How To Test Engine Compression (1999-2001 1.6L Mazda Protegé)

Interpreting Your Compression Test Results

It's not unusual to see the compression values vary a bit, especially in high mileage engines.

If the low compression value is within a certain range, it won't cause any problems and you probably won't notice anything.

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:

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

What usually causes a low or no engine compression test result is that that specific cylinder has bad cylinder head valves or worn piston rings.

The cool thing is that we can find out without having to tear down the engine by doing a ‘Wet’ compression test.

Doing a ‘Wet’ compression test simply involves adding about two tablespoons of oil to the confirmed ‘dead’ engine cylinder.

The engine oil that you're gonna' add to the cylinder will help determine if the low compression reading 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. 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. 2

   Install the compression tester onto the cylinder.
   
   Hand tighten the compression tester only, do not use any type of tool to tightened it.

3. 3

   When all is set up, have your helper crank the engine.

4. 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.

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 1999-2001 1.6L 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.