Wet Compression Test – Next Level, Mechanical Engine Testing

Wet Compression Test - Next Level, Mechanical Engine Testing
Wet Compression Test - Next Level, Mechanical Engine Testing

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

Above all, 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 seeing 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 to make, 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. But, 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 (less than 10 percent), it’s a sign of a healthy engine.

Leaking Piston Rings Or Worn Cylinders

  • If the compression increases with the wet test (more than 10 percent). Then, the results identify the problem as, the piston rings and/or cylinder walls. Because, the oil added to the cylinder from a wet compression test, is now providing a wet seal for the rings. So, if they are not sealing on their own, the oil creates a seal and increases the compression.

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. All confirmed by a wet compression test.

Valve Timing Or Camshaft Issues

Slipped Timing Belt
Slipped Timing Belt
  • So, what if all of the cylinders are low and inserting oil into the cylinder, does not increase compression. Then, the most likely cause, is the camshaft timing is 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 adjacent cylinders on the same bank are low. As a result, you should ,suspect a faulty head gasket.

Conclusion To, wet compression test

So, you don’t need special skill, to do a compression test. But, you need to know how to, interpret your results correctly. Finally, when good compression exists, you can expect a cylinder, to gradually and evenly raise pressure.

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. Because, it tells you how well your engine’s cylinders are sealing. But, instead of measuring pressure, it measures pressure loss.

Thank You !