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How to Run Dry and Wet Compression Tests

Posted by Scott Goldfarb on

If your diesel engine has not been providing adequate power lately, you may want to consider running a compression test. A compression test will help you determine the condition of your valves, cylinders, and rings, and it can be completed by following some simple steps.

Tips Before You Test

Before beginning to run a compression test on your engine, you will want to take note of these tips:

  • To run the test you will need to remove the spark plugs. Spark plugs are commonly made of aluminum. Because of this, they are susceptible to thread damage if they are removed from a hot engine. To eliminate the potential for damage, remove the spark plugs before you warm the engine for the test. Be sure to remove all the spark plugs.
  • Gather the necessary supplies:
    • A ratchet wrench and extensions
    • A spark plug socket
    • Motor oil
    • A spark plug wire puller
    • A compression tester
  • Protect yourself. Wear protective clothing, gloves, and facial gear when working around a hot engine.
  • The throttle and choke plates should remain fully open during the test for accurate results.
  • Do not perform a compression test around any flames.

How to Run a Dry Compression Test

Once you have prepared your engine and yourself, you are ready to run the compression test by following these six steps.

  1. Warm the engine and disable the ignition and fuel.
  2. Fully depress the accelerator while cranking the engine.
  3. Connect the tester to a cylinder and turn the engine over six to eight compression strokes.
  4. When the tested cylinder comes up on its compression stroke, you will be able to watch the cranking speed start to slow down.
  5. Take note of how fast the compression increases, and write down the highest reading.
  6. Run the test on each of the other cylinders, with the same number of compression strokes, noting the highest reading for each.

Interpreting the Dry Compression Test Results

A compression of 135 PSI or higher is the goal. Although 135 PSI or higher is the goal, engines with readings over 100 PSI are acceptable. Engines with readings below 85 PSI are in need of work.

You also want each of the cylinders to give similar readings. Engines with more wear may have cylinders with differing readings. Ideally, the cylinders will be within 20% of each other's PSI readings. The greater the difference between the cylinders' readings, the greater the chance of leaky valves, broken rings, sticky valves, or a combination of these problems.

Another thing you can watch for is the PSI it takes for each cylinder to pump up. A typical cylinder will produce around 40 PSI on the first stroke, with each additional stroke around 35 PSI. If a cylinder has trouble pumping up and increases by less than that, such as 10 PSI per stroke, the cylinder needs attention.

How to Run a Wet Compression Test

Knowing whether or not the PSI readings are in range for the cylinders is helpful, but by adding one more step, you can narrow down the cause of the trouble if your readings are low. This step is called a wet compression test.

To perform a wet compression test on a low-reading cylinder, pour one tablespoon of engine oil through the spark plug hole and into the cylinder. After cranking the engine two revolutions to spread the oil, retest the cylinder following the steps above.

Interpreting the Wet Compression Test Results

If, when running the wet compression test, the compression does not increase much (only 5 PSI or so), the problem can be narrowed down to the valves, pulled head studs, or a warped cylinder head.

If the compression increases by 40 PSI or more, the problem is a poor ring to bore sealing. The piston rings are most likely leaking.

If two adjacent cylinders have low compression readings and they do not increase after running the wet compression test, the most likely culprit is leaking between the cylinders from the head gasket.

What Can Affect the Results?

Excessive oiling can create a false high compression reading. Other factors that can influence the reading are:

  • Altitude
  • Temperature
  • Cranking speed
  • Worn camshafts

Now What?

Compression tests can be a helpful tool in narrowing down the cause of your engine's poor performance so you can fix it and get it back to working order. If you are capable of fixing the problem yourself, you have narrowed down the places to look. If you need a mechanic to take over from here, you can let them know your results to guide them in the right direction.

After you have completed the tests, be sure to add an anti-seize compound to the threads of the spark plugs when you replace them. Use a torque wrench to get the spark plug to the torque listed in your vehicle repair manual.

As a large diesel parts supplier, we have everything from new fuel injectors to delivery valves for sale. Check our inventory for all your diesel engine needs.


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