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 (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 found that you've got one or several cylinders with low/no compression, the next step is to find out why. To be a bit more specific, low/no compression is usually due to one of two things affecting that particular cylinder. Either the cylinder head valves are bad in that cylinder or its piston rings are worn out.
We can check this by adding about two tablespoons of oil to the confirmed ‘dead’ engine cylinder..
After adding the oil we'll check that cylinder's compression again. If the compression reading shoots up, then this tells you that you've got worn piston rings causing the compression problem. If the compression reading does not change (from that of TEST 1), then that cylinder's cylinder head valves are behind the low/no compression problem.
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.
You don't have to add a lot of oil. 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, either the compression value will go up (from the one you recorded before) or it 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 2.0L Mazda 626 (2.0L Mazda MX6).
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.
More 2.0L Mazda Tutorials
There are several more ‘how to’ tutorials that I've written that are 2.0L Mazda specific, that may be of further help. You can find the ones that are located here, at this site, by going to the Mazda 2.0L Index Of Articles.
Here's a small sample of the tutorials you'll find in the index:
- How To Test For A Blown Head Gasket (2.0L Mazda 626).
- How To Test The TPS (1994-2002 2.0L Mazda 626).
- How To Test The MAF Sensor (1996-1997 2.0L Mazda 626).
- How To Test The Fuel Pump (1994-1999 2.0L Mazda 626).
If this info saved the day, buy me a beer!