Interpreting The Results Of The Engine Compression Test
The idea behind the engine compression test (in the case of a misfire condition or rough idle condition) is to find out if any one engine cylinder is not contributing 100% of its power to the overall engine output.
If one or more cylinders have compression value lower than 15% of the highest compression value, your GM 4.3L (or 5.0L, 5.7L) equipped vehicle will run rough or cause a misfire code to set and turn on the check engine light.
Figuring this 15% difference isn't hard, you can easily figure out 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, so that this calculation can make more sense to you, let's say that my 4.3L C1500 Pick Up (or Astro or Blazer, etc.) gave me 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 interpret your test results:
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 GM 4.3L, 5.0L, or 5.7L 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?
This is one of the most overlooked tests to find the root cause of a misfire code, rough idle or an engine miss or a blown head gasket.
Over the years, I have solved many unsolvable misfire codes, rough idle, lack of power issues by doing a simple engine compression test and if you're faced with something similar, I highly recommend doing an engine compression test.
Related Test Articles
To see the 4.3L, 5.0L, and 5.7L specific articles here at troubleshootmyvehicle.com, go to: GM 4.3L, 5.0L, 5.7L Index Of Articles.
Here's a sample of the tutorials you'll find in the index:
- How To Test The ‘Spider’ Fuel Injector Assembly (4.3L, 5.0L, 5.7L).
- Troubleshooting The Fuel Pump (GM 4.3L, 5.0L, 5.7L).
- Coolant Leaking From Intake Gaskets (GM 4.3L, 5.0L, 5.7L).
- How To Diagnose Misfire Codes (GM 4.3L, 5.0L, 5.7L).
- MAP Sensor Test (P0106, P0107, P0108) (GM 4.3L, 5.0L, 5.7L).
If you need and/or want to test the distributor ignition system to see if it's the source of the misfire condition (or misfire code: P0300, P0301, P0302, P03030, P0304, P0305, P0306, P0307, P0308), you can go to this link (found at easyautodiagnostics.com):
- How To Test A Misfire / No Spark-No Start Condition GM 4.3L, 5.0L, 5.7L Distributor Ignition System (1996-2004).
If this info saved the day, buy me a beer!