With compression test u will know where is problem inside the engine.
like which cylinder 1, 2, 3 or 4 and piston & ring or valves & seals.
Example of Toyota Corolla 1300 cc
(a) Warm up and stop the engine.
(b) Remove the No. 2 cylinder head cover
(c) Remove the 4 ignition coils
(d) Remove the 4 spark plugs
(e) Disconnect the 4 fuel injector connectors.
(f) Inspect the cylinder compression pressure.
(1) Insert a compression gauge into the spark plug hole.
(2) Fully open the throttle.
(3) While cranking the engine, measure the compression pressure.
980 kPa (10.0 kgf/cm2, 142 psi)
784 kPa (8.0 kgf/cm2, 114 psi)
Difference between each cylinder:
98 kPa (1.0 kgf/cm2, 14 psi) or less
• Use a fully-charged battery so the engine speed can be increased to 250 rpm or more.
• Inspect the other cylinders in the same way.
• Measure the compression pressure in as short a time as possible.
(4) If the cylinder compression is low, pour a small amount of engine oil into the cylinder through the spark plug hole, and then inspect it again.
• If adding oil increases the compression, the piston rings and/or cylinder bore may be worn or damaged.
• If the pressure stays low, the valve may be stuck or seated improperly, or there may be leakage from the gasket.