I have 2004.5 Chevy 3500 dually with 13,500 miles. I tow a 14,000 lb fifth wheel. Up to now(10 months of ownership), the engine temperature has never gone above 190 degrees, even while towing in 85 degree weather.

Two weeks ago, at -4 degrees, I left the block heater on overnight. The next day, at 4 above zero, after 40 minutes of driving, with no load, my engine temperature went up to 210. Then suddenly, went back to 190. Since then, it has bounced between 190 and 208, with outside temps 20-40 degrees.

Dealer replaced both thermostats, flushed cooling system, says everything is fine. I'm worried about hooking up the fifth wheel in warm weather. I've never had a vehicle that would flucuate 15-20 degrees in temperature.

Yes, they also checked the gauge with a thermometer.

Any suggestions?