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Cooling System, Page 1 of 2

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Your engine needs a cooling system to protect it from self-destruction. Burning gases inside the cylinders can reach a temperature of 4500°F (2500°C) and produce enough heat to melt a 200 lb. engine block.

About one-third of the heat produced in the engine must be carried away by the cooling system. Some is utilized for heating the passenger compartment. Moreover, strange as it seems, your vehicle's air conditioner produces heat in the process of cooling and dehumidifying the air. This heat must also be dispersed by the cooling system.

How the cooling system works
See Figures 1, 2 and 3

The main parts of the engine cooling system are the radiator, pressure cap, hoses, thermostat, water pump, fan, and fan belt (except on electric fan engines). The system is filled with coolant, which should be a 50-50 mixture of antifreeze and water. No matter where you live or how hot or cold the weather becomes, the mixture should be maintained the year around.

The water pump and engine cooling fan are mounted on the same shaft and driven by a belt connected to the engine. The pump draws coolant from the bottom of the radiator and forces it through passages surrounding the hot area-the cylinders, combustion chambers, valves and spark plugs. From there the coolant flows through a hose into the top of the radiator, then downward through tubes attached to cooling fins and surrounded by air passages. Heat is transferred from the coolant to air forced through the radiator passages by the fan and the forward motion of the vehicle.

Figure 1 Most late-model automotive radiators have coolant recovery systems as shown on left. Early model non-recovery systems are shown on right.
Most late model automotive radiators have coolant recovery systems as shown.

Figure 2 The internal combustion engine converts about one-third of the heat it develops into power. Another third is lost in the exhaust system and the remaining third must be carried away by the cooling system. The operating temperatures (given in the Fahrenheit scale) are typical of a modern engine.
The internal combustion engine converts about one-third of the heat it develops into power.

Some GM vehicles may be equipped with a reverse-flow cooling system which operates differently than a conventional type cooling system. The specialized components of this system include a gear-driven water pump with cast internal cross-over passages, an inlet-side thermostat and a pressurized high fill coolant reservoir. In this system, coolant is routed from the water pump, directly to the cylinder heads. When the heads are adequately cooled, any accumulated vapors are vented off, and the coolant then circulates through the engine block. After the coolant leaves the engine block, it returns to the water pump, moving through an internal passage into the radiator. There is a thermostat on the inlet side of the pump which meters the coolant temperature as it flows from the radiator and tries to enter the water pump casting. The water pump, which is driven by the camshaft, which is the center of the system, has cast internal passages which route coolant through the engine without sending it through the intake manifold, eliminating possible leaks. The gear-driven pump ensures coolant flow even if the drive belt breaks. The reverse-flow cooling system is advantageous because it reduces the overall cooling system pressure and basically eliminates pitting or disintegration of the water pump and seal. In addition, routing the coolant to the cylinder heads first promotes higher bore temperatures and less ring bore friction, increasing output and horsepower. Due to the increased temperature of the cylinder walls, which created higher oil temperatures, engines with reverse-flow cooling systems may be equipped with an engine oil cooler or use synthetic oil.

Figure 3 Some vehicles employ a "reverse-flow" type of cooling system, which operates differently than a conventional system.
Some vehicles employ a reverse-flow type of cooling system.

What is coolant

When draining the coolant, keep in mind that cats and dogs are attracted by ethylene glycol antifreeze, and are quite likely to drink any that is left in an uncovered container or in puddles on the ground. This will prove fatal in sufficient quantity. Always drain the coolant into a leak-proof container. To avoid injuries from scalding fluid and steam, DO NOT remove the radiator cap while the engine and radiator are still HOT. The best way to dispose of coolant is through an approved recycling center.

Ethylene glycol

Coolant in most late model vehicles is at least 50-50 mixture of Ethylene glycol and water. This mixture in older vehicles was required not only in the winter to prevent freezing, but also to prevent corrosion in aluminum cooling systems, and to provide lubricants to the water pump. Modern vehicles with air-conditioning must also use it in the summer as well.

Late-model vehicle manufacturers also require their engines to run at a higher temperature because it results in better engine efficiency and improves the effectiveness of emission control devices. This temperature is controlled by the thermostat, most of which are in the 192°F (89°C)-195°F (91°C) range.

