You can see it coming. There's a light mist seeping out of gaps between the hood and the front fenders. Then you're hit with the unmistakable scent of seriously scorched antifreeze. There's no doubt about it: The engine compartment is about to get a steam bath and there's not much you can do about it. The truth is, cooling system grief is unpleasant, but it may not be that difficult to diagnose. Here's the rundown on cooling system troubleshooting, beginning with a brief look at how the system functions.
Cooling system overview
The cooling system in your car or light truck includes the radiator, the radiator cap, the water pump and the fan, along with the thermostat and the hoses, which obviously connect the radiator to the engine. In cars with a mechanical water fan, the system is driven by the same drive belt that turns the water pump. In most applications, the water pump moves coolant from the bottom of the radiator into the engine.
From there, the coolant is then circulated through internal water jackets (cast inside the engine block and heads). The coolant removes excess heat from the engine and the now-hot coolant is pumped out through the thermostat and the upper radiator hose to the reservoir at the side or top of the radiator. The hot coolant is pushed slowly through the radiator core, which is nothing more than a series of tiny tubes surrounded by fins.
Heat is radiated from the water to the surrounding air by way of the fins. When the now-cooled coolant reaches the bottom or opposite-side tank of the radiator, it's transferred back to the engine by way of the water pump and the process starts again (keeping in mind the practice is constant).
The job assigned to the fan is to ensure a sufficient flow of air through the radiator core while the vehicle is stopped (engine idling) or when the vehicle is moving slowly. With no fan (electric or mechanical), the car radiator cannot cope with the heat transfer if the car is stopped or moving slowly. The end result would be a boil-over.
Something that occurs regularly with many vehicles is a buildup of scale and rust inside the cooling system. This creates trouble since it takes away the ability to transfer heat effectively. Typically, it plugs the radiator tubes (think of it as blocked artery disease for the cooling system). The reason it occurs is because the vehicle was likely operated for some time without proper coolant (or straight water) or the coolant wasn't changed. Believe it or not, coolant can wear out, just like oil. Consult your owner's manual for information on scheduled car cooling system maintenance.
Checking coolant levels
When the time comes to troubleshoot the cooling system in your car or light truck, examine the obvious first: If the vehicle is low on coolant, it will tend to heat up, and it will usually show up as a warning lamp coming on ("Hot" or "Check Coolant") or the temperature gauge will read high. The bottom line here: Check the coolant level first. But be careful! Use extreme caution when checking the coolant level in the radiator. First, allow the engine and radiator to cool to a safe level. Then, put on rubber gloves and the proper safety eyewear, like goggles. Last, place a towel over the radiator cap while slowly loosening it. If you don't heed this warning, you’ll run the risk of injury (severe burns) from extremely hot coolant.
If the coolant level is down in the system, inspect the upper and lower radiator hoses for leaks. Look for rust stains near the water pump and radiator. Those stains just might lead you to a slow leak. The various engine freeze plugs are another seldom-considered portion of the cooling system that you should examine for leaks. These are small, soft metal plugs that are press fit to the sides of the engine block and sometimes in the ends of cylinder heads to fill holes left by the casting process. They also serve as cooling system safety valves.
If an engine freezes due to a lack of antifreeze coolant, the idea is that the freeze plugs will pop out first before the block or cylinder heads crack. Over time, these freeze plugs can corrode and rust, which can create leaks. Examine them carefully.
Radiator caps that are incorporated to seal the cooling system maintain the system as closed until pressure reaches the maximum (cap rating). At this point, the radiator cap relief valve opens and the pressure inside the system is released to an overflow tube. Later model cars and light trucks incorporate a coolant recovery system. Here, the car radiator is closed at all times and under pressure. As the pressure builds up in the system (with the engine running), the radiator cap pressure relief valve still opens, but instead of forcing the fluid out the overflow tube and onto the ground, coolant is forced into a holding tank. When the temperature drops in the radiator (with the engine turned off), the drop in cooling system pressure creates a slight vacuum. This allows the coolant in the recovery tank to be drawn back into the radiator.
The job of the thermostat is to maintain a specific temperature level within the cooling system. The thermostat is designed to restrict the flow of coolant through the system until the coolant temperature in the engine reaches the appropriate level. At this point, the thermostat opens, allowing all of the coolant to circulate through the car radiator. Obviously, different engines mandate different thermostats, each with a specific operating range.
In order to test the thermostat, remove it from the engine. Most thermostats are stamped with the temperature rating. Next, simply suspend the thermostat in a pot of hot water on your stovetop. A cooking thermometer can be used to determine the water temperature along with opening point of the thermostat. As the water in the pot reaches the temperature rating of the thermostat, the valve on the thermostat should open. When allowed to cool, the thermostat should close. If it doesn't, replace it.
Anywhere in the snow zone of North America, extended sub-zero cold can cause hoses and drive belts to become brittle. You should check the belts and hoses carefully. Winter is the wrong time of year to be stuck on the side of the road with cooling issues. There's more to consider, too: Under certain conditions, the temperature of the coolant in the radiator may drop enough to freeze – this is especially true if plain water or antifreeze diluted with too much water is used in the system.
If your vehicle overheats in very cold weather without any external indication of cooling issues, this may be the reason. Obviously, the solution is to increase the concentration of antifreeze, but in order to get you back on the road, a quick (temporary) fix is to block off a part of the radiator with a piece of cardboard. The cardboard restricts the flow of air through the cooling fins, which in turn sufficiently raises the temperature of the coolant within the radiator and prevents freezing. There's a catch though: If you block too much of the radiator, the engine will overheat.
Water pump and belts
The final pieces of the cooling system puzzle include the water pump and the belts. Pumps are usually very reliable. They're rather simple devices, and the only issue you might come across is a seal leak or perhaps a bad bearing. If a seal leaks, you can usually spot it with a simple inspection. Take a close look at the bottom of the pump. Many pumps have a small hole on the base: If the pump has a bad seal, water (coolant) will usually exit at this point. Keep in mind that pumps aren't easily repaired - they require specialized tools). It's best to replace a pump with a bad seal or bad bearings, which you can easily hear if they're noisy.
Finally, check the fan belt tension. On a v-belt drive, it's usually best to keep the belt sufficiently tight so that you can move it downward only about half an inch. Both the fan belt and the pump itself will often give an indication of trouble by making squealing noises. Fans can either be mechanical or electric. They seldom cause trouble. Nonetheless, be sure an electric fan actually functions.
With the vehicle parked and the engine at idling and at operating temperature, watch the fan. As the engine idles and the temperature of the cooling system increases, the fan should eventually start. Be careful: Keep your hands away from the fan! If the fan doesn't start, look for trouble in the wiring or electrical system. Relays or fuses are good places to check. If the vehicle has a clutch fan – one that disconnects as the engine turns high RPM – the drive can fail. This can cause the vehicle to overheat by preventing the fan from turning at low engine speeds. In this case, the clutch mechanism isn't repairable. It must be replaced.