I have an optical liquid level sensor in my air/coolant seperator/expansion tank.  It worked OK in tests.  After ground testing the engine for about one hour while auto-tuning and taxi testing, I drained some coolant out of the system to expose the coolant level sensor to make sure it still worked.  It did not indicate low coolant!  I removed the probe and found it had a scum deposit - when wiped clean it worked.  My engine came out of a 2004 RX-8 and had 9400 miles on it.  My cooland is 60% distilled water and 40 % Prestone - yellow - antifreeze.  I have not added any water-wetter but will check today on John Deer's product.  The coolant is always slightly cloudy.  The engine had some antifreeze in it when I recieved it and I flushed the coolant system.  I also changed the antifreeze after about 4 hours of ground operation.  Is there any solution - additive to keep the system cleaner, is there a better coolant level dector - or do I need to hire a scum cleaner to keep it clean??  I guess I could give up and just watch the coolant pressure for clues.

Thanks,
Dennis H.