1. A compression test gives a measure of the condition of the rings and valves in a cylindar. If the cylinder can hold pressure (compression) at an acceptable level, the condition is good. If the pressure is low, then further test can determine if the rings are worn, the valves not sealing, or the cylinder walls worn. Note: many advocate something called a "leak-down" test where air pressure is applied to a cylinder and the leak down of the pressure is observed over time.

2. Any repair shop can do this test but it's one you could do yourself. The compression guage is not expensive. If you can change your spark plugs, you can do this test. The Bentley manual and most other manuals give directions on how to do the test. If you don't have a manual, check your public library.

3. According to the Bentley manual, the minimum pressure is 128psi and the maximum difference between cylinders is 28 psi. On the last compression test I did on my '88 240 with the B230 engine, I got 192 psi in all four cylinders.