Good quality antifreezes also contain water pump lubricants, rust inhibitors, and other corrosion inhibitors along with acid neutralizers. Ethylene glycol antifreeze mixtures should not remain in the cooling system beyond one year.

Propylene glycol

Appearing in the early 90's a new, less-toxic antifreeze/coolant emerged. This is a propylene glycol base. As compared to ethylene glycol, propylene glycol is less toxic and safer for humans, pets, and wildlife in the environment. Its coolant and engine protection properties are similar to the ethylene glycol coolant listed above. Most of the coolant providers now offer a choice between ethylene glycol- or propylene glycol-based products.

Silicate-free coolant

The cooling systems on some 1996 and later vehicles were originally filled with silicate-free coolant. The fluid is easily identified because of its orange color (instead of the green we have come to expect from most types of ethylene glycol antifreeze). If your cooling system is filled with DEX-COOL® or other silicate-free coolant, then no periodic service is required, other than fluid level checks, for 100,000 miles (160,000 km) or 5 years, whichever comes first. However, if you add a silicate coolant to the system (even in small amounts) premature engine, heater core or radiator corrosion may result. In addition, the coolant will have to be changed sooner (12,000 miles (19,300 km) or every year, just like other vehicles not using DEX-COOL® or other Silicate-free coolant).

Controlling the temperature

It's important to get the coolant up to normal operating temperature as quickly as possible to ensure smooth engine operation, free flow of oil, and ample heat for the occupants. When the engine is cold, the thermostat blocks the passage from the cylinder head to the radiator and sends coolant on a shortcut to the water pump. The cooling fluid is not exposed to the blast of air from the radiator, so it warms up rapidly. As temperature increases, the thermostat gradually opens and allows coolant to flow through the radiator.

Cooling systems on older vehicles were limited to a maximum temperature of 212°F (100°C)-the boiling point of water.To get rid of the extra heat generated by more powerful engines, automatic transmissions, and air conditioning, modern vehicles have pressurized systems using a 50-50 mixture of antifreeze and water which enables them to operate at temperatures up to 263°F (129.4°C) without boiling. At this temperature, plain water alone would boil away.

Transmission oil cooler

Automatic transmission oil is cooled by a small, separate radiator, usually located in the lower tank or alongside the main radiator. While the only purpose of the engine coolant is to keep the block and upper engine components cool, the transmission oil has three functions. It hydraulically operates the transmission, lubricates the transmission internal components and also keeps the transmission within proper operating temperature range.

Most vehicles can benefit from the installation of a transmission oil cooler, especially if the vehicle is used to pull a trailer or for some other kind of heavy service.

Engine oil cooler

Some vehicles, particularly high performance or diesel engines, are equipped with an engine oil cooler. The oil cooler can be located in one of the radiator tanks (similar to the transmission oil cooler), or mounted separately, near the front of the engine, on aftermarket applications. Also, there is usually an adapter mounted between the engine block and the oil filter. If the cooler is mounted in a radiator tank, heat is dissipated from the oil to the coolant. A separate oil cooler usually looks like a small radiator and heat is dispersed from the tubes and fins in the cooler to the air passing through the cooler. The purpose of the cooler is to reduce the temperature of the oil, preventing oxidation and increasing the oil's lubricating and protecting properties.

Electric cooling fans
See Figure 4

Many late model vehicles are equipped with an electric cooling fan or an auxiliary electric cooling fan. These fans are usually operated by a thermostatically controlled switch located in the cylinder head, radiator tank, or intake manifold. When the thermostatic switch reaches a specified temperature, the contacts close providing current to the motor. As the temperature lowers to the specified range, the contacts open thus shutting off the cooling fan.

The thermostatic switches may be used to complete the ground side of the fan motor, they may trigger a relay to provide power to the fan motor, or they may send a signal to a computer to energize the fan motor.

On some air conditioner equipped vehicles, the fan motor is energized at all times the air conditioner is operating. This increases the airflow through the air conditioning condenser and the radiator.

Figure 4 Typical auxiliary electric cooling fan mounted next to a mechanical fan.
Click on picture to enlarge view

What to do when the engine overheats

See Figures 5, 6, 7, 8 and 9

Never remove the radiator cap under any conditions while the engine is hot! Failure to follow these instructions could result in damage to the cooling system, engine and/or personal injury. To avoid having scalding hot coolant or steam blow out of the radiator, use extreme care whenever you are removing the radiator cap. Wait until the engine has cooled, then wrap a thick cloth around the radiator cap and turn it slowly to the first stop. Step back while the pressure is released from the cooling system. When you are sure the pressure has been released, press down on the radiator cap (still have the cloth in position) turn and remove the radiator cap.

