I have about 2300 miles on my newly built 400. The temperature gauge has recently started to rise to the last hash mark on the right of the gauge while highway driving. Since the rebuild it would stay right around the "M" when fully warmed, regardless of driving habits. I noticed the rise in temp a couple nights ago. I had been driving on the interstate around 70 MPH for over half an hour. Looked down and noticed it being high. I ran the heater for over twenty minutes with no change. Outside temp was around 55. Strange thing is, when driving in stop and go traffic and idling it returns to normal. When back on the interstate it rises again. I have had no symptoms of it actually overheating yet, but it makes me nervous.

At rebuild, I replaced the water pump and thermostat. Thermostat is a Robert Shaw. New hoses. Radiator flushed and filled with 50/50 mix. Yes, I checked the level of coolant once cooled and system is full. I have no leaks. I run a fan shroud on the belt driven, non clutch fan. Now, I did install a nostalgic air parts a/c system a couple months ago and the condenser is mounted outside the radiator, but again, It has not affected the temp before now, even while driving longer distances at higher speeds in 85 degree temperatures.

Any ideas what may be the cause? I understand that many responses will be to install an aftermarket gauge. I plan to do this at some point, but I am concerned about running hot for prolonged periods and causing damage right now. The fact that the factory gauge does seem to be working(regardless of how accurate it may be) shows that there is a dramatic increase in temp. Thanks for your help.

 

Start simple first.

If it is the original sending unit try swapping it out. It might just be tired and unreliable. The wire might be tired too.