Coolant loss and aeration can be caused by reasons other than combustion gas leakage past the head gasket fire ring seal.
Air can enter the system by:
Filling the system improperly.
Allowing the coolant level to drop too low.
Improper coolant system maintenance.
Venting the system improperly.
Through a cracked air compressor head.
Any cooling system leakage points.
Combustion gases can enter the cooling system at:
The head to block joint.
An injector adapter/seal.
A cracked cylinder head.
A cracked cylinder liner flange.
Through a pitted cylinder liner.
One way to detect air or combustion gases entrance in the cooling system is to visually inspect the system’s components.
The simplest is to check the coolant level. Significant coolant aeration is unlikely if the system is full and no coolant has recently been added. If it is low, there may have been an overflow discharge, a result of air or gases in the cooling system.
A more thorough procedure is to pressurize the system [75 to 103 kPa (11 to 15 psi)] and check for external leaks:
Hoses and lines.
Clamps and connections
Water pump seal.
Radiator core, header and tanks.
Gaskets and drain plugs.
All of these could be sources of leaks and should be repaired immediately.
A properly functioning radiator cap is crucial to sealing and maintaining the cooling system pressure. If the cap is not seated tightly in the filler neck or the pressure relief valve opens at a pressure that is too low, the system pressure and the boiling point of the coolant will be reduced. This can allow coolant to escape through the overflow hose.
It is very important to make sure the cooling system is filled to the proper level. BE CAREFUL NOT TO OVERFILL THE SYSTEM. It will purge itself to reach equilibrium and coolant will be discharged through the overflow.
When the radiator is filled initially or when the coolant is changed, premixed coolant should be added no faster the 20 liters (5 gallons) per minute, This reduces the chance of trapping air bubbles in the system and causing the coolant level to be too low.
Inspect the coolant level in the top tank. Bring the coolant to the proper level before testing.
An easy method to test for air or combustion gases in the cooling system is the Pressure Test using a pressurizing pump with a gauge, part no. 9S8140 or equivalent.
Remove the pressure cap and run the engine until the thermostat opens and the engine reaches operating temperature of 91 to 100°C (195 to 210°F). This will vent the normal coolant expansion.
After the temperature stabilizes install the pressurizing pump and gauge kit.
Use the pump to pressurize the system to nominal pressure of 35 to 50 kPa (5 to 7 psi). Continue to run the engine at a constant temperature and rpm (between 1500 to 1800 rpm) and monitor the gauge.
Because the pressurizing kit does not have an automatic pressure relief valve do not exceed 103 kPa (15 psi) or cooling system components may be damaged.
If the pressure reaches 75 kPa (11 psi) or greater within 5 to 10 minutes release it back to 34 to 48 kPa (5 to 7 psi). If the pressure rises again to 75 kPa (11 psi) or greater there probably is an internal leak that will require removal and inspection of cylinder head, gasket, injector adaptors or cylinder liners.
If there is no combustion gas leak, the gauge will remain at 34 to 48 kPa (5 to 7 psi), varying approximately 7 to 14 kPa (1 to 2 psi) when the fan comes on.
Air and gas in the system can also be checked by another simple test called the Bottle Test.
The equipment needed to perform this test is a bucket of water, a calibrated half liter or pint bottle and a length of hose attached to a modified radiator cap.
After the pressure test the engine should already be at operating temperature [90°C to 99°C (195°F to 210°F)] and the expansion air and gases vented. Install the modified radiator cap and hose. Submerge the bottle in the bucket, filling the bottle completely with water. Invert the bottle keeping the mouth under water. Place the loose end of the hose into the water-filled bottle.
Continue to run the engine at constant temperature and rpm (between 1500 to 1800 rpm). If there is a combustion gas leak, the gases will make their way to the radiator, through the hose and into the inverted bottle.
If more than 0.5 L (0.13 gal) of water per minute is displaced, air or gas entering the cooling system is excessive. Under a load condition on a dyno, 0.75 L (.195 gal) per minute would be excessive. There is probably an internal leak that will require removal and inspection of the cylinder head, gasket, injector adaptors or cylinder liners.