The first 2500 Model T's came with water pumps. After those first 1909 cars, the next 15,004,533 Model T Fords came from the factory with no water pump. Why no pump? The Ford thermosyphon cooling system was designed to run without it. Heat rises. As water (or coolant) is heated in the engine's water passages, it becomes less dense (lighter) and rises up through the hose into the radiator's top tank. As water in the radiator is cooled, it becomes more dense (heavier) and sinks. Over half of the water in the radiator is above the level of the engine's water passages, and its weight pushes that cooler water at the bottom up through the pipe and back into the engine where it's again heated and rises into the tank, and on and on.

Why does a radiator quit cooling as it should? One reason is accumulated foreign matter in the radiator and water passages that (1) interferes with circulation and (2) keeps the coolant from making direct contact with the heated cast iron of the block and the cooling tubes of the radiator. This is why it's important to use a good anti-freeze or rust inhibitor even in climates without freezing weather. Modern antifreezes contain rust inhibitors to prevent corrosion.

If a radiator isn't doing its job, the first step in trying to remedy that is a thorough cleaning of the cooling system. That includes the radiator, pipe, and engine water passages. Folks use all sorts of things for cleaning out the system. That includes radiator flush from the auto parts store, 50-50 vinegar and water, CLR, Simple Green, Dawn dish soap, and others. Whatever you use, apply it in both radiator and engine passages. I prefer doing them separately.

Plug the outlet and fill the radiator with whatever cleaning agent you're using and let it sit for several hours. It helps if the solution is warm, up to about 150º F. After soaking, unplug the outlet and flush the radiator with hot water.

Cleaning out the engine water passages is simliar. Plug the water inlet on the side of the engine and fill up with your cleaning solution, and let it soak for several hours. When it comes to flushing out the crud, it's a good idea to use a device like this that will shoot compressed air and water through the passages. It's made with a rubber plug that fits the water outlet on top of the engine. A piece of tubing with a hose fitting puts in water, and an air nozzle shoots in compressed air. A small hole though the plug allows air to escape and lets water in to fill up the passages. The flushing is done like this.

Another route of attack is done with the water passages dried out. Use a piece of steel cable with a frayed end. Put the other end in a drill chuck and use it as a rooter, shoving it into the freeze plug holes and other openings to knock loose all the crud you can reach, then blow out all the loosened debris with compressed air or suck it out with a shop vac.

But you may find that cleaning isn't enough to cure overheating. For this you can blame wear and tear and the passage of time. A radiator has tubes which carry the coolant. The tubes pass through thin metal fins. Heat from the coolant spreads to the fins, and the heat from the fins dissipates as it is radiated into the passing air. But on many of these old machines, decades of vibration have caused the fins to separate from the tubes. Without that direct physical contact, heat isn't passed from the tubes to the fins and into the air, and the radiator no longer radiates.

When the radiator no longer does its job, even when clean, you have two choices. One is expensive, and the other is even more expensive. The most costly cure, but often most effective, is simply to buy a new radiator. This is what I did with my black era touring car shown in the flushing video. Even after all the cleaning and flushing, the old aftermarket honeycomb radiator wasn't doing its job. So I bought a new replacement. The cost was about $800.

A less costly measure is to have the old radiator recored. That's what I chose to do with the brass radiator in my 1915 roadster. Cost was one factor, but my main reason for that choice was that the original tank was still good, and I like the original better than the tanks on the new reproduction radiators. A new core allowed me to keep the original tank, and it saved me a few hundred dollars. The recore cost me about $800, and a new brass radiator is over $1200.