My caller has a flathead that was just rebuilt. It runs great, can idle all day and not overheat, but when he drives it at highway speeds it slowly gets warmer and warmer - until it's overheating. What is the problem?

Overheating of this sort is usually from combustion gases leaking into the coolant. If there is coolant in the system, the water pump impellers are turning, and the radiator is not plugged, suspect combustion gas leaks.

In the above example, the reason I suspect combustion gases leaking into the coolant is because the overheating comes with increased load on the engine. This can happen even though the leak is small, and there may not be any water in the oil or visible water vapor in the exhaust. The cylinder pressure goes up with throttle opening. This increased pressure may cause enough hot gases to push into the coolant that the radiator can't get rid of the heat fast enough. Then the temperature goes up and up, unless the load is decreased.

Most radiator shops can test for combustion products in the coolant. One common device draws air from the cooling system through a test fluid. Typically, if it changes from blue to yellow, there are combustion products present. A tailpipe sniffer as used in emission testing, can also be used. The technician needs to know what the "background" level of HC from coolant is in order to interpret the numbers he sees (+/-200 ppm for ethylene glycol).

If you have combustion products in the cooling system, how do you fix that? By repairing head gasket seals, or the flaws in the block and/or heads. To avoid the problem, have your block completely empty, then thoroughly cleaned, then magnetic particle inspected. If there are any combustion chamber cracks, repair them or get another block. The gasket surfaces on block and heads must be flat and without blemishes in the fire ring area. The heads must also be crack free - in flatheads, this is usually the problem.

When the studs or bolts are out of the block, a small chamfer around each threaded hole will keep the gaskets from being "held up" by metal pulled up by the binding forces. Clean assembly, with proper gaskets and sealers, and torquing to the correct values in the correct sequence in multiple steps should finish the job.

What if there are no combustion gases in the coolant? Then look at the cooling system. The radiator should be clean inside and out. The fan should be as close to the radiator as practical. Baffles that fit in front of the radiator so that all incoming air must go through the radiator can help.

Non-alcohol anti-freeze/coolant will raise the boiling point: Water Wetter additive will allow the water to transfer more heat. Both have corrosion inhibitors.

The water pumps must circulate the water into the radiator for cooling. Occasionally water pump impellers are loose to the shaft on otherwise good pumps. There should be restrictors, like thermostats or plates with 5/8 inch diameter holes, to maintain some pressure in the engine to help prevent steam pockets and "teakettling".

Head gasket design has evolved to help with pressure gradients as well as flow. An overflow recycler with a proper radiator cap can help to recapture lost coolant. Timing that is not correct and lean fuel/air mixtures can produce extra heat. Big cubic engines produce more heat but are not difficult to cool if there are not combustion gas leaks. Restricted exhaust systems, at any point, add to the heat burden from the engine. (You knew I would get around to headers, right?)

What if you have the reverse of the problem given earlier?

The engine cools when down the road but not at idle or in stop-and-go traffic? The lack of fan air flow is probable, because it cools when it has ram air flow. Fan distance to the radiator, shrouds to make all air pumped by the fan come through the radiator, and slipping/loose fan drive belts are things to check.

My question to you is: In regards to the elimination of heat, does it matter how fast the coolant moves through the radiator?