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What Is A Compression Test? (When Is It Needed & How To Do It)

August 30, 2022

Does your vehicle lack power or run rough? 

Your engine’s performance is typically affected by four factors — air fuel mix, spark, timing, and compression.

If your engine can’t compress the air fuel mixture to the point that it self-ignites (for diesel engines) or is ignited by the spark plugs (for gasoline engines), you’re looking at a compression issue. 

To verify this, your vehicle needs a compression test. 

But what is compression testing? 
And, how do you perform one? 

In this article, we’ll take you through what’s a compression test, when you need to get it done, how to perform it, and the issues it can help you diagnose. 

This Article Contains: 

Let’s begin!

What Is A Compression Test

A compression test is a test method to assess the condition of your engine’s valves, valve seat, cylinder head, head gasket, and piston rings. 

If the material of these intake parts wears out and leads to reduced compression in one or more engine cylinders, there won’t be enough force to move the cylinder pistons and crankshaft. Moreover, compression loss in a cylinder could lead to a misfire or cause your engine to fail entirely. 

To avoid this, a compression test is conducted on each cylinder using a compression gauge or tester. 

The engine compression or compressive load is measured in PSI. Ideally, a healthy engine will have a compression (cylinder pressure) of over 100 PSI per cylinder. Also, there shouldn’t be more than a 10% variation between the highest and the lowest compression readings. 

So, when is compression testing required?
Let’s find out.

When Do I Need A Compression Test

Usually, you need a compression test if your car experiences any of the following symptoms: 

You should also get a compression test done if you see your Check Engine Light turned on and your Engine Control Module records a misfire code like P0301, P0302, P0303, P0304, or P0305

That said, you shouldn’t wait for such alarming signs to get compression tests. You could also get them done as preventive maintenance with every tune-up to maintain good compression and a healthy engine. 

Curious to know how this test is performed? 
The following section covers that in detail. 

How To Perform A Compression Test? (A Step-By-Step Guide)

While it’s easy to perform compression tests yourself, interpreting the results and getting to the root cause requires technical know-how. 

If you’re unsure how to perform the test, it’s best to have a certified technician do the job for you. 

But if you still wish to understand compression testing of an engine, here’s a general step-by-step guide on this test method: 

Step 1: Gather Your Supplies

Here are some of the tools you’ll require for compression testing: 

Step 2:  Warm Up Your Engine 

While you can perform compression tests on cold engines, you should ideally be running the test while the engine is warm.

Why? 
The piston rings, valve seat, head gasket, and other engine component material expand as they heat up. This creates the required compression ratio inside the engine. So if you perform this test on a cold engine, the pressure reading might be inaccurate.

Step 3: Disable The Ignition And Fuel Delivery Systems

Once your engine is warm enough, shut it off completely. 

Next, remove the fuel pump relay switch and the coil pack harness. 

In older vehicles with distributors, you’ll have to disable the ignition coil by disconnecting the positive terminal of the coil or unplug the ignition coil completely. If you need to remove the spark plugs, disconnect each spark plug wire, but mark each wire as per cylinder number.

CAUTION: Avoid removing only the coil-to-distributor wire as the ignition coil is still charged and can shock you if it finds ground. 

Step 4: Remove The Spark Plugs 

Use a ratchet with an extension to remove all the spark plugs. 

This is to avoid another compression stroke from slowing the engine spin during the test. 

Note: Both diesel and gasoline engines may require a special spark plug adapter to remove a glow or spark plug.

Step 5: Install The Gauge In The First Spark Plug Hole 

Plug in the engine compression tester into the first spark plug hole. 

Ideally, start with the cylinder closest to you and move backward. Follow the same sequence for the other engine bank as well. 

Step 6: Crank The Engine For Short Durations 

Ask someone to crank the engine while you note the compression readings. 

You’ll need to crank the engine several times over 3 to 5 seconds. Allow the engine to run until you notice at least six puffs on the gauge, then release the key. This will help you get the maximum compression reading on the gauge. 

Step 7: Note Down The Readings 

Note down the compressive load in PSI for each cylinder. 

A good compression reading is between 125-175 PSI for gasoline engines, while for a diesel engine, the standard is 275-400 PSI. 

Step 8: Repeat The Process On Each Cylinder 

Repeat inserting the compression tester inside each spark plug hole, cranking the engine, and recording the compressive load. 

If the pressure reading is low in one cylinder, the issue is with a single cylinder. But if the readings are low on multiple cylinders, it’s a sign of other engine issues. Also, the cylinder pressure readings shouldn’t be more than 10% apart. 

So what issues do these readings indicate? 

What Problems Can I Diagnose With A Compression Test

A lower compressive force inside the cylinders could indicate three things: 

A. Worn Or Damaged Piston Rings

If the pressure reading is low on a single cylinder or two cylinders that aren’t adjacent to each other, it’s usually due to a faulty valve, valve seat, or piston ring. 

You can confirm this by adding a teaspoon of engine oil to the particular cylinder via the spark plug hole (also known as a wet compression test). The oil will create a seal around the piston and cylinder wall. 

Re-run the test to see if the pressure increases. If it does, the piston ring could be the culprit. If it doesn’t, the reason could also be a cylinder head deformation. 

Note: Multi valve or turbo engines could also experience low compression due to a bent or burnt exhaust valve, resulting in a misfire. 

B. Blown Head Gasket

If two adjacent cylinders have low compression, it’s likely due to a blown head gasket. In that case, look for other stress signs, like milky exhaust fumes. 

C. Valve Timing Issue 

If all the cylinder pressure ratings are below 100 PSI (for gasoline) and 275 PSI (for diesel), it could be due to bad valve timing. Check that all valves are opening and closing correctly and that the timing belt is not broken. 

Naturally, when you get a compression test done by a mechanic to identify these issues, there’s a cost involved. 

How Much Does Compression Testing Cost? 

Usually, an engine compression test could cost anywhere between $141 and $178.
This cost is essentially the labor charges that may vary as per your location and your vehicle make and model. 

Based on the test readings, the mechanic may also suggest certain auto repairs that would cost extra. 

Wrapping Up 

A compression test is a surefire way to verify which of your engine cylinders is acting up. But it requires technical knowledge to read the test result and perform needed repairs accordingly. 

For this, you need an expert auto repair service like RepairSmith

RepairSmith offers convenient online booking with upfront pricing and a 12-month 12,000-mile guarantee on all auto repairs. 

Contact us, and our ASE-certified mechanics will drop by to quickly perform a compression test or any other repair and maintenance service right in your driveway!