also is the shroud in place and all the gaskets. the shroud directs the airflow to radiator. maybe a lower temp stat.

If you have a front liscense plate on try removing it. I have electric fan so slow driving it cooled fine but after a while at 70-80 temp would start to rise. After removing plate it runs cool at any speed.

You probably have a radiator hose collapsing under load. Put it in park, parking brake engaged, and crank it up. Open the hood and have someone raise the rpm while you watch the hoses. it will most likely be the lower hose closing shut. I have seen this many times. Your symptoms are exactly as I would expect with this problem.
Scott
ranger32007-01-26 19:00:17

I had exactly the same thing that ranger3 is talking about.  I put one of the unifit hoses with the spring molded into the rubber (looks like the section of a bendable straw when it's pulled out) and the problem went away.

Follow the rangers advise on checking cause you will be able to see it collapse on the framerail if it is the hose.

Good Luck!

How do you know it's overheating? You can't always go by what the gauge says...they lie!

You may very well have a hose collapsing at that speed. That would slow down the flow of coolant thru the engine, and create a hot running condition. That's why the lower hose almost always has a spring inserted in it. See if you can mash the lower hose together in the middle. If you can, the spring is gone, and that is probably the source of the problem.

At highway speeds, and that RPM, the fan clutch is really just along for the ride, as there should be enough air flowing thru the rad. to cool it. That is, if the air is actually going thru the rad., and not around it! Check to see if all of the foam seals are still around the shroud, and the radiator support.

Removing the license plate on a '78 wouldn't do any good, either...there's a huge rubber thing behind it that the plate bolts to. It's part of the bumper system. But that idea works well on the chromies!

I will check it in the AM.  That's what I'm thinking also..

Also make sure your air dam is inplace. It directs the air coming under the bumper up into the radiator. As stated above, make sure your foam seals are inplace around the radiator. That way the air flow is forced through radiator and not around it.