Interpreting Your Compression Test Results
It's not unusual to see engine compression values differ from one another, especially in high mileage engines.
In most cases, and depending on how small or big the difference, nothing happens to engine performance.
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.
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 a 2.7L Honda Accord I'm working on produced the following compression readings:
| Cylinder | Pressure |
|---|---|
| #1 | 165 PSI |
| #2 | 95 PSI |
| #3 | 155 PSI |
| #4 | 175 PSI |
| #5 | 165 PSI |
| #6 | 170 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 cylinder compression value is generally caused by one of two things:
- The cylinder's piston compression rings are worn.
- The cylinder's cylinder head valves are worn or damaged.
Thankfully, we don't need to tear the engine down to find out. We can do a 'wet' compression test instead.
This test simply involves adding about 2 tablespoons of engine oil to the cylinder with low or zero PSI compression and retesting its compression.
If the compression value increases, then you can conclude that the low or zero compression value is due to bad piston rings.
If the compression value does not increase, then you can conclude that the problem lies in the cylinder head valves.
OK, let's start:
- 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 while you observe the gauge.
- 4
Once the needle on the gauge stops climbing, have your helper stop cranking the engine.
- 5
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. - 6
Repeat steps 1-4 on any other cylinder with a low compression test result.
Let's examine your test results:
CASE 1: The compression value shot up. This tells you that that cylinder's piston compression rings are worn out and are behind the low compression test result you got in TEST 1.
CASE 2: The compression value stayed the same. This confirms that that cylinder's valves are behind the low to 0 PSI compression result you obtained in TEST 1.
More 2.7L V6 Honda Accord Tutorials
You can find a complete list of 2.7L V6 Honda Accord tutorials in this index:
Here's a small sample of the tutorials you'll find in the index:
- Ignition System Wiring Diagram (1995-1997 2.7L Honda Accord).
- How To Test The Head Gaskets (1995-1997 2.7L Honda Accord).
- P0420 -What Does It Mean? (1996-1997 2.7L Honda Accord).
- How To Test The Ignition System (1995-1997 2.7L Honda Accord).
If this info saved the day, buy me a beer!
