‘Wet’ Engine Compression Test

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

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.

    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.

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!

More 2.0L Mazda Tutorials

There are several more ‘How to Test’ articles 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:

  1. How To Test For A Blown Head Gasket (2.0L Mazda 626).
  2. How To Test The TPS (1994-2002 2.0L Mazda 626).
  3. How To Test The MAF Sensor (1996-1997 2.0L Mazda 626).
  4. How To Test The Fuel Pump (1994-1999 2.0L Mazda 626).