How to Check Engine Compression (Step-by-Step)

Remove the spark plugs, thread a compression gauge into each cylinder, disable the ignition and fuel, crank the engine 4 to 6 times per cylinder, and record the PSI. Most healthy gas engines read 120 to 180 PSI with no more than 10 to 15 percent difference between cylinders.

How to Check Engine Compression

Knowing how to check engine compression is one of the most useful diagnostic skills you can have. It tells you the mechanical health of an engine in about 20 minutes. If compression is low in a cylinder, something is wrong internally. If readings are consistent across all cylinders, the engine is in good shape.

Most gas engines should read between 120 and 180 PSI per cylinder. A variance of more than 10 to 15 percent between cylinders is a problem worth investigating.

Here is exactly how to do it.

What You Will Need

• Compression gauge (available at any auto parts store, usually $20 to $40)
• Spark plug socket and ratchet
• Socket extension
• A helper to crank the engine, or a remote starter switch
• Notepad or phone to record readings

Some gauges come with adapters for different thread sizes. Make sure yours fits your spark plug openings before starting.

Step-by-Step Compression Test

Step 1: Warm up the engine
Run the engine until it reaches normal operating temperature. A warm engine gives more accurate compression readings.

Step 2: Disable ignition and fuel
You want the engine to crank without starting. Pull the main ignition fuse, or disconnect the fuel pump fuse. Check your fuse box diagram for which one applies to your vehicle.

Step 3: Remove all spark plugs
Use your spark plug socket to remove every plug. Label them so you know which cylinder each one came from. Inspect them while you are at it. Oily, wet, or heavily fouled plugs are their own clue.

Step 4: Open the throttle
Have someone hold the gas pedal to the floor, or prop it open. You want full airflow into the engine while cranking for accurate readings.

Step 5: Thread in the compression gauge
Start with cylinder one. Thread the gauge adapter into the spark plug hole by hand. Do not use a wrench. Snug is enough.

Step 6: Crank the engine
Have your helper crank the engine for about 4 to 6 revolutions. Most gauges have a hold needle that stops at the highest reading reached.

Step 7: Record the reading
Write down the PSI for that cylinder. Release the gauge pressure and move on to the next cylinder. Repeat for all of them.

Step 8: Compare results
Once you have all readings, look at the range from lowest to highest. A healthy engine will be consistent across all cylinders.

What the Numbers Mean

A compression test checks the internal health of your engine. It measures how much pressure each cylinder can build and hold. Think of it like taking your car's blood pressure.

• Consistency is the main goal. You want all the cylinders to show roughly the same number.
• Low pressure means a leak. If a number drops too low, air is escaping. This usually points to worn out rings, bad valves, or a blown head gasket.
• Watch the gaps. Even if the numbers look high enough, a big difference between two cylinders means trouble is starting. If one cylinder is much weaker than the rest, the engine will run rough and lose power.

Dry vs Wet Test: What Is the Difference?

If you find low compression in a cylinder, the next step is a wet test. This tells you whether the problem is the piston rings or the valves.

Dry test: The standard compression test described above. This is your baseline.

Wet test: Add about a tablespoon of clean engine oil into the low-compression cylinder through the spark plug hole. Then test that cylinder again.

Here is how to read the result:

• Compression goes up after adding oil: The piston rings are worn. The oil temporarily seals the gap between the rings and cylinder wall, which is what raises the number.
• No change after adding oil: The problem is with the valves or the head gasket. Oil cannot help seal a valve that does not close properly.

This is a simple but very useful step that narrows down the diagnosis significantly.

Low Compression: Rings, Valves, or Head Gasket?

Worn piston rings: Rings wear over time and let combustion pressure escape past them. Signs include blue exhaust smoke, oil consumption between changes, and a wet test result that improves with added oil.

Bad valves: A burnt or bent valve does not seal the combustion chamber properly. You may notice rough idle, misfires, or sluggish throttle response. Adding oil during a wet test will not change the compression reading.

Head gasket failure: If two adjacent cylinders both show low compression, a failed head gasket between those cylinders is the most likely cause. You may also notice coolant in the oil (milky residue on the dipstick), unexplained coolant loss, or white exhaust smoke.

A leak-down test is the next step if you need to pinpoint the exact source. This involves pressurizing each cylinder and listening for where air is escaping.

Why We Compression-Test Every Engine We Sell

At JDM State Motors of NC, every engine gets a compression test before it is listed for sale. It is the most direct way to verify internal engine health, regardless of mileage or appearance.

If an engine does not pass our check, it does not go up for sale. This is part of how we make sure customers are not getting stuck with a unit that fails shortly after install.

If you want the compression numbers for a specific engine in our inventory, contact us and we will pull the data for you.

Frequently Asked Questions

How do you check engine compression?
Remove the spark plugs, thread a compression gauge into each cylinder, disable ignition and fuel, and crank the engine 4 to 6 revolutions per cylinder. Record the PSI for each. A healthy gas engine reads 120 to 180 PSI with consistent results across all cylinders.

What is a good engine compression reading?
For most gas engines, 120 to 180 PSI per cylinder is normal. All cylinders should be within about 10 to 15 percent of each other. A reading significantly lower in one cylinder points to an internal problem worth diagnosing.

What does low compression in one cylinder mean?
It usually means worn piston rings, a bad valve, or a head gasket issue affecting that cylinder. A wet test with engine oil helps narrow it down. If compression rises with oil, the rings are the issue. If it stays the same, look at the valves or head gasket.

What is the difference between a wet and dry compression test?
A dry test gives you the baseline reading. A wet test adds oil to a low-compression cylinder to help identify whether worn rings or bad valves are the cause. Oil helps seal worn rings and raises the reading. It has no effect on valve issues.

Can I check compression myself?
Yes. A compression gauge costs $20 to $40, and the job takes under an hour for most vehicles. You need basic tools and someone to crank the engine. It is one of the more accessible DIY engine diagnostics available.

Browse Our Compression-Tested JDM Engines

Every engine we list passes a compression test before it goes up for sale. Shop our full inventory, browse Honda engines or Toyota engines, or request a fitment quote if you are not sure what you need.