Interpreting Your Compression Test Results
If the compression values you obtained from TEST 1 vary between each other, the next step is to find out if they're causing an engine performance problem.
Generally, a compression value that varies by more than 15% of the highest value you obtained will cause a problem.
This problem will manifest itself as a misfire and cause a rough idle condition.
You can easily and quickly find out if any low compression value is causing a problem by doing some math.
You can do this 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 my engine compression test produced 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
After interpreting the results of your 'dry' compression test, and you have one or several cylinders with low compression, the next step is to do what's called a 'wet' compression test.
This involves adding a few drops (2 tablespoons) of engine oil to the cylinders with the low engine compression result.
Now, you may be asking yourself, why a wet compression test? Well, because it'll help you to determine 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.
Depending on whether the compression pressure rises (on your compression tester) or not, you'll be able to say that the problem lies in the piston's rings or in the cylinder head valves.
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 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 2.4L Nissan Altima (Frontier or Xterra).
CASE 2: The compression value stayed the same. This test result tells you that the low compression in this specific cylinder is due to bad cylinder head valves.
Related Test Articles
You can find all of the Nissan 2.4L specific test articles here: 2.4L Nissan Index Of Articles
Here's a sample of the tutorials you'll find in the index:
- How To Test The Fuel Injectors (Nissan 2.4L).
- How To Test The Starter Motor (Nissan 2.4L -Frontier, Xterra).
- How To Troubleshoot A Blown Head Gasket (Nissan 2.4L, 2.5L).
If this info saved the day, buy me a beer!
