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My '68 GTX 4 speed had similar problems with highway vs idle speed. On hot summer days it would run around 200-210 on the highway and at idle would easily get to 225 and keep going. Even after going through every cooling system component including a 3 row recore of the 26" radiator, nothing changed. I even got to the point of swapping back to an orig cam, rejetting the carb, advancing the timing, running a 98-100 octane mix, using infra-red heat guns, tighter gripping fan belts or better fitting pulleys, removing any vacuum leaks, and running 90% water with Redline water wetter. All of that helped....a little. After 3 yrs of trying I finally sold the car w/o ever solving the problem. No doubt an alum radiator with high capacity water pump would have helped some. It was annoying that between my Mopar mechanic and myself we could never find the smoking gun. The setup was basically stock with the proper 7 bladed fan/shroud and stock viscous fan. I do have to wonder if the new Year One repro fan shroud was part of the problem as they are not as deep as the originals, hence the fan blade protrudes out a tad. Could have made a difference. The idle engine vacuum ran around 8-12" as I recall which I felt was a symptom. Normal would have been around 14" or greater. My hunch was that a less than optimum fuel/air ratio was the source of the overheating.



Exactly right if everything else was right.

With todays alcohol fuel our fuel systems need to be richer with plenty of timing to run cool. 10% alky required jetting up even though guys will swear there right jetting wise because they have o2 sensor in the exhaust, there still lean and don't even know it and blame everything except the tune up.