Compression Test – How To Do It – What Can It Tell You

Compression Test
Compression Test

A Compression test is a good way to measure the condition of the rings, cylinders, and valves.

If your engine is running rough or is lacking power; consider doing a compression test.

It is important to perform a compression test every time a tune-up is done, as part of preventative maintenance.

You really don’t need special skills to do a compression test. But, you do need to know how to interpret your results correctly.

By performing a compression test; internal engine malfunctions; such as bad valves; piston rings or excessive carbon buildup; can be detected before they cause irreparable damage.

It benefits the owner to be aware of these problems so they can make an informed decisions about the results. Late model engines are mostly made of aluminum and are prone to spark plug thread damage.

Stripped Spark Plug Threads
Stripped Spark Plug Threads

This is very common when you remove a spark plug on a hot engine.

Before warming up the engine, remove the spark plugs and add a dab of anti-seize compound to the threads. Install and tighten the spark plugs to the torque listed in your vehicle repair manual using a torque wrench. This will make it easier to remove the plugs next time around.

How To Do A Compression Test (Dry)

  1. Start with a warm engine; the ignition and fuel disabled; and all spark plugs removed.
  2. In addition always wear protective clothing and gloves while working around a hot engine. You might also want to invest in some quality steel toe welding boots to protect your feet, especially if your vehicle requires welding.
  3. While doing the test; the throttle and choke plates must be fully open for an accurate test.
  4. Have your helper fully depress the accelerator while he is cranking the engine.
  5. Avoid any flames around the engine during the test.
  6. First of all, connect the tester to a cylinder and turn the engine over 6 to 8 compression strokes.
  7. You will be able to hear the cranking speed slow; as the tested cylinder comes up on it’s compression stroke.
  8. Note how fast the compression increases and write down the highest reading.
  9. Test all cylinders the same way; with the same number of compression strokes.

What Does All This Info Tell You

There is more than one school of thought; on what the compression of the average engine should be. As a general rule a compression of 135 PSI or better is excellent. Similarly, A compression of 85 PSI or lower is extremely bad. The most desirable situation is that all cylinders give the same or close to the same reading.

Furthermore, That reading should be above 135 PSI. Uneven readings are not uncommon on high mileage or worn engines. Also, The difference between the highest and lowest readings should be no more that 20%. As long as the lowest reading is 100 PSI or better; then the engine is acceptable.

Greater differences between cylinders indicate worn or broken rings; leaky or sticky valves or a combination of all.

While observing the gauge during the cranking of the engine; you should have noticed the way each cylinder pumped up. Usually, a cylinder will produce about 40 PSI on the first stroke and about 35 PSI on each additional stroke.

Broken Piston Rings
Broken Piston Rings

Problem cylinders may have trouble pumping up; and may increase by only something like 10 PSI per stroke.

It may be possible to crank these cylinders enough times to come close to the other cylinder’s total PSI. This is why we try to crank all cylinders the same amount. As a result poor rings usually cause this condition. Be aware that a cylinder suffering from excessive oiling; even from bad rings; can yield high compression test results. Other symptoms may give you a clue to the problem (a smoking engine).

There are some variables that affect the readings obtained from compression testing;

  • Cranking speed
  • Altitude
  • Temperature
  • Worn camshaft lobes
  • High performance long duration profile camshafts

How To Do A Compression Test (Wet)

Many automotive books describe a dry and a wet compression test together. Usually these tests must be interpreted together; to isolate the trouble in cylinders or valves. To perform the wet test; a tablespoon of engine oil is poured into the cylinder through the spark plug hole.

So, You squirt the tablespoon of engine oil into a cylinder that is reading low. Crank the engine two revolutions or so to spread the oil; then retest the cylinder. If the compression comes up markedly; 40 PSI or more; the trouble is poor ring to bore sealing.

Adding Oil To Spark Plug Hole For Wet Compression Test
Adding Oil To Spark Plug Hole For Wet Compression Test

If compression doesn’t increase much; about 5 PSI, then the problem is probably with the valves. It could also be pulled head studs or a warped cylinder head.

What Does All This Info Tell You

Normal

  • As a result, compression builds up quickly and evenly to the specified compression for each cylinder.

Piston Rings Leaking

  • Consequently, compression is low on the first stroke. Compression builds up with the following strokes; but does not reach normal. Your compression improves considerably when you add oil.

Valves Leaking

Sticking Valves From Carbon
Sticking Valves From Carbon
Valve Leaking
Valve Leaking

Your compression is low on the first stroke. Compression usually does not build up on the following strokes. Your compression does not improve much when you add oil.

Compression Test Showing Leak Between Cylinders
Compression Test Showing Leak Between Cylinders

If two adjacent cylinders have lower than normal compression and injecting oil into the cylinders does not increase the compression; the cause may be the head gasket leaking between the cylinders.

Conclusion

Testing compression is really nothing more than a way to figure out which cylinder has a problem. If you do find any issues; the next step would be doing a cylinder leak down test. A cylinder leak down test is similar to a compression test; in that it tells you how well your engine’s cylinders are sealing. But instead of measuring pressure; it measures pressure loss.

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