Interpreting Your Compression Test Results
It's not uncommon for the compression value of each cylinder to vary from one another. Up to a certain point, a difference between compression values won't cause an engine performance problem.
But if the compression values vary by more than 15%, you'll have a rough idle issue or a misfire problem.
You can do this (figuring out the 15%) in one of two ways:
- You can calculate this 15% difference with pen and paper.
- You can use my low compression calculator. You can find it here: Online Low Engine Compression Calculator (at: easyautodiagnostics.com).
If you want to manually calculate the 15% difference, here's what you'll need to do:
- STEP 1: Multiply the highest compression value by 0.15 (this is the decimal value of 15%).
- STEP 2: Round the result to the nearest one (for example: 25.6 would become 26).
- STEP 3: Subtract the result (the number that was rounded) from the highest compression value.
- ANSWER: The result of this subtraction is the lowest possible compression value any cylinder can have.
Now, let me give you a more specific example: Let's say that I got the following compression readings:
My next step is to do the following calculation:
- STEP 1: 175 x 0.15 = 26.25.
- STEP 2: 26.25 = 26 (rounded to nearest one).
- STEP 3: 175 - 26 = 149.
- ANSWER: 149 PSI. Any cylinder with this compression (or lower) value will misfire.
Since cylinder #2 is only producing 95 PSI, I can now conclude that it's 'dead' and causing a misfire.
To find out if the lowest compression value you got from your engine compression test is within a good range, you'll need to do the same calculation. Of course, you'll need to use the highest compression value you got and not the one in the example.
Once you've found the 'dead' cylinder, the next step is to find out what's causing the low compression value. For this step, go to: TEST 2: 'Wet' Engine Compression Test.
TEST 2: 'Wet' Engine Compression Test
One of two things will cause a low cylinder compression value:
- The affected cylinder's intake/exhaust valves are damaged or worn out.
- The affected cylinder's piston rings are damaged or worn out.
Thankfully, you don't need to take the engine apart to find out since a 'wet' compression test will easily tell you where the problem is.
In a wet compression test, you add about two tablespoons of engine oil to the cylinder with low compression and then retest its compression.
If the cylinder's compression value increases, you can conclude that the low compression value is due to piston ring problems.
If the compression value does not increase, you can conclude that the problem is due to bad intake or exhaust valves.
OK, let's get started:
Add a small amount of engine oil to the cylinder that reported low compression or no compression in the 'dry' compression test.
You don't have to add a lot of oil. The amount should be about 1 to 2 tablespoons of oil.
Install the compression tester onto the cylinder.
Do not use any type of tool to tighten the compression tester. Hand tight is fine.
When all is set up, have your helper crank the engine.
You'll get one of two results:
1.) The compression value will go up (from the one you recorded before).
2.) The compression value will stay the same.
Let's take a look at what your test results mean:
CASE 1: The compression value shot up. This test result confirms the low compression value is caused by bad piston compression rings.
CASE 2: The compression value stayed the same. This test result confirms the low compression problem is caused by worn or damaged cylinder head valves.
More 1.7L Honda Civic Tutorials
You can find a complete list of 1.7L Honda Civic tutorials in this index:
Here's a small sample of the tutorials you'll find in the index:
- How To Test The Crank Sensor (2001-2005 1.7L Honda Civic).
- Maintenance Required Light Reset (2001-05 1.7L Honda Civic).
- How To Do A Cylinder Balance Test (2001-2005 1.7L Honda).
- How To Test The TP Sensor (2001-2005 Honda 1.7L).
If this info saved the day, buy me a beer!