Coolant Loss in STR 3.0 Diesel

About every 600k's I have to top up with about 300ml's of coolant in the overflow tank (I only ever check when the engine is cold). The engine starts up smoothly each morning and runs like a dream. The problem has been around since I purchased the STR 6 months and 11,00k's ago. The temp gauge is always steady and reads just under halfway on the gauge.

There are no visible leaks in the radiator or hoses (can't see if water pump is problem). The oil is clear with no sign of water, there are no bubbles in the radiator when I rev the motor. There is no sign of coolant in the overflow pipe from the heater.

I have replaced the radiator cap (which increased coolant use slightly) and am going to remove, flush and clean radiator at the local radiator man. The radiator man has visually checked the hoses and radiator, but did not do a pressure test as the motor was hot and pressured up when he checked it.

When I put the radiator back in, I will replace the thermostat and top and bottom radiator hoses and heater hoses. I am also thinking of replacing the water pump (all this will be happening when Mr Nizzbits returns as I have been quoted $460 to purchase a new water pump from Bursons).

Does any body have any ideas on the cause?

get a gas tester and see if your getting combustion gasses in the radiator.

you sure your not overfilling the overflow tank? its quiet hard to see the cold mark in it.

Where do I get a gas tester?

I fill to the line just below the neck on the bottle - thanks for your quick reply Tweak'e

just below the neck ????

thats WAY overfilled. have a look the hot mark is about 3/4 and cold mark is about 1/2 way down.

If you know any fridgies ask if you can use their gas detector, as these detectors pick up oxygen deplitcion, so if you have carbon monoxide (combustion gasses) you will know.

Just had the CO2 test which returned negative. Had pressure test - dropped 1 pound in five minutes, but no sign of coolant leak.

More pressure tests Monday (hot and cold) then removing radiator, flushing and new hoses.

Will keep you posted,