2001 Mercury Mariner 50-60HP Factory Service Manual, Page 50Get this manual
GENERAL INFORMATION
6Crank the engine over until the compression reading peaks on the gaugeRecord the reading7Check and record compression of each cylinderThe highest and lowest reading recorded should not differ by more than 15 (see example chart below)A reading below 120 psi might indicate total engine wear problemCompression Pressure (Minimum) 135 psi (950 Kpa, 9.5 kgcm2) Example of compression test differences: Maximum (psi) 180 150 8Remove compression gauge9Install spark plugsMinimum (psi) 162 127.5