Checking Cyliner Compression

            Before attempting to check your vehicles compression, I suggest you go out and buy the proper service manual for your application.  This will be more specific and give you the correct specifications on what to do.  I will outline a basic procedure for checking compression.

            For those of you who do not understand the principle behind compression I will try to explain it.  A vehicle needs three things to work: an air/fuel mixture, spark, and compression.  If you take one of these away or decrease the performance in any of these areas your vehicle will not be running correctly or at its full potential. 

Compression works like this:

1. The vehicle sucks in air through the air intake and mixes the air with the fuel either through carburetion, or fuel injection.

2. The vehicle then pulls the air/fuel mixture into the cylinders (we will not get more specific on this since we are just dealing with compression here).

3. The piston then travels upward compressing the air/fuel mixture almost to the point of explosion.

4. The spark plug then ignites the mixture causing the piston to move downward, thus creating the vehicles power.  The cycle repeats.

            Now that we have discussed the basics behind compression, we will discuss several things that can effect compression.  There are several different things that can cause a vehicles compression to be low.  First, the vehicle can have worn piston rings, causing gasses to escape past the piston.  Second, the vehicle can have worn valves, or valves that don’t seal properly causing gasses to again escape out the valves.  Third, the vehicle can have a blown head gasket, which causes the gasses to escape between the head and the block mating surface.  Finally, the engine can have a hole in a piston or block causing the gasses to escape this way.

The procedure for checking compression

1. First you are going to want to acquire the proper service manual for your vehicle.  After doing so you are going to want to thoroughly read through the section on checking compression and make sure that it is something you can handle and are comfortable with.
2. After familiarizing yourself with the manual you are going to want to go out and buy the proper tools. You will need: sockets, extension, swivel, wrenches, and a compression tester.  This is all depending on your application.  You are going to possibly have to invest in a socket set such as this one 49 Piece Socket Set all depending on your application.
3. Now that you have the tools, and the information you need, you will need to open your hood and mentally go through the process.  You will need to figure out what size sockets you will need, what size socket for your spark plugs, and how you are going to do the procedure.
4. Now that you have figured out how you are going to do everything, you are going to want to start your car and let it warm up to proper operating temperature.  This is vital, because it will throw off your readings if you don’t.
5. Now start by disabling the coil or distributor, whichever your vehicle has.
6. Next remove the first spark plug that you want.  It really doesn’t matter which one as long as you keep track.  I would recommend going in the firing order of the vehicle.  This helps, so you can write down the cylinder and the compression it has for your results.  The firing order can be determined by looking in the manual.
7. Now remove the first spark plug.  I would recommend this being the one from cylinder number one.  Do this by removing the spark plug wire from the plug and setting aside.  Then use your ratchet and spark plug socket to remove.  Depending on the location you might need to use your extension and swivel.  Depending on your application again be careful to not touch your hand on the hot exhaust manifold.
8. Once your spark plug has been removed you are going to need to take your compression tester out and match your spark plug threads to the proper adapter in the kit.  Install your compression tester by slowly rotating the threads into the spark plug hole.  Make sure to be careful so you don’t cross thread the threads.
9. Now that your compression tester is in place, have a buddy help you out.  Have them get in the vehicle while you are outside.  Take your compression tester in one hand so you can see it, and hold the throttle completely open in the other hand.  Have your friend crank the car over for 5 to 10 complete engine rotations.  Record your readings from your compression tester.  Now press the bleed button on the side of the tester and remove it.  Unscrew the threaded end from the hole.
10.  Replace the spark plug back into the number one hole, making sure not to over tighten.  Now replace the spark plug wire onto the plug.
11.  Move onto cylinder number two.  Repeat the process through the entire engine doing all of the cylinders and recording your readings.

Note:  As you move along on your testing you might come into contact with a spark plug that is in a tough place to reach.  To get at these you may need to use an extension and or swivel.  This may not even work, so you may need to use wrenches.  Just make sure to take your time, and be careful.  You don’t want to break any spark plugs off or mess up their threads.  If you are really stuck, take a break and calm yourself, sometimes all you need is to relax for a bit and go back at it.

            Now that you have recorded all of your numbers on your sheet you are going to need to do some calculations.  If you did it according to the cylinder numbers, it will make this much easier because you will then know where a problem is if you have one.  You want to take your numbers and make sure that they are all within 10% of each other, this is the general rule.  If they are then you are in a good luck.  You still will want to consult the service manual to make sure they are in the same range as that.  If they are not within specifications by the service manual, or they are not within the 10% range, then you will know which cylinder is not holding compression because you recorded it on your sheet.  In another section we will go into more details on what to do if you have a cylinder low on compression.  We will discuss how to do a cylinder leak down test.

            I hope that this step by step overlay of compression testing will help you come to some conclusive results on whether your vehicle is lacking compression or not.

Written by: Cody Mammenga an NDSCS Student