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.
Contents of this tutorial:
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:
- 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.
- 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.
- Bad gas mileage.
- This is caused by the simple fact that the engine is NOT running with all 4 cylinders.
- 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:
- The engine cranks very fast.
- This fast cranking speed is very noticeable.
- 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.
- 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).
- Fuel pump is working and providing pressure.
- 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 two recommendations to you:
1) Which one to buy: The engine compression tester that I have always used is the Actron CP7827 Compression Tester Kit. My only complaint about this engine compression tester is that it does not come with a case to store it in.
Engine Compression Gauge Testers
2) Where to buy: You can buy an engine compression tester just about anywhere, but you'll end up paying more for it (especially at your local auto parts store). The above links will help you comparison shop. I think you'll agree it's the better way to save money on the compression tester!
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!
This is what you'll need to do:
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!
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.
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!
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.
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 3 cylinders.
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:
- 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.