I am having the same problem with my Cobra. At about 30 MPH and above it runs cool as a cucumber—160 degrees with a 160 degree thermostat but in slow traffic or at idle it gradually increases the temp to where it would boil over if the engine continued to run. When the engine is turned off and the electric fan is on, it cools right down without problems. The infrared thermometer shows that there is about a 40 degree gradient between the radiator top return fitting and the bottom supply to the water pump which one writer said was normal. (By the way those IR thermometers are the greatest things since nylon tie straps. ) The IR thermometer also shows that the dash temperature gauge is reasonably close.

Overtime I have found and fixed a backward head gasket—it’s a Ford Cleveland, a hole in the return hose and a leaking thermostat housing gasket. None of these repairs fixed the problem.

I have rerouted the plumbing two or three times to reduce/prevent air in the heads, have gone through two water pumps, two radiators, four electric fans including about a 2400 CFM Spal-now a 3400 Zirgo, and an unnumbered amount of different thermostats and radiator caps. I have tried Water Wetter and several different anti freeze combinations. Nothing has worked.

There is a good chance the problem may not be in the cooling system at all. But last week, I talked with the good folks at PRC (Performance Rod & Custom) whose company is about 8 miles north. I wanted to try a triple pass two row aluminum radiator. They recommended a dual pass, with a Spal fan and shroud, and another way of directing the plumbing to reduce the likelihood of air in the heads.

When the unit is in, I’ll let you know what happens. Good luck on yours

Rick