Replenishing Radiator Fluid After a Coolant Leak on Motor MythBusters

The predominant pop-culture car overheating scenario: A lone car drives down a dusty highway, surrounded by parched earth, heat haze blurs the view ahead and POOMFSSSS! Steam starts billowing from under the hood. Obviously, there’s a coolant leak, but the intrepid travelers have no reserve of radiator fluid to top off and escape the desert heat. The Motor MythBusters might have a solution for this problem: peeing in the radiator.

Related: This isn’t the Motor MythBusters‘ first experience with bathroom-related car myths. Find out if birds prefer to poop on red cars, and catch up on the other 12 episodes of Motor MythBusters only on MotorTrend+! Subscribe or renew today for only $44.99 for one year and receive a limited edition Hot Wheels Roadkill General Mayhem (bundle valued at $80)! 

Peeing in a car’s radiator is such a common myth that we can’t attribute it to any one movie or internet forum. But would urine actually work as a radiator fluid substitute if a car had a coolant leak? To test this myth, the Motor MythBusters not only have to find out if urine has a high enough boiling point to function as an effective coolant, they also have to figure out how much urine it would take to top off a car’s cooling system. Then there’s the question: Is urine too corrosive to work in a car’s cooling system?

How Does a Car Cooling System Work?

Not all cars have a water-cooled engine. Vintage Porsches and Volkswagens are famous for being air-cooled, but the principle for cooling in either configuration is the same: heat convection. Heat convection is the transfer of thermal energy in a fluid—liquids, gases, and plasma are all fluids—through physical movement. Weather patterns are Mother Nature’s grandest example of heat convection, trying to equalize temperatures all over the planet. In cold climates, windchill factor is often reported with the weather—basically, how much colder it feels because of the breeze.

In a water-cooled engine, the engine block and cylinder heads are filled with passages that pass by cylinder bores and through the heads. Liquid coolant is pressurized and forced to circulate through those passages by a pump, pulling heat out of the engine, which is also connected to an external radiator. As the hot coolant passes through the radiator, the cooler air circulating around the radiator pulls heat out of the coolant, which then gets pumped back into the engine.

For this system to work effectively, the liquid coolant must be able to absorb and dissipate heat quickly while not boiling under extreme conditions, and the radiator needs adequate airflow to remove enough heat for the engine to operate properly. That’s why auto manufacturers included fans in the design of water-cooled engine cooling systems nearly from inception, to keep air flowing around the radiator when the car isn’t moving.

What Makes an Effective Coolant?

Water has a boiling point of 212 degrees Fahrenheit at sea level and most cars operate optimally in the 180-220 degree range. You might think that precludes water from being an effective coolant, but we mentioned sea level for a reason, where the air pressure is 14.7 psi. As the pressure around water increases—say by heating it up in a closed system, like a car’s cooling system—the boiling point increases as well. Grandma’s old pressure cooker could get a pot of water well above 212 degrees, but as soon as the pressure is released, superheated water explodes into steam.

Auto engineers took care of this by adding glycol and other additives to the water in a car’s cooling system, raising the boiling point as well as preventing freezing in cold climates. This is all great if you carry a jug of antifreeze coolant with you at all times, but what else can you put in a car’s cooling system that will get you down the road without overheating? Any liquid that goes into a water-cooled car’s cooling system needs to absorb and dissipate heat quickly, plus have a high boiling point and low freezing point.

Can You Pee in a Car’s Radiator?

There’s also the metallurgy of a car’s cooling system to take into account; standard antifreeze coolant is non-corrosive and urine can be alkaline or acidic, depending on the individual’s diet. Urine is also electrolytic, making it a good conductor. A car’s cooling system can contain a multitude of metals, including steel, aluminum, brass, and iron. When dissimilar metals are in contact with electrolytic liquids together, the corrosion process can speed up dramatically.

We’ve barely even discussed the volume of liquid it takes to fill a car’s cooling system. Between reservoir tanks, coolant passages, hoses, and pumps—does the average human bladder, even when full, contain enough urine to adequately fill a radiator and then some? The Motor MythBusters have their work cut out in testing urine as an effective liquid coolant, but at least they don’t have to worry about where the closest restroom is.

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