My coolant temperature has been rising  little by little over the 440 hours I have been flying my 13B. Last summer on the way to Arlington fly-in, I departed St George Utah at 100 degrees OAT and the coolant temp quickly rose to 230, my absolute maximum allowable. I could not climb above pattern altitude, so I landed. I cobbled together a water dribble system out of drip irrigation tubing, and it allowed me to continue the trip. After returning home I decided something had to be done. After checking everything I could think of, I removed the radiator, and after careful inspection, I found debris clogging much of the core that was visible. Some of it was bits of silicone used to help seal the couplings, and some of it looked like scale from the water jackets.I cleaned out as much as I could get to, and I made a screen out of stainless mesh and inserted in line inside one of the 1 1/2" silicone hose couplings. I flew it like that for about 10 hours while I had a new radiator made by Griffin Radiator. Coolant temps were better, but still not acceptable. I got the new radiator installed yesterday, and I found the screen already 3/4 clogged with more scale debris.
Today I flew it with the new radiator, and at OAT of about 65 F a full power climb out at 80 KIAS resulted in the coolant temp stabilizing at 167 F. I am not running a thermostat, and part throttle cruise coolant temp was about 130F. Descending out of 10000 ft  the coolant temp dropped to about 110F, and my cabin heat was practically nil. Looks like I need cowl flaps now! I am actually looking forward to some 100 degree days this summer to see how it cools now. If a 35 degree rise in OAT results in the same rise in coolant temp, I predict that my max coolant temp will not go above 202 F.
My coolant temp rise problem has been so gradual, while other things were being changed around, that the possibility of a clogged radiator did not even cross my mind.
Lessons learned: 1. Keep good flight data and refer to if often, the only way to catch gradual trends.
2. A cooling system in an aircraft definitely needs some sort of screen or filter. I may try one of these http://www.babymall.com.au/tefba/
3. 230 F does not seem to have hurt my 13B, I have probably 10 total hours flying at that temp.
 
Keep the shiny side up!
 
Chuck Dunlap
N616RV 13B