Air conditioning, automatic transmission, and power-operated accessories put an extra burden on the engine cooling system. The temperature light or gauge is designed to activate as the engine begins to overheat. This gives the driver a chance to correct the cause of overheating with minimum delay. If you are stuck in heavy traffic and the temperature gauge shows the engine is overheating or the temperature light comes on, shut off the air conditioner.

If your vehicle has a belt driven fan, whenever you come to a stop, shift into neutral and speed up the engine a little to increase circulation of the coolant and air flow from the fan. Most front wheel drive vehicles now have electric cooling fans. Increasing the engine speed won't help on these vehicles.

If the temperature light turns on or the temperature gauge indicates overheating when the air conditioner is running, follow these steps:

  1. Turn off the air conditioning, then turn the heater on and the blower motor on high. Turning the heater and blower motor on will help get rid of some of the extra heat from the engine.
  2. If the light doesn't go out in about a minute, pull over in a safe place and set the parking brake. Then place the transmission selector lever in park.
  3. Don't turn off the engine, instead, speed up the engine (on vehicles with belt driven fans only) so it sounds as if it's idling twice as fast as normal. Lift the engine hood and check for fluid leaks at the radiator hoses, radiator, or radiator overflow outlet. Check to see that drive belts are intact, fan is turning (either the mechanical or electric fan), and radiator cap is sealed. The overheating should subside.
  4. When the overheating has passed, proceed on the road a little slower, and don't resume normal driving for 10 minutes.
  5. If the radiator starts boiling over, pull off the road as soon as possible. Shut off the engine. When the boiling stops, raise the hood, but don't touch the radiator cap. Allow the system to cool.
  6. On vehicles without coolant overflow systems, place a cloth over the cap and slowly turn it to the first notch to relieve the pressure. Remove the cap, start the engine, and slowly add water, or a water/coolant mixture if possible. Replace the cap.
  7. On vehicles with coolant overflow systems, simply add a water/coolant mixture to the reservoir.
  8. Never open the radiator cap when the engine is hot; the release of pressure will precipitate boiling and further overheating and may scald anyone nearby in the process. If the engine is losing coolant, or a fan belt is broken or loose, or if the overheating persists, stop the engine until the cause of the overheating is corrected.
  9. At the first opportunity, check the system to find out why it overheated. Refill with the correct mix of the antifreeze and water.

Figure 5 Loss of coolant is not usually a mystery if you know what to look for. Coolant leaks usually show up as a puddle on the garage floor or driveway. External coolant leaks are likely to occur at loose hose clamps, leaking hoses, leaking radiator, leak at the thermostat, leak at the radiator petcock, loose water pump bolts, a failed water pump seal, faulty radiator cap, loose freeze plugs, or a leaking heater core (this will sometimes leak coolant inside the passenger compartment).
Loss of coolant is not usually a mystery if you know what to look for.

Figure 6 Vehicles with coolant overflow systems use a plastic bottle to catch the expanding coolant. These are sealed systems and the radiator cap should not be removed. Check the coolant level and add coolant through the reservoir.
Vehicles with coolant overflow systems use a plastic bottle to catch the expanding coolant.

Figure 7 On vehicles without coolant overflow systems special care should be taken when removing the radiator cap. Allow the system to cool and use a rag to prevent burns and slowly release the system pressure.
On vehicles without coolant overflow systems special care should be taken when removing the radiator cap.

Figure 8 Coolant level should be 1-2 inches (25-50mm) below the filler neck on systems without coolant recovery (left). This allows for coolant expansion within the tank. On coolant recovery systems (right), maintain the level at the mark indicated on the reservoir.
Coolant level

Figure 9 Lever-type radiator caps were used on systems without coolant overflow. These make releasing the pressure safer and easier. When the system is under pressure, simply lifting the lever will release the system pressure allowing the safe removal of the cap.
Lever-type radiator caps were used on systems without coolant overflow.

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©1998 W. G. Nichols - Chilton's Easy Car Care