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How to Do Compression Test and Read the Results

If you notice that your engine is running rough or your vehicle is lacking power when you are driving it, then you probably need to do a compression test.

Running a compression test will not require you to have any special skill. But you will still need to know how to read and interpret the results from the test.

Performing a compression test will be able to tell you if you have any malfunctions in the internal engine. The malfunctions can include damaged valves, broken piston rings, or excessive carbon buildup. You will need to check if those things occur before they can cause worse damages.

The owners who realize about this problem will be able to take repairing decisions from the results. Aside from that, spark plug thread damages can also arise if your engines are of late models, which are mostly made from aluminum.

Dry Compression Test

How To Do Compression Test (Dry)

  1. With the engine warmed up, disable your fuel and ignition. Proceed by removing all spark plugs.
    Please note that before you warm up your engine and remove the spark plugs, you need to apply a miniscule amount of anti-seize compound on the threads. Continue to tighten the spark plugs using a torque wrench according to the specification of your vehicle repair manual. In doing this, you will make it easier for you to remove the plugs when you need to do it again in the future.
  2. Make sure that you wear protective gear the whole time you’re working with your engine. Using some high-quality steel toe boots can also protect your feet, especially from the heat of the engines.
  3. To get an accurate result from the compression test, make sure that the choke plates are fully open when you are doing the throttle test.
  4. Get a helper to do the job. Ask them to press the accelerator while they are cranking the engine.
  5. Do the test conservatively so you can avoid any flames that might occur during the process.
  6. Get a tester and connect it to the cylinder. Proceed by turning over the engine to six or eight compression strokes.
  7. Pay attention to the noise that the engine creates. You might be able to hear cranking at low speed. This is the sign of the cylinder coming up to its compression stroke.
  8. Read the result on the compression tester and write down the highest number.
  9. Do the test on all of your cylinders and make sure they have the same number on the compression strokes.

What Does All This Info Tell You?

There are many opinions regarding what the average number of compression strokes should be. But the general rule is a compression of 135 PSI is always better. A lower number such as 85 PSI is considered a bad rating. But the condition that you want is the same reading number for all the cylinders.

When you read the result, make sure that the number is at least 135 PSI. You might also get uneven readings on engines that are old or worn. If the numbers are different, make sure the difference is not more than 20%. But you don’t need to worry about your engine condition as long as the reading is above 100 PSI.

You also need to observe the gauge response when the engine is being cranked. A normal cylinder will produce about 40 PSI on the first stroke. Meanwhile, for each additional stroke, the number should be about 35 PSI.

If you get uneven numbers from the reading, you can also try to crank the cylinders a few more times. Usually, those numbers can occur from the poor ring condition. The reason might be excessive oil deposits around the rings. If this condition is true, then you can also notice another common symptom which is an occurrence of smoke when you crank the engine.

Some other variables that can affect the readings of the compression test include:
● High-performance long-duration profile camshafts.
● Cranking speed.
● Worn camshaft lobes.
● Altitude.
● Temperature.

How To Do Compression Test (Wet)

Before running the compression test, squirt the cylinder with small amount of engine oil. At the first test, you will get a low reading number. Proceed by cranking the engine two times to spread the oil, and then repeat the compression test. If you get a reading of about 40 PSI or above, then the engine problem is probably caused by poor rings on the bore sealing.

Wet Compression Test

If the problem is the valves, then you will notice that the compression test number will not increase that much. Another reason could be from pulled head studs or the condition of a cylinder head that is warped.

What Does All This Info Tell You?

If the compression test rating is normal, then it will also mean that the compression engine can build up quickly and evenly, depending on the specification of each cylinder.

Piston rings leaking
If you notice that the compression number is low on the very first stroke, then there might be leaks on the piston rings. The number might also build up on additional strokes, but it will not reach a normal number. You can probably improve the reading by adding oil.

Valves leaking
If this happens, then the reading of the compression test will usually not build up even on many tries of compression strokes. Adding oil will also not improve the compression test numbers that much.

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