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Engine Diagnostics - How To Perform A Compression Test

Understanding how to properly perform a compression test is key to making an accurate diagnosis of an engine mechanical or performance issue. In this article, we'll discuss what a compression test is, different types of compression tests, how to ensure that our results are accurate, and how compression testing integrates into diagnosis to lead us in the right direction toward identifying the root cause of the issue.

What is a compression test?

Before we jump straight into performing a compression test and evaluating results, we need to understand exactly what compression is and what a compression test tells us.

Engine compression is the compacting of the air/fuel mixture in the cylinder and combustion chamber as the piston travels from bottom dead center (BDC) to top dead center (TDC) through the compression stroke. Sufficient compression of the air/fuel mixture is critical to efficient combustion and smooth engine performance. Compression is measured in units of pressure, most commonly psi.

As a side note, an engine's compression ratio refers to the ratio of the cylinder and combustion chamber volume with the piston at bottom dead center to the volume with the piston at top dead center, which is expressed as 1. If an engine has a compression ratio of 10:1, the BDC cylinder volume is 10x that of the TDC volume.

A compression test is ultimately an evaluation of how well the combustion chamber is sealed in each cylinder, and therefore the ability of the engine to compress and efficiently combust the air/fuel mixture. Compression testing is performed by using a compression tester to measure the peak pressure in each cylinder with the engine cranking or running. The compression tester is connected to the cylinder in place of the spark plug in order to measure cylinder pressure. A compression tester is a relatively simple tool consisting of adapter for various spark plug threads, a tube with a Schrader valve, and a pressure gauge. The Schrader valve allows airflow only in the direction of the gauge so that the peak pressure reading is held on the gauge during testing.

When To Use Compression Testing

Compression testing is commonly used during diagnosis of various engine performance issues where an engine mechanical problem is suspected. Insufficient compression in one or more cylinders can cause engine misfires, rough idling/running, air/fuel ratio issues, or the engine to be unable to start. These are common examples where compression testing can be used to identify an engine mechanical concern, but any issue where low compression is a possible cause is a candidate for compression testing.

Compression testing is not typically the first test we'll do in any given diagnosis as depending on the engine it can be quite time consuming. We want to address any other possible causes that that are quicker and/or easier to eliminate first. This will of course depend heavily on the specific issue and where we are in the diagnosis, as well as where we've prioritized an engine mechanical concern on our possible cause list.

For example, if we're diagnosing a single cylinder misfire, we would generally want to eliminate the spark plug, ignition coil, and ignition coil circuits prior to performing a compression test to check for an engine mechanical issue. On the other hand, if we were diagnosing a crank/no start where the engine sounds like it lacks compression when cranking, it may be more efficient to jump right to a compression test.

If we're at the point in a diagnosis where a compression test is the logical next step, we first need to decide what type of compression test makes the most sense based on the situation. We'll go over each type of compression test and when they should be used in detail below.

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Dry Compression Testing

In almost all situations, we'll want to start with a dry compression test to get baseline readings and perform further testing from there if needed. Dry compression testing is performed while cranking the engine to read the maximum compression pressure of each cylinder.

To Perform a Dry Compression Test:

Disable the ignition coils and fuel injectors. This can be done by fuse/relay removal or disconnecting the ignition coils and fuel injectors. Some vehicles have a compression testing mode available through the OE scan tool that will accomplish this without removing any components.
Remove all spark plugs.
Install the compression tester and gauge to the first cylinder.
Hold the throttle in the wide open position.
Crank the engine until the tester gauge reading stops increasing.
Repeat for the remaining cylinders.

There are two primary specifications that we need to pay attention to when evaluation compression test results; minimum pressure and cylinder variance. Minimum pressure is fairly self-explanatory; this is the minimum allowable pressure for any given cylinder. The variance specification is the maximum allowable pressure difference between any two cylinders. An example of compression pressure specifications is 180 psi minimum with 15 psi max variance.

If we find that our compression readings are outside of one or both specifications, our next step is to determine why. Typical next steps in this situation are a cylinder leakage test or a wet compression test, both of which we'll discuss below.

Wet Compression Testing

The primary purpose of a wet compression test is to isolate the piston rings as the cause of low compression or excessive cylinder variance. A wet compression test is performed the same way as a dry compression test with the addition of a small amount of engine oil (typically about a tablespoon) added to the cylinder being tested. The added oil will temporarily provide a better seal between worn piston rings and the cylinder wall and cause compression readings to increase.

Keep in mind that in the case of a crank/no start condition, excessive unburnt fuel in the cylinders can wash the oil from the cylinder walls and cause low compression readings even though the piston rings are not worn. Generally, compression needs to be well below the minimum specification for the engine to be unable to start, so be careful not to be mislead by low compression readings in these situations.

