Advance Essentials: Cooling System Basics


Your automotive cooling system has two jobs. First, it prevents cooling until the engine heats up to the proper operating temperature. Second, it cools the engine and maintains proper operating temperature.

Should it fail, the engine could overheat and self-destruct in as little as 10 minutes. Sometimes, drivers see the “HOT” warning light activated on the dashboard and bypass a tow, continuing to drive home to do the repairs themselves. Huge mistake. That act of driving the vehicle home to be spared the cost of a tow could actually wind up costing $3,000 or more, with the engine ultimately paying that price.  

So what is the best way to prevent cooling-system failure? Simply remember to change your coolant with regularity and replace your hoses as they age. We’ll walk you through the operation of the major cooling system components, explain what each does, give you the warning signs of impending failure, and supply some hints on how to shop for replacement parts. 
It all starts with the automotive thermostat

An automotive thermostat is really just a valve to control the flow of coolant through the engine. It contains a spring, stainless-steel pin, rubber gasket, and a wax pellet. The wax inside the pellet expands as the coolant reaches 195 degrees Fahrenheit. The expanding wax pushes the stainless-steel pin against a solid retainer, forcing the pellet backwards just enough to open the valve. Once that automotive thermostat opens, the coolant flows and cools the engine. When the engine cools down, the wax contracts and the spring forces the automotive thermostat valve closed.


Here’s how things can go wrong:

  • Continue to use worn coolant — the formula loses it anti-corrosive properties, leading to corrosion attacking all the metal parts in the engine and cooling system, including the stainless-steel pin
  • In turn, that corroded pin destroys the automotive thermostat’s rubber sealing gasket, allowing the wax to leak out
  • The automotive thermostat can then fail in either the open or closed position  

In the “stuck open” position, the automotive thermostat prevents the engine from reaching full operating temperature. So if you notice that the temperature gauge on the dashboard never reaches the “normal” range, or the heater never expels hot air, suspect a “stuck open” automotive thermostat.

Conversely, a “stuck-closed” automotive thermostat can cause the engine to overheat quickly. So if you pull out of your driveway and barely cover any distance before seeing that “HOT” warning lighting up on your dash, suspect a “stuck closed” automotive thermostat.

Your car’s automotive thermostat is the part that fails most often. Luckily, it’s also the least-expensive part of the cooling system. Simply remember to replace it with a premium STANT thermostat every time you change your coolant. Why the premium STANT thermostat brand? Because we’ve seen for ourselves that STANT thermostats are the world’s best thermostats. Ask for it by name: STANT thermostat. Buying a new thermostat

All automotive thermostats have a temperature rating stamped onto the bottom of the wax pellet. Some drivers assume they can cure an overheating engine by either removing the automotive thermostat or installing an automotive thermostat with a lower temperature rating. Wrong! A colder automotive thermostat won’t cure a cooling system’s underlying problem. In fact, it can lower gas mileage, cause poor acceleration, instigate misfires, even damage the catalytic converter. Always select the automotive thermostat with the temperature rating specified by your vehicle manufacturer.

Again, we recommend investing in premium-brand automotive thermostats, and one great choice is STANT thermostats. The price difference between an economy automotive thermostat and a premium STANT thermostat is only a few bucks. Don’t forget the tube of RTV silicone, fresh coolant to replace the liquid that you lose, and a shop manual for proper installation technique. Water pumps make the coolant go ‘round and ‘round

Water pumps are pretty simple devices. With a drive pulley on one side, an impeller on the other, and a bearing/seal in the middle, you have all the makings of a water pump. The water pump system can fail in two ways:

  • The water pump seal can fail, causing the water pump to leak — if you notice coolant leaking from your water pump, and it’s more than just an occasional droplet, you’ve got a water pump that’s about to fail so get set to replace it as soon as possible
  • The water pump impeller inside the water pump can fail — the plastic variety are especially prone to premature failure
  • Air in the cooling system can cause a metal impeller to fail as well

If you’ve got an overheating problem, and you’ve checked the radiator fan and radiator, plus replaced the thermostat, chances are you’ve got a failing water pump. Replace it ASAP. Choose from a new or rebuilt water pump, depending on the age of the vehicle and your budget. Be sure to also include a tube of RTV water pump sealer and fresh coolant to replace what you lose during the water pump repair. The radiator does all the hard work

To remove heat, the radiator fan blows outside air across thin fin tubes filled with hot coolant. If the fin tubes are clogged on the outside of the radiator, air can’t flow past them and the radiator fan can’t do its job to cool down the tubes. Start troubleshooting by inspecting the radiator’s exterior. If you see bugs or dirt clogging the fins, hose down the radiator with a garden hose and spray nozzle. If that doesn’t solve the overheating problem, check the condition of the radiator coolant. With the engine cold, remove the radiator cap and examine the coolant. If what you see after twisting off the radiator cap looks brown, muddy, rusty, or has debris floating in it, you’ve most likely got a clogged radiator. Before replacing the radiator cap, you can try cleaning the radiator. You can do that with a radiator cleaning chemical and a cooling system flush. Put the radiator cap back on and see if the overheating is stopped. If that still doesn’t do the trick, no doubt the system is so corroded and plugged, the only option left is to replace the radiator.

Replacing a radiator isn’t difficult, but you’ll need a special transmission cooling-line removal tool to dislodge automatic transmission lines. Check our tool selection for the correct removal tool for your vehicle. Next up: drain the coolant, disconnect the transmission lines and swap out the radiator. We recommend replacing both the upper and lower radiator hoses at the same time you do this radiator swap. For this additional task, you’ll also need to have on hand hose clamps, a new radiator cap, fresh coolant, and a bottle of transmission fluid as recommended by your vehicle’s manual.       Coolant temperature sensor

Many late-model cars and trucks use electric radiator fans. To cycle the fans on and off, the computer relies on signals from the engine coolant temperature sensor mounted at the top of the engine. When the computer registers that the coolant temperature sensor reading has risen above a certain point, it turns on the radiator fans. However, if the engine coolant temperature sensor fails, the computer never turns on the radiator fans, leaving the engine to overheat.

How do you know whether the problem is the engine coolant temperature sensor or the electric radiator fan? Here’s a simple temperature sensor test. Start the vehicle and let it warm up a bit (but don’t let the dashboard temperature gauge go beyond “normal” range). Next, turn on the A/C and set it to “MAX” or “RECIRCULATE” mode. Now look at the radiator fans to see if they’re running. If they are, that confirms the fan relay and radiator fans are, indeed, working properly. If they don’t run, check the fuse. If that’s good, your problem may be a faulty engine coolant temperature sensor. Temperature sensors fail quite often. You can test the temperature sensor with a digital multi-meter and a specification chart. Ultimately, it’s just easier to replace that engine coolant temperature sensor, which are relatively inexpensive — just unscrew the old temperature sensor and swap in the new temperature sensor. Radiator fan

Radiator fans come in two varieties, mechanical and electric. Mechanical fans are powered by the car’s engine, and the fan itself rarely fails. But here’s the clincher: most mechanical fans are attached to a thermostatically controlled clutch mechanism which fail quite often. If you have a mechanical fan and your engine overheats in city driving but is fine at highway speeds, you may have a failed clutch. Another sign of clutch failure: excessive fan noise, even when the vehicle is on idle. Clutches are easy to replace. Just remove the fan, the clutch retaining nuts, then swap in the new unit.

Electric radiator fans have a fairly low failure rate. But if the fan continually blows the fuse, it’s a safe guess that it’s on its way out. Replace the entire fan assembly. But first, consult your owner’s manual and follow the manufacturer’s recommendations to ensure warranties are not voided.