If an engine runs hotter on the highway and cools off at slow speeds, is that not enough airflow through the radiator, not enough water flow or the cooling capacity too small?

It drops around 10 - 15 degrees within abut 2 minutes when getting off of the highway and going back roads. That does not seem right to me. The temperatures are not overly excessive right now, but I think something funky is going on.

partly plugged rad

running a T stat?
New/Old radiator/shroud?

"running a T stat?" Good question. I have seen cases where a car with no thermostat ran at acceptable temperature at idle and low speeds but ran hot at highway speed. Adding a thermostat solved the problem. The theory is that coolant was flowing through the radiator too fast to adequately cool.

The radiator is a recored original - not plugged. Ran a Stant 180 degree thermostat and now has a Milodon 180 degree high flow thermostat. No real change.

It has the original shroud with the original aluminum 7 blade fan and a new fan clutch assembly.

It is a 67 Coronet 383 with AC. Has aftermarket condenser and fairly large tranny cooler in front of the radiator.

The motor is the original 383 and is fresh with 60 miles on it. 8.9:1 compression with a little cam, Holley Street Dominator Intake and Holley 750 vacuum secondary carb.

That theory is completely at odds with accepted thermodynamics.

Generally, overheating at highway speeds is water flow related.

But you don't claim to be over heating, just running "hotter",.

You you have numbers?

Highway speeds generate more heat in the engine, so what might be "ok" at idle and no load fails at a higher load. Your recored radiator means nothing on the surface, was it flow tested? What core was put in? Airflow out of the engine compartment can be an issue as well. Air can't go thru the radiator if it can't get out of the engine compartment.

You bring up some very good points.

What bothers me is it acts "funky". A few weeks ago when it was in the high 80's drove it about 20 miles. A little back road driving and then 12 miles of highway driving and it was a nice steady 195. Got off the highway and turned around and right away it started climbing to 215. Drove it 7 miles at 60 mph and when got off the exit was stuck at a light for a bit. Climbed to almost 220. 4 mile back road trip to the house it dropped well into the 190's. This was with the Stant Thermostat.

Yesterday it was cool - in the lower to mid 70's. With the new Milodon thermostat same exact route and it acted very similar. Never exceeded 203 on the highway, and driving back home on the back roads it went down to 190. Idling extended period of time in the driveway was 195.

I don't know the answers off hand on the radiator. I do have that written down somewhere and did a bunch of research before I settled on this core. I might call Glen-Ray for another radiator if I feel the radiator is suspect.

I agree on getting the heat/air out of the engine compartment. Dealing with that exact issue with my cousin's 67 Firebird with a blower.

I am wondering if the aftermarket condenser and tranny cooler is chocking the air to the radiator at highway speeds...........

i read somewhere that there was a factory bulletin to remove some 7 blade clutch fans.With the
fan clutch slipping at high speeds the big fan was turning slow and actually blocking air flow.
An easy test would be to try a standard 7 blade mechanical fan.I've never had anything but bad luck with clutch fans...they all ran 200 - 205 on the highway and hot at idle as well.

is your temp gauge accurate?, I would assume that the temp "change" amt is accurate but its good to have an accurate baseline to work with, that the numbers are correct. You might take the rad to a shop & have them flow it. we'll get started by eliminating some potentials. EDIT I'm assuming that the lower ain't partly collapsing & the timing is in the ballpark.

on the highway you have the most air flow & the most water flow (at least the impeller is turning faster) so their is faster water flow if their ain't an issue which I'm thinking their is, so when you slow down & their is less air flow and less water flow & it runs cooler then that tells me that at the higher speed when the eng is producing more heat that the air flow is OK & that something (the radiator) is restricting the water flow.

Actually that was my next step! Glad someone else agrees. I have several 7 blade mechanical fans in the attic.

You sound pumped! holler how it goes.

How about the air fuel mixture? Lean is hot. Different load on the engine from low speeds to freeway speeds.

Also this ethanol fuel leans the mixture. Run the current gas out or real low if possible and get ethanol free gas and try the same driving.

I have a car that the ethanol free fuel makes a difference on the engine temp. 383 2bbl with A/C complete but not working, so the condenser is there blocking airflow on the 22" radiator. Which means this car is right one edge of overheating or running cool all day.

After reading you had a clutch fan, I too was going to suggest a switch to a fixed blade fan, I don't run nothing but on my toys, every clutch fan I've had always resulted in high running temps

The condenser and tranny cooler ain't helping either, but they're necessary evils, if you find they're the cause, depending on their size, perhaps they could be relocated to expose more surface area of the radiator to incoming air, esp were hot water is initially entering the radiator...

might want to read
https://board.moparts.org/ubbthreads/ubbt...tml#Post2335341

Especially page 2

Did you say you had a aluminum fan? I didn't think the factory ever used aluminum fans, is it a flex fan? If it is then I would think that it would flatten out at speed, and block the air flow.

Some 1967 and older BB AC cars had aluminum blade clutch fans. They do not flex, they are a rigid blade.

Sounds like you have tried 2 T stats and not much difference. T stat is not the issue.

In my experience, hot at speed indicates not enough radiator. Not enough capacity or plugged up.

To check for a clogged rad, start with a cold engine. Run it and check to see if the core heats up evenly. You only have a small window to check, as radiate heat will spread to clogged section.

I second this idea. It's cheaper to do this first and usually easier than changing stuff. At least richen it up with ethanol fuel.

Acting "funky" sure sounds lean to me.

Both of my cars have clutch fans and have driven them in 115° Vegas heat more than once for over an hour at a time in stop and go traffic.
. Yes I have had clutch fans go bad, but it does not effect the temp at speed only idle or low speed.

Good luck with it. BTW I have AC(condenser in front of the radiator and a trans cooler too in front of the radiator) I change the fuel jetting on my 340 when I take it out of town, Mine acts funky too if I leave it jetted lean here at home after having it jetted for 6000'+ out west.