Because cylinder leakage testing is a more comprehensive test, we typically opt for it over a wet compression test.

Running Compression Testing

The purpose of a running compression test is to evaluate the airflow through each cylinder rather than the cylinder's sealing ability like the dry and wet compression tests. Running compression testing is most useful when diagnosing engine misfires or other performance concerns that occur under load. A restriction to or lack of airflow through a particular cylinder will show up as a lower running compression reading compared to the other cylinders. This type of issue will typically not show up on a dry or wet compression test as the cylinder airflow requirements are relatively small for these.

Running compression testing is best reserved as a secondary test to vacuum testing. Vacuum testing is done by simply connecting a vacuum gauge to the intake manifold and monitoring the reading with the engine running, typically at idle. A rapid fluctuation in the vacuum gauge indicates an airflow issue in one or more cylinders. Running compression testing is then used to isolate the cylinder with the airflow issue.

The most common causes of unstable engine vacuum and/or low running compression are worn camshaft lobes, bent or burnt valves, and restrictions such as excessive carbon buildup on the valves or intake runners. Keep in mind that any issues that prevent the valves from fully sealing will show up on dry and wet compression tests.

To perform a running compression test:

Remove the spark plug and disable the ignition coil of only the cylinder being tested.
Disable the fuel injector of only the cylinder being tested.
Install the compression tester and gauge to the cylinder.
Start the engine and allow it to idle.
Release the pressure from the gauge and allow the reading to stabilize.
Repeat for the remaining cylinders as needed.

Pro Tip: A running compression test can be performed with the Schrader valve removed from the compression tester to get a more dynamic reading, but we will not be able to accurately perform a snap throttle test this way.

Snap Throttle Compression Testing

A snap throttle compression test is an extension of the running compression test that further evaluates the airflow capabilities of an individual cylinder. The purpose of this test is to amplify and identify any cylinder airflow issues as opening the throttle significantly increases the amount of air flowing through the intake manifold.

To perform a snap throttle test, quickly snap and release the accelerator pedal during a running compression test, and note the reading on the compression tester gauge. As mentioned above, the Schrader valve needs to be installed in the compression tester for this test.

The possible causes of low snap throttle compression are the same as those listed above for low running compression.

Possible Causes of Low Compression

Possible causes of low compression readings will depend on the type of compression test being performed. Below are some common causes of low readings for each type of test and recommended next steps.

Low Dry or Wet Compression

Worn piston rings (dry compression only: cylinder leakage or wet compression test
Worn cylinder wall: borescope cylinder, cylinder leakage test
Cracked piston: cylinder leakage test
Poorly sealing valves: cylinder leakage test, vacuum test, running/snap throttle compression test
Blown head gasket: cylinder leakage test, testing for exhaust gasses in the cooling system
Bent connecting rod: borescope cylinder to observe TDC position, cylinder leakage test to confirm no sealing issues are present.

Low Running/Snap Throttle Compression or Rapid Vacuum Fluctuation

Worn camshaft lobe: visually inspect camshaft lobes
Excessive carbon buildup: borescope valves through intake, inspect intake runners
Bent valve stem: visually inspect with borescope, dry compression test, cylinder leakage test

Cylinder Leakage Testing

Cylinder leakage testing is the most common next step when low static (dry or wet) compression readings are found, which is why we decided to include it as a supplement to the compression testing information in this article. The purpose of cylinder leakage testing is to pressurize the cylinder and detect any leak points.

Cylinder leakage testers are installed to the engine the same way as compression testers, but have a second line and gauge that connects to and measures shop air pressure. The tool is used to pressurize the cylinder using regulated shop air pressure to test it for leakage.

Pro Tip: While cylinder leakage testers will have a gauge that shows a percentage of cylinder leakage, we typically don't recommend relying on this as a source of useful information; if we didn't know there was leakage in the cylinder based on a compression test, we probably wouldn't be performing a cylinder leakage test in the first place. As mentioned above, the primary purpose of a cylinder leakage test is to identify leak points rather than evaluate the amount of leakage.

To perform a cylinder leakage test:

Rotate the engine so that the cylinder being tested is at top dead center (TDC) of the compression stroke.
Stop the engine from rotating by securing the crankshaft pulley or flywheel.
Remove the spark plug of the suspect cylinder and install the cylinder leakage tester in its place
Set the supply pressure to 100 psi.
Open the valve on the tool to apply pressure to the cylinder.
Listen for air leakage to identify leak points.

Here are some common points of cylinder leakage and how to check them:

Piston rings: remove the oil cap and listen for excessive air leakage into the crankcase.
Intake valve(s): open the throttle and listen for air leakage into the intake manifold.
Exhaust valve(s): listen for air leakage from the tailpipe.
Head gasket: Listen for air leakage into adjacent cylinders and watch for bubbles in the coolant reservoir.

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