Pull and ground all spark plug wires. You don't want an exposed spark jumping around if you have any hint of raw fuel or fuel vapor around. Then pull one plug and install the compression test adapter until the O-ring is hand tight. Install the hose that includes the gauge (some don't have hoses that disconnect from the adapter that screws into the spark plug hole). Crank the motor for about 3 seconds. You should see the needle on the gauge go from zero to somewhere around 110 to 130 psi. It may take a second or two for the needle to quit rising and stabilize. Record the number. Release the pressure (normally a plunger you push), then move to another cylinder and repeat. Make sure you reinstall the spark plug before moving on to the next cylinder. Make sure you treat all 4/6 cylinders the same. If you cranked for 4 seconds on one, then crank for 4 seconds on the remainder.
That was the "dry" test. Now go back and repeat the entire test but squirt about a teaspoon of oil into the cylinder before installing the adapter. This will give you the "wet" test.
For both tests, the difference between the high cylinder and the low cylinder should not be more than 10%. Poor idling of an otherwise great running engine can often be traced to one cylinder that is well below the others. This weak cylinder pulls down the engine to the point it has trouble running at normal idle. If normal idle is say 600 rpm, and it will only keep running at 800 rpm, it is often traced to a weak cylinder.
The wet test should read a few psi higher than the dry test.
Off the top of my head, I can't remember what the "wet" test tested, could be rings, or could be valves. Someone help me out here. I believe it is rings. A scuffed cylinder wall/ring will not rise as much as the good ones. So if 5 of 6 cylinders give you 120 psi dry, and one cylinder gives you 115 psi dry, and when you do the wet test, the 5 cylinders each rise to 130 psi, but the 1 cylinder stays at 115, that gives you a clue as to whether the problem is rings or valves.
Here is an example of the tool:
http://www.sjdiscounttools.com/kdt2428.html
