How to perform an engine compression test

In this article we’ll perform a compression test on a 1995 Monte Carlo with a 3.1 liter V6. I’ll cover what factors determine the need for a compression test and how to set up and perform this test.

Tools for the Job

Skill level/Difficulty, Advanced.
Here’s a list of tools you will need to perform this test;

• 3/8 drive ratchet
• Sparkplug socket
• 3″ and 6″ extensions
• Sparkplug wire pullers
• Compression tester
• Remote starter switch

Refer to Sears/Craftsman tools for a great buy on a complete compression test kit and a remote starter switch to use as well.

Estimated Time

If you’ve never done this job before, this could take approximately 2 hours. An experienced technician should be able to perform this job in 3/4 to 1 hour start to finish. A job like this should pay you 2 hours labor if performed for a customer from a repair shop.

Engine Animated Video

Watch this short clip on basic engine construction to see how compression factors into the running cycles during engine operation.

Factors for Testing

This 1995 Monte Carlo came to me with a bad head gasket. As part of trying to determine the full condition of the engine for repair, I am performing a compression test to make sure that no problems exist in the lower end of the engine.

A compression test can be performed for several different reasons. If the engine is running poorly, smoking, or burning a quart of oil in a short period of time could be a few reasons why to perform this test.

One of the most important times to perform this test would be what I call a “check for purchase”. If you plan to purchase a used vehicle with high mileage, you might consider performing a compression test to verify if any long term problems my exist or pop up unexpectedly after your purchase. Protect yourself and your investment.

What to look for

Compression tests are designed to read the amount of compression developed from each cylinder within the engine being tested. Whether you have a 4, 6, or 8 cylinder engine, the manufactures have designed each engine with a specific amount of compression needed to run smoothly and perform at it’s optimum performance range.

Each engine is different based on options like, turbos, supercharger, old style carburators to newer fuel injected systems.

Even without having a specification to compare to, you can always cross compare all your readings from one cylinder to the others and make sure the readings are within 10% of each other. Any cylinder with more than 10% will cause an imbalance and have poor idle and or acceleration.

Set up to Test

For starters you will need to remove the spark plugs in order to hook up the gauge to test the cylinders individually.

Note: Refer to Replacing Engine Spark Plugs for recommendations on spark plug removal.

(Even though this removal procedure is a 4 cylinder engine, the concept is the same and there is helpful knowledge about the tools to use.)

Gauge Hook Up

Remove all the sparkplugs before testing any of the cylinders.

Once removed, install the screw in adapter into the 1st cylinder to be tested. (install hand tight only)

Connect the adapter to the gauge head via the quick release coupler

Obtaining a Reading

Here is where you could do a couple of different things. You could get someone to help by staying in the car and cranking the engine over with the key while you read the gauge or walk around and crank it every time you change cylinders. But I like to promote automotive independence. Plus here’s another special tool that is a must have.

The remote starter switch allows you to connect and jump power to the starter while cranking the engine from under the hood. This is a great time saver when doing this type of job.

Locate the starter on the vehicle being tested. This one can be accessed from the front of the vehicle.

Connect the starter switch across the 2 terminals at the starter.

The small terminal on the solinoid is designated as the S terminal. The S represents the start side of the circuit. When the key is in the crank position, battery power is supplied to this terminal.

The large terminal on the solinoid is designated as the B+ terminal. This terminal has constant battery voltage already there waiting to energize the motor.

The idea is to energize the motor from the remote starter switch by pushing a button when ready.

Perform the Test

Disable the vehicles ignition and fuel system.

Locate and unplug the ignition coil and remove the fuse for the fuel pump to prevent any problems from happening.

Push the remote starter button or turn the key and listen for about 5 revolutions. Record your reading on a piece of paper as you test all of the cylinders. This will allow you to compare your readings when you are done.

If any of the cylinders are lower than the others, retest that cylinder to see if you just had a bad connection with the adapter or loss of compression from the gauge.
Note:(compression range for a gas engine varies, but are approximately 120 psi per cylinder)

Reason for disabling ignition and fuel

With the sparkplugs removed, the fuel system can still spray fuel into the cylinders and the ignition system is still active and can arc and ignite fuel in and around the sparkplug hole causing a flash fire. Never mind that it could possibly arc to you for an unexpected jolt if not paying attention during the test.

Results

Assuming that you had a reading where one or more cylinders were low on compression the problem can only be one of two things, bad piston rings in the lower end, or worn valves in the heads. Here are a couple of things to check.

Start by squirting a small amount of oil into the cylinder being tested and retest that cylinder in question. If the compression reading goes up, this is a sign that the piston rings in the engine are worn. The only resolution to this sort of problem is to repair or replace the engine.

If the compression reading does not change, another test will need to be performed to determine the state of wear. A cylinder leak down test will need to be performed to verify if worn valves in the engine are the cause of loss in compression.

Stay connected with Auto Repair Facts and I will put together a post on how to perform a cylinder leakdown test soon.