The last thing any motorist wants to see is a TEMP warning light flashing in his/her face, or worse yet, steam billowing out from under the hood of the vehicle. Yet it happens all too often. The hotter the weather, the greater the load on the cooling system and the greater the chance of the engine overheating if the cooling system can’t handle the heat.
About one-third of the energy that’s produced in a typical internal combustion engine by burning gasoline ends up as waste heat that must be absorbed, carried away and gotten rid of by the cooling system. A 3.0L V6 engine pushing a 3,800-lb. car down the highway at 60 mph produces enough waste heat to comfortably heat a three-bedroom house in the dead of winter.
Coolant circulating through the engine block and heads serves as a heat sink or reservoir to absorb heat from the surrounding metal. Heat always flows from an area with the highest temperature to surrounding areas that are cooler. In other words, it spreads outward from the source. So even though the "normal" operating temperature of the coolant is up around 195° to 215° F after the engine has warmed up, it is still cooler, relatively speaking, than the hot metal surfaces inside the block and heads that are exposed to combustion temperatures of 2,000° under light load, and up to 3,000° or higher under heavy load.
At 60 mph, the water pump may be pushing as much as 20 to 30 gallons of coolant per minute through the engine and radiator to keep up with the heat load! When the hot coolant reaches the radiator, it’s cooled by the ram air flow coming through the grille. But if the air conditioner is also running, the condenser mounted ahead of the radiator is probably heating the incoming air up an additional 20° to 40° before it reaches the radiator.
Many cars today have tiny grille openings or no grille openings at all for the radiator. They’re "ground breathers" that use an air dam under the radiator to deflect air up and through the radiator. If the air dam has been knocked off, or if the radiator is full of dead bugs and debris, the coolant isn’t going to receive much cooling as it passes through the radiator.
At speeds below 30 mph, the cooling system usually needs the help of the cooling fan to increase airflow through the radiator. If a belt-driven fan clutch is slipping, the fan may not turn fast enough to prevent the engine from overheating. A missing fan shroud will also reduce the volume of air the fan can pull through the radiator, and may allow coolant temperatures to rise to the point where the engine overheats. With electric cooling fans, a bad fan motor, fan relay or temperature sensor can prevent the fan from coming on when it is needed.
Too Hot To Handle
Piston scuffing can occur if the pistons get too hot, and valves can stick if the stems overheat and run out of clearance in the guides. Thermal overload also puts tremendous stress on the cylinder head, and in particular, aluminum heads because aluminum expands at a much higher rate than cast iron when it gets hot. This, in turn, may crush the head gasket between adjacent cylinders causing the gasket to fail and leak. A hot head can also succumb to warping and cracking if it gets too hot, causing even more problems that will be expensive to fix.
Overhead cam engines can experience cam scuffing, seizure and even breakage if the head gets too hot. A hot head usually swells the most in the middle, causing the head to bow and bind the rotating camshaft.
Lubrication is another area that will suffer as a result of overheating. The viscosity of the oil gets thinner as the oil heats up, and if it gets too hot it can start to oxidize (burn) causing a breakdown of the lubricant. The parts most likely to suffer are the rod bearings, rings, cylinder walls, valve guides and upper valvetrain components.
Fortunately, water has a tremendous capacity for absorbing and carrying away heat. For every degree in temperature rise, water can absorb 10 times as much heat as cast iron and five times as much heat as aluminum. Ethylene glycol, the main ingredient in most antifreeze, has about a 25% lower heat capacity than pure water. That’s one reason why a cooling system should never be filled with straight undiluted antifreeze - it increases the risk of overheating during hot weather. But antifreeze raises the boiling temperature of the coolant when mixed with water, so as long as the coolant is at least a 50/50 mixture of water and antifreeze the cooling system should be able to handle normal loads with no problems.
A 50/50 mixture of water and ethylene glycol antifreeze in the cooling system will boil at 225° if the cap is open. But pressurizing the system with a spring-loaded cap raises the boiling temperature of the mix about 3° for every 1 psi of pressure. With a 15 psi cap, the boiling temperature will be up around 265° F.
Overheating can be caused by anything that decreases the cooling system’s ability to absorb, transport and dissipate heat. This includes a low coolant level, loss of coolant (through internal or external leaks), poor heat transfer inside the engine because of accumulated deposits in the water jackets, a blocked thermostat, a plugged or restricted radiator, a slipping fan clutch, a dead cooling fan, a slipping water pump belt, an eroded or loose water pump impeller or even a defective radiator cap.
