How to do Compression Test on a Gasoline Engine

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When your car starts to misfire, has a rough idle, or does not have power, you know there is a problem and the engine might be the culprit. One of the tests you could do to ascertain that the problem is in the motor is an engine compression test.

This test is done to test the mechanical health of the engine. More specifically, it helps check if there a combustion engine is generating enough compression in the cylinders.

How compression takes place

See, combustion engines have cylinders or sealed chambers where compression and combustion take place. Each cylinder has a piston assembly for compressing fuel/air mix, and intake and exhaust valves for controlling airflow.

The valves provide a proper seal to the cylinders to enable the engine to build compression. At the same time, the valves open and close systematically to allow the inflow of fresh air and outflow of burnt gases to keep the engine running.

The piston rings also provide a seal between the cylinder wall and the piston. The rings must seal properly to prevent compressed gases from escaping into the crankcase, resulting in compression loss.

What does engine compression test tell you?

When one of the parts mentioned above starts to fail, you will experience compression loss in the engine. Subsequently, your car will start to misfire and inevitably perform poorly.

However, these low compression symptoms will not always signal a problem in the heart of the engine. Sometimes an air leak outside the engine block, bad air flow, or low fuel pressure could manifest the same symptoms. That’s why you need to do a cylinder compression test with an engine compression tester to find out if the low compression is happening in the cylinder.

If there is a problem in any of the cylinders, the compression test will reveal it. Some of the problems the test can help to diagnose include broken or leaking engine valves, worn, badly seated, or leaking piston rings, and a defective head gasket.

The compression test also tells you whether the piston rings are sealing properly to prevent combustion gases from escaping through the cylinder wall.

In the following section, I will teach you how to use a compression tester step by step to check engine compression of any petrol engine.

How to use a Compression Gauge to do Compression Testing

This is the procedure for performing a wet and dry compression test on a car motor using a Compression Tester Kit. In this procedure, I am using Innova compression tester.

The Innova kit is an inexpensive and very effective compression test tool designed for spark-ignited gasoline engines that do not exceed 300psi compression pressure. Check out my detailed review of the best compression testers for small engines and motor vehicles

Tools you need

  • Compression tester kit – (I am using Innova Equus compression gauge)
  • Spark plug remover
  • Torque Wrench
  • Ratchet wrench with socket extension
  • Engine oil squirter
  • Pen and paper

Pre-test conditions

Before you start the compression tests, make sure the vehicle battery is fully charged and the starter is in good working condition. This will enable you to crank the engine multiple times without fail. Also, warm up the engine to reach the normal working temperature.

Procedure

1. Put on the safety gear

When working on engines, safety is paramount. Always wear safety eye protection and a pair of safety gloves at least to protect your hands from oil and other engine fluids. Also ensure that you put the transmission of your car in parking (for automatic car) or neutral (for manual car). This will prevent the car from moving when you crank the engine.

2. Remove the spark plugs and ignition wires

Label the spark plug wires and remove them from the engine. Follow up by removing the spark plugs from their respective holes using a ratchet. If your spark plugs are deep into the cylinder, attach the appropriate socket extension to the ratchet to reach them. Be sure to label the spark plugs as well so that you can return them to the right cylinders once you complete the engine pressure test.

remove spark plug

3. Disconnect the ignition system  

Disconnect the wiring harness at the ignition coil. This prevents the ignition system of the motor from generating sparks that do not have anywhere to go. These high-voltage sparks from the coil pack are a dangerous safety risk.

4. Disable the fuel system

Disconnect the fuel system relay or fuse to prevent injectors from spraying fuel into the combustion chamber. Consult the car repair manual to locate the fuel relay or fuse. Once disconnected, crank the engine a couple of times to remove fuel from the fuel lines. Alternatively, disconnect the fuel injectors from wiring harnesses if they are easily accessible.

5. Attach the compression test tool to one cylinder

In this test, I am using the Innova compression tester kit. It is similar to many other compression check tools for gasoline engines.

To get started, attach the spark plug hose to either the 14mm adapter or 12mm adapter. Most gasoline car engines have a 14mm bore size. Then attach the spark plug hose assembly to the spark plug hole by twisting it to thread it in.

Make sure to tighten the hose by hand. Do not use any tool

connecting compression gauge to spark plug hose

Connect the quick attach compression guage to the hose assembly. To do this, pull back the spring-loaded sleeve on the compression tester gauge, insert the end of the hose then pull forward the sleeve until the coupling snaps. Perform a pull test to ensure that the hose is locked into the gauge.

6. Perform a dry compression test  

After fitting the compression test tool, now start the car to begin running compression test. Start by stepping the gas pedal all the way down to the floor to open the throttle plate and crank the engine 5 times.

Have someone film the compression guage while you crank so that you can review cylinder compression behavior on each stroke. Alternatively, have an assistant do the cranking while you observe the gauge. Ideally, the compression should increase quickly and uniformly during each compression stroke until a peak is reached.

Once the compression peak is reached, stop cranking the engine and release the gas pedal. Record the compression test result and wait for about 10 seconds to read the gauge again.

