Interpreting The Engine Compression Test Results

Up to a certain point, it's normal for the compression of each cylinder to vary a bit.

But if the values vary drastically, then you're gonna' have a rough idle condition or a misfire on your hands.

Finding out if the low compression values are within normal operating parameters isn't hard to do. In this section I'll explain the math behind it.

To find out you'll need to multiply the highest compression value by 15% (0.15). Then you'll subtract the number you got from the highest compression value you got. The end result of this is the lowest compression your 2.4L Honda Accord (or Honda Element) can before it starts to misfire.

To explain this a bit better, let's say that I got the following compression readings from my compression test:

  1. Cylinder #1 175 PSI.
  2. Cylinder #2 160 PSI.
  3. Cylinder #3 165 PSI.
  4. Cylinder #4 125 PSI.

My next step would be to:

  1. Multiply 175 by 0.15 (175 x 0.15). This would give me = 26.
  2. Then: 175-26= 149.
  3. So then, 149 PSI is the lowest possible compression reading that any one of the rest of the engine cylinders can have. Any compression reading below this and that engine cylinder will misfire.
  4. This means that cylinder #4 is the one causing the misfire

To further pinpoint the problem, the next step is to do a ‘wet’ compression test on the dead or low compression cylinder.

The wet compression test will let us (you mainly) if the problem is due to worn out cylinder head valves or worn out piston rings.

TEST 2: Wet Compression Test

A ‘Wet’ compression test will help you to find out if the low cylinder pressure or pressures you recorded in the ‘Dry’ compression test are caused by worn piston rings or worn cylinder head valves.

It will also help you decide what you need to replace and how much it's gonna' cost you. To be a bit more specific, this test will tell you if you only need to have the cylinder head worked on (not recommended) or if you only need to have the block worked on. Both of these options are not the ideal since just repairing one and not the other is not a good idea. Why?

Because by repairing only one, but not the other, you're only delaying the inevitable by about six months, which is: you're gonna' have to replace or repair the entire engine anyway.

OK getting back to the test at hand, what you're gonna' do is add about 2 tablespoons of oil to the cylinder (that recorded no or low compression in the previous test) and then check its compression once again.

What will happen is that if the low compression values is due to worn piston rings, the compression value will go up from the previous one you got doing the Dry compression value.

If the compression value does not go up (from the previous one), then you'll know that the problem lies in the cylinder head valves.

OK, to get this pot of water boiling, this is what you need to do:

  1. Add a small amount of engine oil to the cylinder that reported low compression or no compression in the ‘Dry’ compression test
    1. The amount should be about 1 to 2 tablespoons of oil.
  2. Install the compression tester onto the cylinder.
    1. Do not use any type of tool to tightened the compression tester, hand tight is fine.
  3. When all is set up, have your helper crank the engine.
  4. You'll get one of two results, either the compression value will go up (from the one you recorded before) or it will stay the same.

CASE 1: The compression value shot up.. This tells you that the piston compression rings are worn out and thus the problem is in the bottom end.

The reason why the compression value shot up, with the engine oil added, is because the oil is aiding the piston rings to create a near perfect seal.

Since the oil is not letting the compression escape by the rings, this results in your compression gauge reading a higher value than with the ‘dry’ compression test.


CASE 2: The compression value stayed the same.. This confirms that the problem is in the cylinder head valves.

The reason why the compression reading did not shoot up, is because the engine oil can not help the intake or exhaust valve seal the compressed air therefore, the compression value will stay the same.

Engine Compression Test Summary

A compression test may seem like a tremendous pain in the neck to do but it really isn't. Not only that, I can guarantee you that it will either save you time and money by letting you know it's the engine that's bad (and thus keep you from spending money on: new fuel injectors, new COP ignition coils, new spark plugs, etc.) OR...

... Tell you that you need to keep troubleshooting the problem that's causing the rough idle, engine miss, misfire, etc.