Has your favorite mode of transportation lost some of its "zip"? 
Maybe you think its time for a tune up and maybe you have had 
your engine tuned up but it still doesn't have the performance 
it used to have. Or maybe its just a little "nervous" when 
idling. 

There are several things that can be the cause:bad spark plugs,
bad plug wires, worn distributor, leaking valves, or worn or 
broken piston rings and the list goes on.

Its time for a compression test to determine if you have bad 
piston rings or leaky valves. The pounds per square inch of 
compression varies with each engine. If you want to know the 
compression specs for your vehicle, consult your shop manual.

To run a compression test, you need a few basic tools.

First and foremost is a compression tester. With your tester, 
check each cyclinder and record the results. There shouldnt be 
over 20% variance between the highest and lowest reading. If 
there is a greater variance, you may have internal engine 
problems. 

To further isolate the problem, run a "wet compression check". 
This is done by squirting a some oil in each cylinder (I use 
about a teaspoon per cylinder). If the compression comes up to 
specs with the wet check, you have worn or damaged rings and 
are looking at engine rebuild. If the compression doesnt change, 
you have valve problems and usually can solve your issues with 
a valve job. However, bad valves and worn or damaged rings can 
be partners. So, when you pull the cylinder heads, check for 
ring grooves and other signs of cylinder/piston damage/wear. 
You should also consider the mileage on the engine. If it has 
high mileage, many times a valve job will cause the engine to 
start using oil. If there is evidence of wear in the cylinders 
you might want to re-ring the engine as well.

The compression test may not help you pinpoint exactly what the 
problem is, but it will help you narrow down the possibilities. 
And for that alone, it's worth doing!