I have a 327 engine with a fan clutch. It starts out at 170 degress on the street. When I get on the freeway the temp climbs to 210 degress and keeps climbing to 230 or so. If I get off the freeway and street drive, it cools right down to 170. I would think that it would run cooler on the freeway with more are hitting the radiator and engine, and it would run warmer on the streets with less air. I would be thankful for any ideas. Jim

JamesR, the first thing that comes to mind is a failed thermostat. When they fail in a 'partially open' condition, you can go the entire winter and not notice it, thinking that the it's just the cold weather that's causing the engine temp a little longer than usual to come up to normal. When the temperature outside goes up you begin to experience your problem. Because the thermostat is partially open, it still cools the car at reduced loads like city cruising and idle. Check and/or replace your thermostat. They're cheap.
Let us all know what happens regardless.
Good luck,
Louie Hammel

Thanks Louie. I will try that. Its easy enough. I just don't know why it is at high speeds only, Jim

James, it takes more horsepower to push a cra though the air at hiher speeds, and it takes more fuel burned to make more horsepower, and more fuel burned makes more heat. The extra heat isn't being transferred from the coolant to the cooling air fast enough, so the coolan temp climbs. Either the coolant isn't circulating adequately or the heat isn't being transferred in the radiator. stuck thermostat, collapsed/collapsing lower radiator hose, inefficient water pump, clogged radiator (internally and/or externally) are possibilities. Lean carburetion and retarded ignition timing also could have some effect. If you have access to a library, they usually have books on auto repair that can lead you through the troubleshooting. Good luck.

Higher speeds cause more friction = more engine heat. The water is passing thru the system too fast for it to sit in the radiator long enough to properly cool, and in turn cool the motor.

Tuning cause also cause higher temps at higher RPM, but the t-stat is the first place you should start.

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Tommy B.
71-72 Malibu (http://www.geocities.com/beldarr/Chevelle.htm)
"I'm high all right, but on the real stuff
High octane gasoline
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And a shoe shine"

In my experience with these engines getting hotter at speed or cruise, it's always been lean. When you drive faster the airflow actually or should outstrip the engine's ability to make heat. Therefore the efficiency of the cooling system is better and disproportionally higher when cruising down the road (the cooling capacity is a higher ratio to heat production than at low or idle conditions, due to lower coolant flows and lower airflow). So, the real cooling test is when the efficiency of the radiator is the lowest and the engine makes the most heat compared to capacity to cool. This is almost exclusively at idle and slow speeds. So, my educated guess goes to the fuel mixture because the best radiator cannot keep up with too lean conditions in primary fueling situations. It just produces too much heat.

I bet he has put a new or rebuilt carb on lately and changed the mixture.

Jet and/or rod change is needed.

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Steve Jack - ConceptOne Pulleys and Brackets / Engineering & Marketing Technologies

I had these issues with my previous combo.

Since my rebuild, however, the engine runs as cool at 95mph as it does at 45mph.

The cooling system and thermostat stayed the same. The carb was completely rebuilt and rejetted.

I think Steve has something there. It makes perfect sense that the engine would run hotter if the mixture were leaner.

I can't possibly compare apples to apples here since the entire engine was rebuilt and upgraded, but I think he was right about being a tad lean for high speed cruising.

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I would like to thank Team Chevelle for the use of this great website. I would also like to thanks everyone for there help and information. I got the problem solved. It was the carb. jetting. It was running way to lean. That is why it run cool at lower speeds, but got hot at high speed under power. New bolt on after market carbs need to be rejetted to the car. Now I know. Thanks again, Jim