careful not to drop any of them on the floor, or you run the risk of having the spark plug's porcelain insulator crack and then you'll have a misfire on your hands.
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 your helper crank the engine till the needle on the compression gauge stops climbing.
Now, record on paper the value at which the needle stopped and the number of the engine cylinder on a piece of paper. Release the pressure on the gauge and repeat this step one more time.
Repeat this test step on the remaining cylinders.
Let's examine your test results:
CASE 1: One or two cylinders had a much lower compression value than the others. This could be normal because the cylinders will not wear down at the exact same rate or these cylinders are causing the misfire problem.
To further interpret these test results go to: Interpreting The Compression Test Results.
CASE 2: Three or more cylinders had almost no compression or 0 PSI compression. Having three or more cylinders with almost no compression or 0 PSI compression will cause your GM vehicle's 3.5L engine to not start.
Having a compression value close to 0 PSI or 0 PSI is usually due to a problem in the cylinder head valves or piston rings of the affected cylinders.
We can find out which of the two it is by doing a wet compression test. For this test go to: Interpreting The Compression Test Results.
CASE 3: The compression value of all six cylinders was similar and above 120 PSI. This test result lets you know that an engine compression problem is not behind your misfire condition and/or engine no start problem.
I'll explain why: If the engine had a compression problem causing a misfire or a no-start condition, then your test results would have indicated one or more cylinders with low or very close to 0 PSI compression. Since your test results indicate that all 6 cylinders have very similar compression values, you can rule out engine compression as a source of the misfire or engine no start problem you're trying to diagnose.