What engine compression tells you
Engine compression is the amount of pressure your engine builds when the piston moves up in the cylinder with both valves closed. A compression test measures this pressure in each cylinder using a gauge screwed into the spark plug hole. The test takes about 15 minutes per cylinder and costs $100 to $200 at a shop, or you can do it yourself if you have access to a compression tester and a helper.
Low compression in one or more cylinders usually means worn piston rings, a cracked piston, a leaking valve, or a head gasket problem. High compression that is uneven across cylinders can point to carbon buildup or a stuck valve. A compression test does not diagnose the exact problem, but it narrows down where to look and whether the engine is worth fixing or has reached the end of its life.
Most mechanics run a compression test when an engine misfires, won't start, runs rough, or loses power. You might also want one before buying a used car, or if you notice white smoke from the exhaust or oil in the coolant.
Key Takeaways
- A compression test measures the pressure each cylinder builds when the engine cranks, using a gauge threaded into the spark plug hole.
- Normal compression varies by engine, but all cylinders should read within 10 to 15 percent of each other; a shop manual for your vehicle lists the exact range.
- You need a compression tester (around $30 to $60 to buy), a socket wrench, and someone to turn the key while you watch the gauge.
- Low compression in one cylinder usually means a valve leak or worn piston rings; low compression in all cylinders usually means a head gasket leak.
- A wet compression test (adding oil to the cylinder) helps you tell whether the problem is the rings or the valves.
Tools and preparation you need
You will need a compression tester, which screws into the spark plug hole and has a gauge dial or digital readout. Most auto parts stores rent them for $20 to $30 or sell them for $30 to $60. Buy or rent one with a rubber hose and a threaded adapter that fits your engine's spark plug size (usually 10mm, 12mm, or 14mm).
You also need a socket wrench to remove the spark plugs, a helper to crank the engine while you read the gauge, and a safe, level place to work. Disconnect the negative battery terminal before you start. Remove all the spark plugs so the engine cranks freely without firing. If your engine has a distributor cap, remove it to prevent the engine from starting accidentally.
Let the engine cool to room temperature. A warm engine will give false readings. Make sure the fuel pump is off — on older cars, pull the fuel pump relay; on newer cars, turn the key to the "on" position without starting, then turn it off again to reset the system.
The step-by-step test procedure
Screw the compression tester into the first spark plug hole until it seats firmly. Have your helper turn the ignition key to "start" and hold it there for about five seconds while you watch the gauge. The needle will climb as the engine cranks. Write down the highest number it reaches, then release the key.
Unscrew the tester and move to the next spark plug hole. Repeat the test on every cylinder. Each one should reach roughly the same pressure — usually between 120 and 180 psi depending on the engine, but check your vehicle's service manual for the exact range. All cylinders should be within 10 to 15 percent of the highest reading.
If one cylinder reads much lower than the others, or if all cylinders read low, move on to the wet compression test before you draw any conclusions.
The wet compression test and what it reveals
A wet compression test adds a tablespoon of engine oil to a low-reading cylinder, then runs the test again. If the pressure jumps up noticeably, the problem is worn piston rings or a worn cylinder wall — the oil seals the gap temporarily. If the pressure stays low, the problem is a leaking intake valve, exhaust valve, or head gasket.
To run the wet test, pour the oil slowly into the spark plug hole of the cylinder that read low. Screw the tester back in and crank the engine again for five seconds. Compare the new reading to the dry test. A rise of 20 psi or more points to ring wear; little or no change points to a valve or gasket leak.
If all cylinders read low on the dry test and stay low on the wet test, the head gasket is likely leaking between cylinders or into the coolant passages. If all cylinders read low but improve with oil, the rings are worn across the whole engine.
Reading the results and what they mean
Compare your readings to the manufacturer's specification in the service manual for your vehicle. Most engines should have compression between 120 and 180 psi, but some high-compression engines run 160 to 200 psi, and some older or smaller engines run 90 to 140 psi. The exact number matters less than whether all cylinders match.
If all cylinders read within 10 to 15 percent of each other and match the manual's range, your compression is normal. If one cylinder reads 20 to 30 psi lower than the others, that cylinder has a problem — either a valve leak or ring wear. If one cylinder reads much lower and the wet test shows no improvement, suspect a valve or head gasket. If the wet test shows a big improvement, suspect rings.
If all cylinders read low by the same amount, the problem is usually a head gasket leak, worn rings across the whole engine, or a timing issue. If compression is higher than the manual specifies and uneven across cylinders, carbon buildup or a stuck valve may be the cause.
When to do a compression test versus other diagnostics
A compression test is most useful when the engine misfires, won't start, or runs rough, because it quickly tells you whether the problem is in the ignition, fuel, or engine mechanical condition. If the engine starts and runs normally, a compression test is less urgent unless you are buying a used car or the engine has high mileage.
A compression test does not replace a scan tool that reads diagnostic trouble codes, which can point to a specific sensor or system problem. If your check engine light is on, read the code first — it may save you time. A compression test also does not replace a leak-down test, which pressurizes the cylinder with compressed air and listens for where air escapes; a leak-down test is more precise but costs more and takes longer.
If you suspect a head gasket leak, a compression test is a good first step, but a coolant system pressure test or a chemical test for exhaust gases in the coolant may give you a clearer answer.
Frequently Asked Questions
Can I do a compression test on a running engine?
No. The engine must be off and cool. Crank it with the starter motor while you read the gauge. If the engine starts and runs, you cannot get an accurate reading because the combustion pressure will spike the gauge and may damage it.
What if the compression is low in just one cylinder?
Run the wet compression test on that cylinder. If pressure jumps up with oil, the rings are worn in that cylinder only. If pressure stays low, that cylinder has a leaking valve or a head gasket leak between that cylinder and another one or the coolant jacket.
Does high compression mean the engine is in good shape?
Not necessarily. High compression that is even across all cylinders is normal and usually a sign of a well-maintained engine. High compression that is uneven — one or two cylinders much higher than the others — can mean carbon buildup or a stuck valve, which will cause misfires and rough running.
How often should I check compression?
You do not need to check compression on a regular schedule. Check it when the engine has a specific problem like misfiring or rough running, when you are buying a used car and want to know the engine's condition, or if the engine has very high mileage and you want to know how much life is left.
What if my compression readings are all over the place?
Uneven readings across cylinders usually mean different problems in different cylinders — one valve leak here, ring wear there. Write down each reading and run the wet test on the lowest ones. This tells you whether you are dealing with multiple valve problems, multiple ring problems, or a mix of both.