Keeping Cool

Radiator Caps

Cooling systems are pressurized to raise the boiling point of the coolant, since normal engine temps are significantly higher than the boiling point of water at atmospheric pressure. The pressure level is usually maintained by a blow-off valve incorporated into the radiator cap. When system pressure exceeds a predetermined level, the valve opens and bleeds off pressure until it reaches the desired level, at which point the valve closes. If the spring in the cap weakens or the rubber seal deteriorates, system pressure drops and the coolant can boil, which then aerates it and reduces its effectiveness drastically. This condition will also allow coolant to be purged into the overflow container, though it will not be drawn back in when the system cools. Don't ignore that hissing and gurgling you're hearing every time you shut off the engine.

Fans

Without airflow, a radiator is relatively inefficient, and will serve mainly as a tank for increasingly hot coolant. Since automobiles cannot rely solely on the airflow generated by vehicle movement, a fan is used to create airflow. The standard in cooling fans for many decades was the engine-driven mechanical fan. Using the engine to spin the fan seems an obvious choice, but variations in engine rpm can be an issue. Clutched mechanical fans can alleviate some of these issues, though electric fans eliminate engine rpm as a cooling system variable altogether. Electric cooling fans were generally avoided by manufacturers prior to the '70s, mostly because of reliability concerns, both for the fans and the vehicle's electrical system. Both have improved drastically over the years. Airflow generated by the fan(s) should also be directed to be truly effective.

A fixed-blade fan is the most basic type and probably the least effective. To generate sufficient airflow at low rpm, the fan blades must be appropriately shaped, which becomes a detriment at high rpm, drawing excessive power from the engine. Flex fans are a simple alternative that still use fixed blades, though they flatten out as engine rpm climbs. In theory, this provides maximum airflow at idle and reduced drag at speed, but beware--not all flex fans are created equal, and some can actually sap more power from the engine than a stocker.

Electric fans are quite common on modern passenger cars and trucks, even in applications where a mechanical fan could be fitted. Manufacturers prefer electric fans because they are the easiest to control. A typical OE electric fan system will use temperature sensors to trigger the fans on and off, and most activate the fan(s) automatically if the A/C is turned on. Problems with electric fans are usually with the temperature switches and fan relays. Know where these are located on your vehicle so you can test them if a problem arises on the road. Temp switches can often be jump-wired in an emergency; carrying a spare fan relay is an even better idea.

To be really effective, fans should be housed in a shroud. The shroud concentrates the airflow generated by the fan and is usually shaped to spread it over as much of the radiator's core as possible. Without a shroud, the fan is only moving air over a section of the core equal to the area inside the circumference of the fan's blade rotation. The fan should also be positioned so that the blade tips are about halfway out of the lip of the shroud.

The clutch fan enables the use of relatively large, multibladed fans that are only driven directly when needed. The clutching is usually controlled thermostatically; heat coming off the radiator causes a bi-metallic spring (similar to a choke spring) to expand, turning the clutch spindle and tightening its engagement. These are very effective both at cooling and at minimizing the power drain from the engine, but the clutches do wear and can fail. A seized clutch fan usually won't cause a cooling problem, though it will draw an excessive amount of power. More commonly, a failing clutch will slip. If your engine is running hot at idle, try placing a small piece of paper over the front of the radiator (or A/C condenser) with the engine at idle and see if it is drawn to the surface. If not, shut the engine off and spin the fan by hand. If it freewheels with little resistance, you've likely found the problem.