Overheating can also be caused by anything that increases the normal heat load on the engine and cooling system. This can include a slipping clutch or automatic transmission, a dragging brake, an exhaust restriction (plugged converter, muffler or pipe), running the A/C at maximum while sitting or creeping in heavy traffic during unusually hot weather, pulling a trailer, hauling an unusually heavy load of cargo or passengers, mountain driving or extremely aggressive driving.
Cooling System Checks
Getting all the air out can be fun if the radiator cap isn’t the highest point in the system. Some vehicles have special bleeder valves to help you vent trapped air, but most do not. If the heater is the high point in the system, you can loosen and burp a heater hose to release trapped air. Another trick is to raise the front end of the vehicle to tilt the cooling system so the radiator cap becomes the highest point in the system.
Air can get into a cooling system one of two ways: either because of a coolant leak or because the cooling system wasn’t completely filled following a coolant change.
A simple drain and fill can leave half or more of the cold coolant in the block - along with air pockets that can be difficult to dislodge. One way to avoid this is to use a coolant exchange machine that replaces the old coolant with new or recycled coolant while the engine is hot and idling.
On many vehicles, the recommended refill procedure is to fill the cooling system at the cap, run the engine until it warms up, then shut it off, let it cool and add more coolant to the reservoir as needed until the system is full. This can be a time-consuming procedure that may have to be repeated more than once to make sure the system is completely filled. One alternative here is to install a thermostat that has a "jiggle pin" vent. This type of thermostat is used on many import applications, and allows air to vent past the closed thermostat valve when the cooling system is being refilled.
If an engine has overheated because of coolant loss, your first order of business should be to find and fix the coolant leak. Possible leak points include hose, the radiator, heater core, water pump, thermostat housing, head gasket, freeze plugs, automatic transmission oil cooler, cylinder head(s) and block.
Make a careful visual inspection of the entire cooling system, and then pressure test the cooling system and radiator cap. A pressure test will reveal internal leaks such as seepage past the head gasket as well as cracks in the head or block. A leak-free system should hold pressure for at least a minute or more.
Also, check the water pump. There should be no visible seepage around the shaft or vent hole, nor should there be any noise or play in the pump bearing.
Check all of the hose. Any hose that has age cracks, bulges, soft spots or other visible damage needs to be replaced. Hoses more than five years old should also be changed.
Sometimes a lower radiator hose will collapse under vacuum at high speed and restrict the flow of coolant from the radiator into the engine. This can happen if the reinforcing spring inside the hose is missing or damaged.
A bad thermostat is often blamed for causing an engine to overheat. But in many cases, the thermostat is a victim and not the cause of the problem. Severe overheating can often damage the wax element in a good thermostat, which is why it should always be tested following an episode of overheating even if you’ve diagnosed something else as the cause (like a coolant leak).
You can check the thermostat by starting the engine and feeling the upper radiator hose (or use an infrared noncontact thermometer to read its temperature). The hose should not feel uncomfortably hot until the engine has warmed up and the thermostat opens. If the hose does not get hot, it means the thermostat is not opening.
Another way to test the thermostat is to remove it and dip it into a pan of boiling water (it should open). The exact opening temperature can be checked by using a thermometer.
If the thermostat needs to be replaced, install one with the same temperature rating as the original. Using a cooler thermostat (160 or 180) in an attempt to "cure" poor cooling performance can increase fuel and oil consumption, ring wear and emissions. Vehicles with computerized engine controls use engine temperature to go into closed loop and to control other emission and drivetrain functions, so if the engine never reaches a normal operating temperature it can cause additional problems for your customer.
With mechanical fans, make sure the drive belt is properly tensioned and is in good condition (replacement is recommended for belts that are five or more years old). If the fan has a clutch, inspect it for signs of fluid leakage (oily streaks radiating outward from the hub of the clutch). If the clutch allows the fan to spin freely with little or no resistance when the engine is off, or you can make the fan wobble by wiggling it, the clutch is bad and needs to be replaced.
High mileage fan clutches are an often-overlooked cause of overheating. The shear characteristics of the silicone fluid gradually deteriorate over time, with an average loss in drive efficiency of about 200 rpm per year. Eventually, slippage reaches the point where the fan slips too much to effectively cool the engine causing the engine to run hot. On average, the life of a fan clutch is about the same as a water pump. So if the engine needs a water pump, better replace the fan clutch, too.
If you suspect a radiator blockage, use an infrared thermometer to "scan" the surface of the radiator for cold spots when the engine is hot and idling. If there appear to be blockages that are reducing cooling performance, the radiator will have to be removed for cleaning or replacement.
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Larry Carley, Underhood Service, May 2001
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