How to Do an Engine Compression Test

A compression gauge test can reveal the condition of valves, head gasket, pistons, cylinders, rings and more.
A compression gauge test can reveal the condition of valves, head gasket, pistons, cylinders, rings and more. | Source

An engine compression test will tell you many things about the mechanical condition of your engine. The test can detect, for example, pressure leaks through rings, cylinders, valves, or a head gasket without taking apart the engine. So you'll save a lot of time and money in diagnostics.

But, why would you want to do a compression test?

  • Perhaps you want to know whether it's time to replace your car.
  • You've noticed popping noises coming from the intake or exhaust systems, too much blue smoke coming out of the tail pipe, or a rough idle you've been unable to diagnose.
  • or because you want to know the mechanical condition of a used car you want to buy.

You can compression test the cylinders with the help of a compression gauge. All you have to do is connect the gauge in place of a spark plug and test each cylinder pressure in turn. Once you have the results, you compare your gauge reading to manufacturer specifications and interpret your results.

Usually, the manual for your particular vehicle make and model will have the correct compression for your engine. You can buy an inexpensive, aftermarket manual online or through one of your local auto parts stores.

Although you don't need special skill to do a compression test, you need to know how to interpret your results correctly.

Here's how.

Table of Content
I. Preparing for The Compression Test
II. Engine Compression Test
III. Interpreting Your Results
IV. Wet Compression Test
A compression gauge test can help you detect a blown head gasket.
A compression gauge test can help you detect a blown head gasket. | Source

I. Preparing for The Compression Test

WARNING: You'll need to remove the spark plugs while the engine is warmed for this test. If a spark plug seems too hard to remove, DON'T FORCE IT LOOSE. This may deform the mounting plug threads on an aluminum cylinder head. To avoid this, before warming up the engine, remove one spark plug at a time and add a dab of anti-seize compound to the spark plug threads and tighten the plug to the torque listed in your vehicle repair manual using a torque wrench. This will make it easier to remove the plugs next time.

1. You should perform this test with a fully charged battery. If battery power decreases as you test the last two cylinders, for example, you'll get a false low compression reading on those cylinders.

2. Make sure you have the correct compression gauge for your engine. Diesel engines need a gauge with a higher compression tolerance than that of a gasoline engine gauge.

3. First, warm up the engine. You can take your car for a 20-minute drive on the highway, or open the garage door and let the engine idle. This will allow the valves to sit tight and the pistons to expand and seal properly.

4. Turn off the engine and open the hood.

5. Block the throttle open using a screwdriver to make sure the engine has enough incoming air during your test. Depending on the type of fuel system, you'll need to remove the air filter box from the top of the carburetor, or disconnect the air cleaner duct from the throttle body.

6. Remove the fuel pump fuse or fuel-injection fuse (on electronic fuel injection systems) to keep fuel from going into the combustion chambers. To locate the fuse, consult your car owner's manual or repair service manual. You may find the fuse in the "power center" or "fuse block" under the hood, or in the fuse box or side kick panel under the dashboard.

On a diesel engine, disconnect the fuel shutoff solenoid. This will disable the injection pump.

7. Start the engine to use up the remaining fuel in the system lines. Wait until the engine stalls.

8. Disconnect the spark plug wire closest to the front of the engine. Grab the wire by the boot, and gently twist it back and forth as you pull it off the spark plug. If you can't reach the boot, use a set of spark-plug wire pliers. Then, label the wire with a number to identify its place on the engine.

9. Repeat step 8 on the remaining spark plugs wires.

. . . if a spark plug seems too hard to remove, DON'T FORCE IT LOOSE. This may deform the mounting plug threads on an aluminum cylinder head.

10. Clean the well around each spark plug to prevent dirt and debris from falling into the cylinders when removing the spark plugs. Dirt and debris inside a cylinder will scratch or score the wall and cause oil and compression leaks. To clean the spark plug well, you can use compress air or a bicycle pump and a soft brush.

11. Now remove the spark plugs using a ratchet, ratchet extension and spark plug socket. Removing all the plugs will allow the engine to turn easily during your tests. Keep track of the place of each spark plug on the engine so that you reinstall the plugs in their corresponding cylinder. This won't affect your results but it's nice to keep things in order in case you want to 'read' the spark plugs to check combustion quality in a particular cylinder later on. Your repair manual tells you how to read the plugs.

On a diesel engine, you'll need to disconnect either the injectors or the glow plugs to connect the compression gauge into, depending on your particular engine. Consult your vehicle service manual, if necessary. Also, you'll need to use a heat shield for the gauge, if you need to connect to an injector port.

12. Disable the ignition system by unplugging the spark wire from the coil or disconnecting the electrical connector on the coil pack or the feed wire, depending on the type of coil your engine uses. Consult your vehicle service manual for this, if necessary.

