Interpreting Your Compression Test Results
If you're reading this section, then your compression test results indicate you've got one or more cylinders procuing a low compression value.
Up to certain point, a low compression value will not cause any problems.
But if the compression value varies by more than 15% of the highest one, then you've got a cylinder that will cause a misfire.
Alright, finding out if the low compression value is causing a problem can be done 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:
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
If you've reached this point, you've found one or several cylinders with low or no compression. You might be wondering what's causing this compression test result. In this test section we're going to find out with a very simple and easy test.
This test involves adding about two tablespoons of engine oil to the cylinder that gave you a low or no compression test result in TEST 1. After adding the oil to the cylinder (with low or no compression), we're going to test its compression again.
Why add the engine oil? Because the engine oil will help to determine if the problem is in the piston rings, or if the problem is in the cylinder head valves.
OK, this is what you need to do:
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.
Install the compression tester onto the cylinder.
Do not use any type of tool to tightened 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 see what it all means:
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 (block) of the engine in your 1.8L Mazda Protegé.
CASE 2: The compression value stayed the same. This confirms that the low compression problem of the affected cylinder is due to worn or damaged cylinder head valves.
If this info saved the day, buy me a beer!