Interpreting The Results Of The Engine Compression Test
OK, so you tested all 8 cylinders and you've got those readings on paper, what now?
Well, the next step is to find out if the lowest compression reading you got is causing a problem (like a misfire trouble code: P0300, P0301, P0302, P303, P0304, P0305, P0306, P0307, P0308).
To find out, you've got to figure out if the lowest compression value varies by more than 15% of the highest. Because if it does, the cylinder can be considered dead.
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 Ford F150 (or Crown Victoria, or Expedition, or etc.) produced 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 Compression Test.
TEST 2: ‘Wet’ Engine Compression Test
So, you've found the ‘dead’ cylinder or cylinders, what next? The next step is to do a ‘wet’ compression test and find out if the low compression value (or 0 compression value you got) is due to bad cylinder head valves or bad piston compression rings.
This involves adding a few drops (2 tablespoons) of engine oil to the cylinders with the low engine compression result and repeating the compression test.
The results you obtain from this second ‘wet’ compression test will help you determine if the low compression you recorded in the ‘Dry’ compression test are caused by worn piston rings or worn 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 take a look at what your test results mean:
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 Ford 4.6L, 5.4L equipped vehicle.
Here's why: The engine oil helped the piston rings seal better, thus bringing up the compression value almost back to normal. If the problem were in the cylinder head valves, then the engine oil you just added wouldn't make a difference at all (on the compression value).
CASE 2: The compression value stayed the same. This confirms that the problem is in the cylinder head valves.
Here's why: If the cylinder head valves and their seats are worn out (or maybe even bent from a broken timing belt), no amount of engine oil is gonna help seal the compression in, in the cylinder. So, if the compression value, for the specific cylinder you're testing did not go up (after you added oil to it), then this is a dead giveaway that you've got cylinder head valve damage.
Why An Engine Compression Test?
Now, usually all 8 engine cylinders wear out evenly. But every now and then, either thru' lack of maintenance or other repair issues that happen along the way of several thousands of miles and/or years, one or several engine cylinders wear out at an accelerated pace.
When this happens, no matter what gets replaced, like: spark plugs, ignition coils, fuel injectors, spark plug cables, etc, nothing solves the misfire codes (P0300, P0301, P0302, P303, P0304, P0305, P0306, P0307, P0308) or the misfire condition.
As mentioned earlier, testing the compression of all 8 cylinders is one of the most overlooked troubleshooting tests. But, before jumping into this test, you should first verify that the misfire is not spark related.
So test the ignition system first. At the end of this article you'll find some more test tutorials (Ford 4.6L, 5.4L V8 specific test articles) that'll help you to further test a misfire condition.
Related Test Articles
Now, if the compression readings on your Ford or Mercury indicated that everything is OK, and yet the car or SUV or pick up is still misfiring, the following tutorials might help:
- How To Use A Noid Light (Ford 4.6L, 5.4L).
- How To Test The Fuel Injectors (Ford 4.6L, 5.4L)
- How To Diagnose Misfire Codes P0300-P0308 (Ford 4.6L, 5.4L).
- How To Test The Alternator (Ford 4.6L, 5.4L).
- How To Test The Starter Motor (Ford 4.6L, 5.4L)
- How To Test The Ford 4.6L, 5.4L Coil-On-Plug Ignition Coils (at: easyautodiagnostics.com).
If this info saved the day, buy me a beer!