13. Install the compression gauge on the first cylinder in place of the spark plug by hand, and tighten the gauge connector snugly to seal the cylinder so that compression won't leak. Depending on your particular vehicle model, you may need to use an adapter to connect the gauge to the spark plug hole. Choose one with the same thread size as your spark plugs. Some compression gauges come with a set of adapters for this purpose.

NOTE: Some gauges only have a rubber nipple at the tip instead of a threaded connector. You push and hold this nipple manually against the spark plug opening when conducting the test.

Diagnose hard to find internal engine problems using a compression gauge.
Diagnose hard to find internal engine problems using a compression gauge. | Source

II. Engine Compression Test

Once you install the compression gauge, you're ready to start your test.

1. Ask a helper to crank the engine (or you can use a remote starter switch; it's an inexpensive device and simple to use. Just follow the instructions on the product's package to connect it). You'll see the pressure build up on the gauge and reach its maximum point after about 4 to 5 compression strokes. Whether you use 4 to 6 strokes to test the first cylinder's compression, use the same number of strokes for the rest of the cylinders.

2. Note how far on the scale the needle on the gauge moves.

3. On a notepad, record the compression reading on the last stroke (amount of compression indicated by the needle).

4. Then, release the pressure on the gauge through the release valve, which also resets the gauge back to zero; disconnect the gauge and connect it on the next cylinder.

5. Repeat steps 1-4 above for each of the remaining cylinders.

6. Compare your results to the minimum and maximum compression manufacturer specifications for your particular engine. You'll find this information on your vehicle service manual.

Watch the next video so that you have a visual reference about the test, if necessary.

III. Interpreting Your Results

When good compression exists, you can expect a cylinder to gradually and evenly raise pressure to within manufacturer specifications during a test.

  • On a healthy gasoline engine, compression usually falls between 125 and 175 psi (pounds per square inch), depending on your particular vehicle make and model. And the difference between the cylinder with the highest and the one with the lowest compression should remain between 15 to 20 psi, according to James E. Duffy in Modern Automotive Technology.
  • On a healthy diesel engine, good compression usually falls between 275 to 400 psi, with a variation of no more than 10 percent between any two cylinders.
  • A compression above manufacturer specifications usually points to carbon build up in the cylinder.
  • If your tests indicate low compression on one or more cylinders, go to the next section Wet Compression Test.
  • With a compression variation beyond 10 to 15 percent between two or more cylinders, your results may indicate worn out components like valves, rings, cylinder, a burned or broken valve or springs or even a blown head gasket—go to the next section Wet Compression Test.
  • Low compression between two adjacent cylinders usually points to a leaking head gasket between those two cylinders. Perform a Wet Compression Test. Go to the next section.
  • Low compression on all cylinders usually indicates a worn out or jumped timing chain or belt; otherwise, cylinders or rings can't hold pressure anymore and the engine needs an overhaul. Go to the next section.

Once you zero in on a potential problem or problems, perform a more specific test to confirm your results. For example you may want to perform a leakage test, if your compression results indicate a leaking head gasket.

NOTE: After your compression test, the Check Engine light might come on. Use a scan tool to reset the light. If you don't have a scanner, you may want to take your vehicle to a local auto parts store to have it reset, or take it to your local dealer. Consult your vehicle repair manual, if necessary.

Common Causes of Low Engine Compression
Cracked block or cylinder head
Valve train problems
Valve or valve seat problems
Worn out rings or cylinder
Blown head gasket

IV. Wet Compression Test

If one or more cylinders registered a compression below the manufacturer specification, perform a wet cylinder compression test on those cylinders to narrow down the potential cause of the problem.

The test is basically the same as before, except this time, you are going to add a tablespoon (about three squirts) of clean, W30 motor oil into the cylinder you want to test. Use an oil squirt can to add the oil to the cylinder through the spark plug hole.

With this test, one of two things will happen:

  • Cylinder compression won't change at all, indicating a compression leak through a head gasket or a valve—or less common, through a cracked cylinder head or block.
  • Cylinder compression will rise, indicating a compression leak through a worn out cylinder or piston ring.

WARNING: Do not use more than a tablespoon of oil on a gasoline engine or you'll get a false high compression reading on the cylinder, even with a bad head gasket or valve. And pouring too much oil in a diesel engine's cylinder will lock the engine or brake an internal component because of the extra high cylinder compression. Consult the service manual for your specific vehicle make and model, if you need more information on this.

WARNING . . . pouring too much oil in a diesel engine's cylinder will lock the engine or brake an internal component . . .

An engine compression test is the most practical way to learn about the mechanical condition of your engine without the need to disassemble it. The test will help you locate potential internal problems affecting engine performance. Once you have your results, you can take an informed decision about how to proceed, for example, what specific components to further troubleshoot to verify your findings and what engine repairs, if any, are necessary.

Test Your Knowledge of Compression Leak Problems

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