Is a compression check the best way to determine if you have a leaking/blown headgasket? If a head gasket leaks then there is no way a cylinder could hold compression right?
Not necessarily. I just replaced a blown head gasket this weekend. The #6 had a leak to the water jacket. All cylinders were 150 to 160 PSI compression. The leak was small enough to cause cooling issues, but not detectible with a comression tester. Also,my cooling system pressure tester showed 1 psi loss over 1 hour. Excellent results. However, when I used my leak-down tester the problem was evident. Other clues were the plug and the header tube appearance.
Leak down tester is one of the best tools I own. Compression tester is a gross measurement tool. Leak-down is a finer method.
Only use a compression tester with a screw in adapter for the spark plug hole. The other type leaks too much to get an accurate reading.Your local auto parts store may have a compression tester to rent. If you do mechanic work on your own car on a regular basis, it would be a good tool to add to your collection.
With the engine warmed up, remove all spark plugs and prop the throttle wide open, crank the engine until it the gage reading stops increasing. On a cold engine, it will be hard to tell what's good & what's not. Some of the recent posts have numbers ranging from 140-170 psi. If the compression is low, squirt some oil in the cylinder and do it again – if it comes up, the rings are worn. There should be no more than 10% difference between cylinders. Use a blow down leak test (puts compressed air inside cylinders) on cylinders that have more than 10% difference.
I generally use a big screwdriver handle stuck in the TB between the butterfly and the TB to prop the throttle open. The plastic is soft enough that it won't damage anything and won't get sucked down the intake either.
A battery charger (not the trickle type) is a good thing to have if you haven't driven the car lately or if you have any doubts about the battery's health. Connect it up while you are cranking the engine and it will help keep the starter cranking at a consistent speed from the first cylinder tested to the last cylinder.
See the link in my sig to build and use your own blow down compression tester.