How To Test Engine Compression (2.0L Mazda 626)

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.

You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (2.0L Mazda 626) (at: autotecnico-online.com).

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.
   • Usually caused by very low compression in one cylinder or...
   • 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):
   • P0300 Random Cylinder Misfire.
   • P0301 Cylinder #1 Misfire.
   • P0302 Cylinder #2 Misfire.
   • P0303 Cylinder #3 Misfire.
   • P0304 Cylinder #4 Misfire.
3. Bad gas mileage.
   • This is caused by the simple fact that the engine is NOT running with all 4 cylinders.
4. Engine pollutes more.
   • 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.
   • This fast cranking speed is very noticeable.
2. The Ignition System is sparking all 4 spark plugs.
   • This tells you that the no-start condition is not caused by a fault in the ignition system.
3. The fuel injectors spray fuel.
   • You can confirm this with a Noid Light test.
   • 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:
   • Blown head gasket.
   • Broken timing belt.
   • Engine threw a rod.

OK, let's get testing.

Which Compression Tester Should I Buy?

There are lot of engine compression testers to choose from and many places to buy them. I'm gonna' make some recommendations to you:

TEST 1: 'Dry' Engine Compression Test

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!

IMPORTANT: 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!

This is what you'll need to do:

1. 1

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

2. 2

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

3. 3

   Remove all four spark plugs.
   
   As you're 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.
   
   Engine cylinder #1 spark plug hole is the closest to the drive belt.
   
   IMPORTANT: 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.

6. 6

   Have your helper stop cranking the engine once the needle on the gauge stops climbing.

7. 7

   Write down the compression value on a piece of paper. Include the number of the cylinder this reading belongs to.

8. 8

   Repeat steps 4 thru 7 on the other 3 cylinders.

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:

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

CASE 2: Low compression in one or more cylinders. To a certain point, it's normal for the compression to vary a little between cylinders (as the engine accumulates thousands of miles).

But if these values vary too much, then you're gonna' have a bonafide misfire on your hands.

The next step is to do some math to find out if this low compression value is within a normal parameter or not. Go to: Interpreting Your Compression Test Results.

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:

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

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:

1. 1

   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.

2. 2

   Install the compression tester onto the cylinder.
   
   Do not use any type of tool to tightened the compression tester. Hand tight is fine.

3. 3

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

4. 4

   You'll get one of two results: 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).