Cooling System Basics

Making sure your engine stays cool--and warm--when it's supposed to

Photo Courtesy: David LaChance Cooling System Basics
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Cooling System Basics from Hemmings Sports & Exotic Car

November, 2009 - Craig Fizgerald

Water and ethylene glycol rush through coolant passages to the most vulnerable parts of your engine. The coolant is called for by a thermostat, pushed through the passages by a powerful water pump, passes through at least four hoses, in most cases runs through a heater core to provide heat to the passenger cabin, and runs back to the radiator to dissipate some of the heat. A fan either running off the crankshaft, or powered electrically, draws cooling air through the radiator to cool fluid that can reach near 300 degrees.

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You've got to keep this system healthy to guarantee that your engine is not only running cool enough during operation, but that it runs warm enough in cooler temperatures so that fuel mileage doesn't suffer. It's a delicate dance between over- and underheating, and a bit of time spent diagnosing potential issues can result in a much healthier engine. Here's a guide to the components. We'll talk about some improvements next month.

Tools required
A basic tool set will take care of most of your cooling system needs, but there are a few specialty tools that are helpful. A bulb-type coolant tester will let you know whether you've mixed your coolant and water properly. A pressure tester (not shown--generally around $75) is handy for chasing leaks on a cool engine.

Coolant is a mix of ethylene glycol and water (generally a 50/50 mix, but read the container to be sure). It not only raises the boiling point and protects from freezing in the winter, but also inhibits rust and lubricates the water pump. Use distilled water when mixing coolant for your radiator. Tap water has minerals and impurities that can cause issues down the road.

Cap acts similarly to the thermostat, regulating the pressure of the cooling system instead of the temperature. Biennial replacement is cheap insurance. By pressurizing the system and adding ethylene glycol instead of straight water, coolant can reach temperatures upwards of 250 degrees. The valve opens when the pressure is exceeded, sending coolant out the overflow.

Upper and lower radiator hoses are the aorta of your cooling system. Spongy, cracked or leaking hoses should obviously be replaced. Factory-appropriate hoses should be used whenever possible. Many cooling issues can be traced to a collapsed lower radiator hose. Factory lower hoses often carried springs inside to prevent collapse, but inferior replacements might not.

Heater hoses are generally -inch in diameter, and route hot coolant from the engine behind the firewall for heat in the passenger cabin. Blocked heater hoses can cause overheating issues. Coolant coming in and out of the upper and lower hoses should be hot. If one of your heater hoses is colder than the other, chances are good that you've got a blockage.

To test the thermostat, wedge a feeler gauge between the thermostat housing and the copper cup, which contains wax and a copper pellet. Suspend the thermostat in a pan of hot water on the stove. As the temperature of the water reaches the thermostat's calibrated temperature, the valve should open and the thermostat will fall off the feeler gauge.

Mechanical cooling fans are usually belt-driven and spin in direct relation to engine RPM. Later fans use clutches that allow the fan to spin slower at higher RPM. They work well, but they work better in conjunction with factory fan shrouds. On older British cars like this Spitfire, the fan shroud is made of cardboard, and generally fell off the first month.

The radiator core is made of flattened copper or aluminum tubes with thin metal fins between the tubes. The fins transfer heat from the tubes into the stream of air forced through the radiator. The core is mounted to two tanks; one on top, one on the bottom. Years of use can corrode the cooling fins.

The water pump has a belt-driven impeller, which forces coolant through the system. Weeping from the pump's drain hole or noises like marbles should result in immediate replacement, as the pump will likely fail. Leaks at the mounting position can be cured with a new gasket and some high-temp RTV sealant.

The heater core is a small radiator with no tanks. Leaks will generally make themselves known through a smell of coolant inside the car, a wet carpet, and a greasy fog on the inside of the windshield. Replacement can be easy, or not, depending upon where it's mounted.


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