How To Do An Engine Compression Test (GM 3.8L)

Testing the compression of each cylinder or a specific engine cylinder is becoming a standard test when troubleshooting hard to diagnose misfire conditions on the GM 3.8L V6 equipped cars and mini-vans.

A low compression condition in any of the 6 engine cylinders will cause a misfire condition and set misfire codes (if the vehicle is OBD II equipped: P0300, P0301, P0302, P303, P0304, P0305, P0306) that no matter what you replace, the misfire will continue to drive you crazy.

This article is part of a series of articles to help you to troubleshoot and repair a misfire condition on your GM (Chevrolet, Buick, Pontiac, Oldsmobile) 3.8L V6 equipped car or mini-van. The key article of the series is: How To Diagnose Misfire Codes P0300-P0306 (GM 3.8L).

You can find this tutorial in Spanish here: Cómo Verificar La Compresión Del Motor (GM 3.8L) (at: autotecnico-online.com).

Symptoms Of An Engine Compression Problem

When it comes to engine compression issues, they often fall into two main categories:

1. Low Compression in Some but Not All Cylinders: This means that while some cylinders in the engine have lower-than-normal compression levels, others may still maintain acceptable levels of compression. This can happen due to various reasons such as worn piston rings, damaged valves, or cylinder head gasket leaks.
2. Zero PSI Compression in Some or All Cylinders: This indicates a severe problem where either some or all cylinders in the engine have no compression at all. This situation typically arises from significant engine damage, such as a blown head gasket, broken piston, or severe valve damage.

Regardless of whether the engine has low compression or zero PSI compression, these issues manifest through several symptoms that affect the performance, reliability, and drivability of the vehicle:

• Hard Starting or No Start: When compression is compromised, the engine may struggle to start or fail to start altogether. This happens because insufficient compression affects the engine's ability to generate the necessary power to start and run.
• Poor Engine Performance: Reduced compression leads to poor engine performance, resulting in sluggish acceleration, rough idling, and overall lackluster performance. The engine may feel weak and struggle to maintain consistent power output.
• Misfiring: Inadequate compression can cause misfiring, where the air-fuel mixture fails to ignite properly in the cylinders. This results in erratic engine operation, with noticeable jerking or hesitation during acceleration.
• Reduced Power and Performance: As compression decreases, so does the engine's ability to produce power efficiently. This leads to reduced power output, making it challenging to accelerate or maintain speed, especially under load.
• Excessive Oil Consumption: Compression issues can contribute to increased oil consumption as oil may leak into the combustion chamber due to worn piston rings or damaged valves. This results in higher oil consumption than normal and may lead to other complications such as fouled spark plugs.
• Audible Engine Noise: In severe cases, compression issues may cause abnormal engine noises such as knocking, rattling, or tapping. These noises typically indicate significant internal engine damage and should be addressed promptly to prevent further deterioration.

Which Compression Tester Should I Buy?

There are lot of engine compression testers to choose from and many places to buy them. I'm gonna' make two recommendations to you:

1) Which one to buy:  The engine compression tester that I have always used is the Actron CP7827 Compression Tester Kit. My only complaint about this engine compression tester is that it does not come with a case to store it in.

2) Where to buy:  You can buy an engine compression tester just about anywhere, but you'll end up paying more for it (especially at your local auto parts store). The above links will help you comparison shop. I think you'll agree it's the better way to save money on the compression tester!

TEST 1: Dry Engine Compression

Every engine cylinder, as you may already know, needs air, spark, and fuel to produce engine power. The other key component, that's never included into this formula, is the mechanical condition of the cylinder, specifically: the compression ability of the cylinder.

So then, this test will help you to verify that the specific cylinder you are testing, is contributing an adequate compression pressure to produce the required power output requirements.

This test article assumes that the engine starts and runs and that you're testing a misfire condition, but even if your engine doesn't start, this test will help you.

NOTE: It's important that the engine NOT be cold, and yet not be hot. So, if the engine has been running for an extended amount of time, let it cool down. If the engine is cold then I suggest you start'er up for about 15 minutes. Now, if the engine in your car or mini-van doesn't start, then this doesn't apply to you and let me tell you that the compression test (in this article) will still work.

OK, without further ado, here's the test:

1. 1

   Disable the fuel system. You can do this by disconnecting all 6 fuel injectors from their electrical connectors.
   
   This is important! In a few minutes, you're gonna' be cranking the engine and you don't want fuel being injected into the engine as you do the compression test.

2. 2

   Disable the ignition system, so as not to produce spark during the tests.
   
   You can easily accomplish this by disconnecting the ignition module from its connector. This is also very important!

3. 3

   Remove all 6 spark plugs. Be careful and don't drop any! This (them hitting the floor) could break the spark plug's ceramic insulator and this will cause a misfire!

4. 4

   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 everything is all set up, have your helper crank the engine while you eye-ball the compression gauge (tester).

6. 6

   Once the needle stops climbing, have your helper stop cranking the engine.

7. 7

   Write the compression value down on a piece of paper.

8. 8

   Repeat steps 1 thru' 6 on the remaining 5 cylinders.

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

CASE 1: No compression in 2 or more cylinders. This test result tells you that the engine has serious internal problems.

The most common issues would be: Broken timing chain. Or a blown head gasket. Or the engine threw a rod.

CASE 2: Low compression in one or more cylinders. To a certain point, it's normal for the compression to vary a little between cylinders (as the engine accumulates thousands of miles).

But if these values vary too much, then you're gonna' have a bonafide misfire on your hands.

The next step is to do some math to find out if this low compression value is within a normal parameter or not. Go to: Interpreting Your Compression Test Results.