Wet Compression Test – How To Do It – What Will It Tell You

Wet Compression Test - How To Do It - What Will It Tell You
Wet Compression Test - How To Do It - What Will It Tell You

After you have done a basic compression test, the next step is a wet compression test.

An engine’s cylinders need a good seal between the rings and the cylinder walls. Also, between the valves and valve seats.

On a healthy gasoline engine, compression usually falls between, 125 and 180 psi, depending on your make and model.

So, doing a wet compression test, takes the basic compression test to the next level.

Also, compression testing is the most practical way, to learn about the mechanical condition of your engine. So, if your engine is blowing blue smoke out of the tailpipe; you could have a bad piston ring. This will also cause low compression in that cylinder, and a compression test will confirm that. The same goes for bent or leaking valves.

Even if you are just noticing a general lack of power; a compression test can help you rule out some of the more serious possible causes.

Once you have your results, you can make 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.

So, you have performed a basic compression test and recorded the readings. Next, we will do the wet compression test and record those readings.

How To Do A Wet Compression Test

The test is basically the same as before, except this time; you are going to squirt about one teaspoon of 30-weight motor oil into the spark plug hole.

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

WARNING: Do not use more than a teaspoon of oil or you’ll get a false high compression reading.

Take a compression reading and observe the difference, between the wet and dry test results.

The results shown below, indicate the particular area that needs attention.

A Healthy Engine

  • If all your cylinders are reading close to each other; and there is no big increase in compression (no more than 10 percent),  it’s a good sign of a healthy engine.

Leaking Piston Rings Or Worn Cylinders

  • If the compression increases with the wet test (more than 10 percent); the results identify the problem as the piston rings and/or cylinder walls. The oil added to the cylinder is now providing a wet seal for the rings. So, if they are not sealing on their own; they will, when the oil creates a seal and an increase in compression will be observed.

Leaking Exhaust Or Intake Valves

Burnt Exhaust Valve In Cylinder Head
Burnt Exhaust Valve In Cylinder Head
  • If the compression stays the same, the results point to the valve train. The added oil will have no effect on compression and therefore, the valves are most likely leaking. Low compression in only one cylinder, typically indicates a bad valve. Exhaust valves burn due to hot gases passing through. Intake valves have the advantage of being cooled by the incoming fuel.

Valve Timing Or Camshaft Issues

Slipped Timing Belt
Slipped Timing Belt
  • When all of the cylinders are low and inserting oil into the cylinder does not increase compression; the camshaft timing is likely off. The timing belt can slip on the sprockets, resulting in staggered and low compression results.

Damaged Piston

  • A hole in a piston will result in no compression in that cylinder. Remove the oil cap or (PCV) valve from its grommet. Blow-by caused by this hole can be seen seeping through these openings.

Excessive Carbon Buildup On Piston

  • Carbon build up on the top of a piston will increase compression readings. It can be seen with a probe inserted into the cylinder.

Head Gasket Leaking

Head Gasket Leaking Between Cylinders
Head Gasket Leaking Between Cylinders
  • A faulty or blown head gasket will leak compression, between two adjacent cylinders. When the other cylinders are within specifications; but, two cylinders next to each other on the same bank are low, suspect a faulty head gasket.


Although you don’t need special skill to do a compression test; you need to know how to interpret your results correctly. When good compression exists, you can expect a cylinder to; gradually and evenly raise pressure to within manufacturer specifications.

TIP: You can also do a, A running (dynamic) compression test.

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.

Thank You !