Jeremy,
On our 2004 C-12 Marquis, 2002 built C-12 motor, I changed the thermostat this year as I did not know when it was last changed as we purchased in 2012.
With the new thermostat in place we now see temps from 182 to 198 and it is constantly fluctuating just driving along on a flat road.
The old thermostat, which was the old model not the updated as new one is, worked perfectly and stayed right at 188 degrees unless we were pulling up an extended grade, we pull a F-150 Ford Crew cab which is most of the time loaded fairly well, always with firewood to the brim when headed to Quartzsite rally, then it would rise up to around 202 til we topped the grade.
When I get back to Yuma where the Coach is in December I am again going to change the thermostat to a new one again. If that one does the same I am then going to reinstall the old one. Might just reinstall the old one and carry a new spare.
I queried Dave Atherton and he basically said I had nothing to worry about as the C-12 can get all the way up to 250 without any problems. Now I do not want that at all but do not like the constant variance of the temp. Dave said that there are plenty of sensors that will derate the engine if it is really overheating.
On the 1991 Contessa with a Cat 3116 we did see 230 several times as we towed same then and there was no place to stop other than middle of the road so we kept going. I was sure it did some damage but engine never changed after each of those sessions and went for 1000's more miles.
Check with Dave to be sure.
