Interpreting Your Compression Test Results
On really high mileage engines, it's not unusual to see engine compression values differ from one another.
Usually if the difference in the compression values is small enough, engine performance does not suffer.
When the difference is too big, you'll have a rough idle or a misfire condition on your hands.
To find out if the lower compression value is causing a problem, you need to find out if it's lower than 15% of the highest compression value you got.
You can do this (figuring out the 15%) in one of two ways: You can calculate this 15% difference with pen and paper or you can use my low compression calculator. You can find the low compression calculator 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:
| Cylinder | Pressure |
|---|---|
| #1 | 165 PSI |
| #2 | 95 PSI |
| #3 | 155 PSI |
| #4 | 175 PSI |
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
A low or zero compression test result is usually due to one of two conditions: worn/damaged cylinder head valves or worn piston rings in the affected cylinder.
The good thing is that you and I can find out by adding about two tablespoons of oil to that cylinder and repeating the compression test.
In a nutshell, this is how this wet compression test works: If the low compression value is due to worn piston rings, the engine oil (that you're gonna' add) will bring the compression value up to normal. If the problem is due to cylinder head valves, the oil isn't gonna' help at all.
OK, 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.
The amount should be about 1 to 2 tablespoons of oil. - 2
Install the compression tester onto the cylinder.
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:
1.) The compression value will go up (from the one you recorded before).
2.) The compression value will stay the same.
CASE 1: The compression value shot up. This confirms that the low compression this cylinder registered in TEST 1 is due to worn piston rings.
CASE 2: The compression value DID NOT go up. This tells you that the cylinder head valves are behind the low or no compression value you registered in TEST 1.
More 1.5L Honda Civic Tutorials
You can find all of the 1.5L Honda articles here: 1.5L Honda Civic Index Of Articles.
Here's a sample of the tutorials you'll find in the index:
- How To Test The MAP Sensor (Honda 1.5L).
- How To Test The Fuel Injectors (1992-1995 1.5L Honda Civic).
- How To Test The Throttle Position Sensor (1992-1995 1.5L Honda Civic).
- How To Test The Igniter, Ignition Coil Accord, Civic, CRV, and Odyssey (at: easyautodiagnostics.com).
If this info saved the day, buy me a beer!
