Inner head gasket / O-ring blown or finish� cooling liquid is going in the
combustion chamber. Do a leak down test. As explain below.

TEST #1: Pressure Tests for the Cooling System
 The first test of the cooling system is done when the engine is cold.  A
 cooling system pressure tester can be installed in place of the radiator
 cap. Pressure in the cooling system and hot coolant are not something to
be
 treated lightly.  Use caution, and never remove the radiator cap from an
 engine that has been run even for even a short time. After installing the
 gauge, pump it up to 10 pounds. Look around for leaks if the gauge shows
 any pressure loss. Common trouble spots include heads, hoses, water pump
 and rear heater core. If you smell something sweet when you turn the
heater
 fans on, it may be a coolant leak at one of the heater cores, or simply
the
 O-ring at the rear heater valve leaking. Because there is often more then
 one leak in the system, it is necessary to continue pressure testing until
 the system holds pressure.

 TEST #2
 Start the engine cold with no pressure indicated on the gauge.  The
 pressure in the cooling system builds to aprox 7 lbs at normal running
 temps looking around for leaks. But if the pressure builds quickly to
 around 15 lbs the pressure is coming from one of the cylinders through the
 inner head gasket. This can also show up as a coolant pressure loss in the
 first test. In extreme cases the cylinder can fill with coolant, and will
 not turn over. This is know as hydraulic lock, and if you suspect this
pull
 the spark plugs and tries to turn the engine again. Pulling the plugs from
 the engine allows water to escape from the spark plug holes.

Good luck, Ben