Yesterday my wife and I were driving down the interstate. The outside temp was probably 85, the sun was shining, and I was moving right along with 75 mph traffic, top down. Occasional bursts to maybe 90 mph, when traffic permitted. '74 L48 small block, M20, 3.36 rear gearset, and the AC was off. The car began to overheat. I slowed down and put on the cabin heater full blast to let it cool off. It finally cooled down a bit when I got off the highway. When I got home, I checked the engine temps with an infrared thermometer, and found it was still almost 240F in the oil pan, and at the middle of the rocker arm covers. Curiously, the front and rear of the covers were slightly cooler. (the heat riser has been removed) Water temp was down to about 195F at the radiator inlet, by that time. The radiator outlet was maybe 165F or thereabouts. (Complete stock fan shrouding still in place. )
I had the engine rebuilt over the winter, at about 9.3 to 1 compression. Otherwise mostly stock, probably making 275 HP, max. I had the radiater boiled out and a small leak repaired at the same time. When I put it all back together , I also put in a rebuilt water pump from NAPA, and a fresh 180F thermostat.
I'm wondering if my problem could be fixed by a high volume water pump, like Edelbrock makes. Or could it be something else? Maybe a restrictive exhaust? Bad carb calibration? Total timing is 32 degrees mechanical at 2800 rpm, plus 15 degrees vacuum advance.
I'm sweating for an answer.

 

We had our 1939 Buick rebuilt and after a few runs, it started to overheat really badly. Tried everything. Had a new core during the rebuild, new thermostat, you name it. After trying absolutely everything, I finally took the radiator out and got it rodded. Turns out it was 80% blocked with crap. Mostly silicon gasket stuff that had squeezed out from when the waterpump and thermostat housing had been tightened up! Just a thought....
Cheers
Grant

 

You should not need a high volume water pump to keep it cool. You didn't mention anything about the seals on the radiator between the rad and the rad support, check them and replace if necessary. Also, at highway speed the Air Dam should be directing air up and into the radiator. If the dam is missing or dammaged it will not get enough air up there to properly cool. The Dam and Seals work together. Check those out first before ripping into the pump.

 

First, does it over heat at idle, or only when you get on it?

 

Generally, on the freeway, you have sufficient aitflow. I would check radiator efficiency / size.

 

Define "overheat." 220 at highway and higher speeds in summer is not overheating. What did the gauge read?

 

If the inlet is 195 and the outlet is 165 the rad and airflow is good, whenever or wherever you got those measurements.
Next time take the gun with you and reread those temps when you think there is a problem.

If you have a 180° tstat and your getting 195° at the rad inlet, then I would check the tstat, might not be opening properly.

 

Is the car losing any coolant or is it at a constant level? I would double check the level at operating temp and make sure all the air pockets are out.

My car has had a tiny leak at the thermostat. The coolant boils off so you don't see it but the system is no longer under the correct pressure so boiling point is lower.

 

Never even runs slightly hot around town or in traffic. It usually stabilizes at about 195F . The coolant is not leaking. The spoiler, shrouding and seals are all good.
Normally, on the highway I run less than 70 mph. There, it usually runs at about 205F.
But that day, with higher sustained speeds, it probably hit 235F(?). Hard to say, exactly. The temp gauge calibration is a bit uncertain. But it was definitely a whole lot hotter than I ever saw it before.
Perhaps it's normal? Or maybe I could stand an engine oil cooler?

 

Fan clutch not disengaging?

 

Yep. That's a tad on the warm side. Try it again this weekend and see if the temp goes up again.

 

Fan clutch.