How To Test Engine Compression (2.0L Mazda 626 and MX6)

What makes testing the compression of your Mazda 2.0L four cylinder engine easy is that the spark plugs are easily accessible and in plain view (you do have to remove the spark plug wires, but that's easy peasy). In this tutorial I'll explain how to check engine compression in a step-by-step manner. More importantly, I'll show you how to interpret your test results to see if there's indeed a problem or not.

Here are the contents of this article at a quick glance:

  1. Symptoms Of Low Or No Engine Compression.
  2. The ‘Dry’ Engine Compression Test.
  3. ‘Wet’ Engine Compression Test.
  4. Which Compression Tester Should I Buy?
  5. More 2.0L Mazda Tutorials

Symptoms Of Low Or No Engine Compression

Engine compression problems usually fall into two categories. Either there's no compression across all cylinders or one (or two) have low compression.

When your Mazda's engine has no compression in all 4 cylinders, it's not gonna start (it'll crank but not start). On the other hand, when one cylinder has low/no compression, the engine is gonna' start... but it's not gonna' idle smooth.

Here are some more specifics:

Engine Starts but Runs with a Misfire:

  1. Also known as an engine miss, rough idle condition.
    1. Usually caused by very low compression in one cylinder or...
    2. Uneven engine compression that varies more than 15% across all 4 cylinders.
  2. Check Engine Light on with misfire codes (if your vehicle is OBD II equipped):
    1. P0300 Random Cylinder Misfire.
    2. P0301 Cylinder #1 Misfire.
    3. P0302 Cylinder #2 Misfire.
    4. P0303 Cylinder #3 Misfire.
    5. P0304 Cylinder #4 Misfire.
  3. Bad gas mileage.
    1. This is caused by the simple fact that the engine is NOT running with all 4 cylinders.
  4. Engine pollutes more.
    1. Low engine compression will cause the air/fuel mixture to not burn correctly. This means that unburned fuel escapes into the exhaust. If the cylinder has no compression, all that raw fuel is being sent into the exhaust.

Your Mazda won't start:

This usually is caused by having 2 or all 4 cylinders with no compression. When this happens, you'll see:

  1. The engine cranks very fast.
    1. This fast cranking speed is very noticeable.
  2. The Ignition System is sparking all 4 spark plugs.
    1. This tells you that the No Start Condition is not caused by a fault in the Ignition System.
  3. The fuel injectors spray fuel.
    1. You can confirm this with a Noid Light test.
    2. Also, you can confirm this, although indirectly, by removing the spark plugs and checking to see if they are fuel soaked (fuel fouled).
  4. Fuel pump is working and providing pressure.
  5. The most common causes of no compression on 2 or all 4 cylinders are:
    1. Blown head gasket.
    2. Broken timing belt.
    3. Engine threw a rod.

OK, let's get testing...

‘Dry’ Engine Compression Test

How To Do An Engine Compression Test (2.0L Mazda 626 and MX6)

OK, in this first test section, we're gonna' find out which cylinders have low/no compression. If you've been wondering where you can get your hands on an engine compression tester, you can buy one or borrow one from your local auto parts store (AutoZone, O'Reilly Auto Parts, etc.).

If you need help deciding where to buy one or which one to buy (and save some bucks on its purchase), take a look at my recommendations: Which Compression Tester Should I Buy?.

IMPORTANT: You'll be working around a cranking engine, so you have to be careful and stay alert at all times. Think safety all of the time!

This is what you'll need to do:

  1. 1

    Make sure the engine is NOT hot, if your engine starts and runs. If the engine is hot, let it cool down before trying to remove the spark plugs!

  2. 2

    Disable the 4 fuel injectors by disconnecting them from their electrical connector. This will prevent fuel from being injected into the cylinders as you crank the engine.

  3. 3

    Remove all four spark plug wires and remove all four spark plugs. I recommend marking the spark plug wires before removing them so you'll know where they go when you put them back on.

    As your taking them out, be careful and don't drop any of them on the floor, or you could cause the spark plugs ceramic insulator to break, and this will cause a misfire!

  4. 4

    Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder (this is the spark plug hole closest to the drive belt). Hand tighten the compression gauge only! Do not use any type of tool to get it tight.

  5. 5

    When the tester is set up, ask your helper to crank the engine. Your job is to keep your eye on the compression tester's gauge. Once the needle on the gauge stops climbing, have your helper stop cranking the engine.

    Write down the compression value on a piece of paper. Include the number of the cylinder this reading belongs to. Now repeat steps 1 thru' 6 on the other 4 cylinders.

  6. 6

    After checking all 4 cylinders' compression, the next step is to interpret the results

Let's take a look at what your test results mean:


CASE 1: Low or no compression in 2 or all 4 cylinders.. This test result indicates a serious internal problem.

The most common issues would be:

  1. Blown head gasket.
    1. To further test this, I recommend the following tutorial: How To Test For a Blown Head Gasket (2.0L Mazda 626).
  2. Broken timing chain.
  3. Engine threw a rod.

CASE 2: Low or no compression in one cylinder.. Getting a no or low compression test result means you've found the cause of the engine's rough idle (misfire).

If you got a low compression reading, the next step is to find out if this reading is varying by more than 15% than the other cylinders (because if it is varying by more than 15%, then this cylinder is considered ‘dead’)... this is how you can find out:

  1. Grab a calculator and multiply the highest compression reading that you recorded by .15.
    1. So, let's say that cylinder #4 gave you the highest reading of 170 PSI. Well 170 X .15 gives you 26 (25.5 rounded off).
  2. Now, the next step is to subtract 26 from 170... which gives us 144 PSI.
  3. So then, 144 PSI is the lowest possible compression reading that any one of the rest of the engine cylinders can have. Any compression reading below this.. and that engine cylinder will misfire.

Now, so that this calculation can make more sense to you... let's say that my Mazda gave me the following compression readings:

  1. Cylinder #1 165 PSI.
  2. Cylinder #2 170 PSI.
  3. Cylinder #3 175 PSI .
  4. Cylinder #4    30 PSI .

The next step is to do the math: 175 x .15= 26, 175-26= 149. So, now I know that cylinder #4 is the one causing the misfire!!

Now that you've found the cylinder with the low compression, the next step is to see if this low compression is caused by worn cylinder head valves or worn piston rings... for this test, go to ‘Wet’ Engine Compression Test.