7. Disconnect the motor compression tester

After recording the compression values from the compression gauge, press the side release valve to release compressed air. Pull back the sleeve on the pressure gauge to disconnect spark plug hose from the compression gauge. Then unscrew the hose assembly from the spark plug hole.

running compression test

8. Perform a wet compression test

After removing the spark plug hose from the spark plug hole, squirt about 0.5 ounches of engine oil into the cylinder with the hand pump oil can and reinstall the compression tester kit.

Crank the engine five times and record the wet compression reading for that cylinder next to the dry compression test result.

Now repeat step 7 to safely remove the engine compression gauge kit and proceed to the next cylinder.

9. Test other cylinders

Repeat steps 5 to 8 to perform wet and dry compression tests on other cylinders. Make sure you record side by side both the dry compression test results and wet compression test results.

10. Complete the compression test

Disconnect the negative terminal of the battery first then restore the fuel system by re-installing the fuel relay fuse or reconnecting the fuel system relay. If you had disconnected the fuel injectors, reconnect their wiring harnesses.

Restore the ignition system by connecting back the wiring harness of the ignition coil. Then re-install the spark plug wires.

11. Tune and reinstall the spark plugs

Did you know that a dirty or worn-out spark plug exhibits the same symptoms of lack of power and misfiring? Now you know.

So make sure to inspect the spark plugs and tune them where necessary before reinstalling them.

To tune the spark plugs, start by cleaning them with fine-grit sandpaper and re-gap them to spec with a feeler gauge or wire gauge. I have covered the process of regapping a spark plug using a feeler gauge in this video.

Find out some of the best feeler gauges for gapping spark plugs in this article.

how to regap a spark plug

Now use a ratchet, a socket extension, and a deep socket to screw in the tuned spark plugs into the spark plug hole. Use a torque wrench to tighten plugs to spec as per the car repair manual.

Overtightening or under-tightening a spark plug may affect significantly the engine performance.

Finally, reinstall the spark plug wire boots, reconnect the negative battery terminal and start the engine.

Now it is time to analyze the compression test results.

Video: How to do a Compression Test with Innova Compression Tester Kit

How to test gasoline engine compression

Engine Compression Test Results Analysis

After completing both the dry and wet compression tests, it is time to analyze and interpret the results to find out what they mean. This analysis of compression results is the most important part of compression testing because it helps you understand the problem in your car engine.

Let’s try to make sense of the compression values you recorded.

Good Compression Results

Cylinder compression is considered normal if the lowest compression reading is at least 75% of the highest reading. For instance, if your highest reading on the compression gauge was 150 psi, the acceptable lowest compression reading would be 112.5 psi.

In addition, compression in a healthy engine should build up progressively in a uniform way on each compression stroke until a peak reading is reached. Otherwise, check the next section to find out the possible problem with your petrol engine.

If the compression is ok but the car still performs poorly, you might want to examine the fuel system. I recommend performing a fuel pressure test [Video] to find out if the fuel pump is working well.

Bad Compression Test Results

Low cylinder compression is mostly the problem with many engines that have compression problems. It refers to the inability of an engine to build maximum compression as specified in the engine repair manual or not build at all.

If interpreted correctly, low compression engine symptoms can point to any of these three engine problems:

1. Leaking or worn piston rings

If you observe very low dry compression readings on the first stroke that builds up on subsequent strokes but does not reach normal value, you might be having bad piston rings.

Worn or leaking piston rings do not seal properly. As a result, they allow compressed air to escape through cylinder wall as the piston moves back and forth. This results in very low compression readings and obviously reduces power.

To further probe the worn piston rings problem, check the wet compression test results. If the compression values are higher than those of the dry test for that particular cylinder, then it is would doubt a piston ring issue. This problem can only be fixed by replacing the worn piston rings with new ones.

2. Burnt, bent or leaking valves

If your car engine builds compression on the first stroke but does not change with subsequent compression strokes, your valves could be defective.

Intake and exhaust valves help to control the entry and exit of air into and out of the cylinder. If the valves do not open or close appropriately, the engine cannot build pressure in the subsequent compression strokes.

Wet compression test results can help to further substantiate a valve problem. If the valves are defective, the compression results will not change even with a wet compression test.

The ultimate fix for a leaking engine valve is to replace it. You can further investigate a valve problem by visually inspecting the cylinder with a borescope. The snake camera allows you to view the inside of a combustion chamber to see how everything looks including the piston, oil rings, and valves.

3. Blown head gasket

A blown head gasket is one of the most feared disasters among petrolheads. This is because of how expensive it can be to fix since the engine has to be ripped apart to replace the gaskets.

Normally, some of the telltale signs of this problem are white smoke from the exhaust pipe, milky engine oil, or oil-contaminated coolant. But the most definitive test is a compression test.

An engine with a blown head gasket will record very low compression results on two adjacent cylinders. The compression values may be 20 pounds (or more) lower than other cylinders. As mentioned, the fix to this problem is replacing the head gaskets with new ones.

Conclusion

There you have it. That is how you do a compression test and analyze the results to diagnose compression-related problems in your gasoline engine.

But always remember to check for air and fuel leaks and clogs first before doing the compression test. These car problems have similar symptoms. A failing spark plug could also show the same signs of lack of power and misfiring. So make sure to check them as well.