Car: 68 442, all stock , 4 core OEM rad(original), New fan Clutch, rebuilt motor Etc.

Driving in traffic/ around town/ On highway up to 60MPH it will stay at 180 all day long no matter how hot it is outside, PERFECT. However if on highway going 70-80 up is starts to creep up to 200. Not sure how high it will get as i don't normally drive at 80 in an old car for long periods

Question: What are possible causes?

 

What RPM is the engine turning at 70+ MPH?

Mine is the same way, stays at 180-190 all the time EXCEPT it starts creeping up to 210-220 at 70+ MPH (during the 110+ summer with the AC on), but that's because the RPM is in the 3400+ range. If I slow down to below 3000 RPM the temp comes down below 200 and stays there. More RPM = more heat. For the temp to stay at 180 you (and I) would need more cooling capacity to dissipate the extra heat.

 

My Olds and even my modern Challenger do what Kenneth describes--coolant temp rises with engine rpm. In the Challenger, five minutes over 3500 rpm sends the coolant temp to 240 and oil temp to 260°.

Most people who race their OEM cars (that is, they want to run consistently at high rpm) need more capable radiators for the same reason.

I always thought my Olds built heat at higher rpm because of the drag on those wide, old-school rings. Obviously there's more nuance to the explanation because modern Hemis use low-tension, narrow rings.

 

Well, 200 is not high. Keep in mind that with a 15-16 psi cap, the coolant doesn't boil until 250 F. If the temp stabilizes, you're fine. If it does keep climbing, then consider blockage in the radiator, especially if it's original. I had exactly this problem with a big block Chevy truck. It would overheat at speed. Turned out that the lower third of the original radiator was plugged, thus reducing the effective radiator area.

 

I experienced the same trouble in my '68 442 with A/C. I wound up having the "OEM" 3-row radiator re-cored. The original core fell to pieces when attempting to "rod" it in the acid bath. I suspect the radiator was replaced at some point along the way, due to an accident. My upper support is clearly for a four-row rad, but the lower support has three-row rubber and steel channels. There is other evidence of body/frame/wheel hub repair work in the front left, along with other hints, so I suspect the three-row rad is not original to the car.

No longer an issue after having the radiator re-cored.

 

If you have access to an infrared laser thermometer use it to check for cool spots in the radiator indicating a blockage as mentioned by previous posters.

 

A coworker recently had a similar issue with overheating only on the highway with the AC on. He found there was a bunch of leaves blocking the air flow in the lower corner of the radiator. Same effect of reducing the radiator area.

 

Thanks guys for all the info.
I will push it to see what it may climb to.
Just finished a two hour trip over the weekend to a show(Each way) And noticed at 70MPH it kind of stays at 200+-, If i slow to 60-65 it comes back down.
I guess with nothing else to occupy my time for 2 hours(driving) i became fixated on what the temps were doing.
With this being the orig. Harrison 4 core it is more than likely it is a little restricted inside due to age.
Maybe a recore is in it's future.