Here's a quick "how do I do a compression test"
Go to your local auto parts store and purchase a compression screw in style gauge. It will be a threaded on one end like a spark plug and a long maybe 1-2 foot vacuum line to a gauge. Any parts store should have it, schucks, autozone etc.
Start by warming your motor up to operating temperature, once there, you can begin your test. turn off the car. You want to be sure your car is at warm and full compression state. Cold engines don't give accurate readings to the exact compression of the motor, I believe they can even be a little bit higher.
Unplug the coil pack harness on top of the valve cover from the coilpack. The plug has a little clip in it that you need to take out to get the plug off, plyers work well here, or your fingers even. The plug and wire are on the drivers side of the coil pack. for those who don't know what a coil pack is, it's the thing that has the spark plu wires going into it.
Once this is unplugged, the spark plugs will not fire when you crank over the ignition, this means the motor should not start up.
Next you unplug each spark plug wire from the plugs and be sure you label in some way each wire so you kow which one plugs into which cylinder.
Start by writing down on a piece of paper the cylinders 1-4 so you can easily record your readings. The one closest to the cam gears, this is cylinder number one since the motor is in sideways and the cam side is actually the front of the motor. It goes from 1-4 in that order.
Remove the spark plug from cylinder 1 and thread in your gauge to the spark plug hole and be sure it is snug. Now have a friend crank over the ignition on the car for 2-3 seconds with the throttle open. As they crank it over you need to be watching the gauge and take note of the highest number it reads. The gauge will build compression as each crank of the motor. This is the compression or that cylinder. repeat this process at least twice per cylinder and write down the highest number recorded for each cylinder.
reverse to put the car back to operating condition-
Let's look at your numbers
Keep in mind that each motors number will be different, your looking for anywhere from 180-220 per cylinder for a strong motor on stock internals. Be sure that each cylinder is similar to one another. Older motors may have slightly lower numbers than these, but it's more important that the cylinders are reading similar numbers amongst each other, within 15% of each other. If you have a slightly lower cylinder than the other 3, then you might have some sort of damage inside your motor.
Here's an example
185, 190,185,120
This would indicate you have some ware on cylinder number 4.
If your numbers look like
165,185,190,187
This isn't bad, because you are within 15% or so variance on the 1 cylinder. Keep in mind that you have ware on the 1 cylinder though. It may be something to further investigate and keep in mind if your going turbo or already turboed. This could snowball down under higher boost.
Another trick you can do to narrow down whether the problem is a sticky valve or lies in your valvetrain is to drop a little oil into the spark plug hole just prior to doing the test, screw in the gauge and re-test. If the number goes up, then your problem may lie with the piston (rings, or ringlands) rather than the valvetrain.
Hope this helps, If I missed anything, pelase cue in.
For more info and specifics after you get your numbers, you may want to inquire about other possibilities and causes.
