t’s in great working condition, hot weather poses no challenge. It’s the cooling systems that are marginal or in need of maintenance or repair that will run into trouble when the dog days of summer arrive.
Probably the most troublesome component in any cooling system is the thermostat. The thermostat’s job is to speed engine warm-up following a cold start and regulate the engine’s operating temperature while running.
Accurate temperature control is critical in today’s engines because emissions control, fuel economy and performance all can be affected by coolant temperature. The engine’s powertrain control module (PCM) monitors coolant temperature via one or more coolant temperature sensors.
When coolant is cold, the PCM adjusts the fuel mixture, spark timing and other emission control functions (such as exhaust gas recirculation) accordingly. When the engine reaches its normal operating temperature (typically 180F to 195F), it readjusts spark timing and fuel mixture, and allows EGR and other emission functions (such as purging the EVAP emissions canister) to occur.
Thermostats are fairly reliable, but may fail after years of service or as a result of engine overheating. The first sign of trouble is usually a temperature warning light or a high temperature reading on the temperature gauge. When a thermostat fails, it usually fails to open. This blocks the flow of coolant and prevents the cooling system from carrying away the engine’s waste heat (which is about a third of the heat energy produced by combustion). Heat builds up in the engine, and sooner or later the coolant starts to boil over.
The danger with overheating is that it also can cause engine damage. Severe overheating, or continuing to drive while the radiator is boiling over, risks damaging the head gasket(s) and/or scuffing the pistons – either of which can be very expensive to fix.
If an engine is overheating due to a bad thermostat, the old thermostat should be replaced without delay. It’s a relatively inexpensive component that usually costs less than $10, and is fairly easy to replace on most vehicles.
A thermostat also may jam and fail to close, increasing the time it takes for a cold engine to reach normal operating temperature. It also may prevent the PCM from going into “closed loop” operation, in which it uses oxygen sensor input to adjust the fuel mixture. This can increase fuel consumption and emissions significantly.
The thermostat usually is located in a small housing where the upper radiator hose connects to the engine block. On some cars, though, it may be located where the lower radiator hose connects to the engine.
The most difficult part of replacing it is draining and refilling the cooling system. Getting all of the air out of the cooling system can be tricky on many late model vehicles. Trapped air can prevent the system from being refilled completely with coolant, which can cause the engine to overheat again. Some cars have one or more bleeder valves in the cooling system to vent air when the radiator is being refilled with coolant.
A replacement thermostat must have the same temperature rating as the original thermostat to maintain the proper engine operating temperature. Remember, the PCM is calibrated for a certain engine temperature, so changing the temperature can screw things up.
Some replacement thermostats have a small air vent with a jiggle pin. This type of design is found on many import applications, and is used to make refilling the cooling system easier. The jiggle pin valve prevents air from being trapped under the thermostat.
Some aftermarket thermostats also have a special “fail-safe” feature that prevents them from sticking shut should the unit ever fail. This can provide extra insurance against overheating.
If a customer’s engine has overheated because of a coolant leak and he is buying coolant, hoses, a water pump or other parts, you should recommend replacing the thermostat, too. Why? Because excessive engine temperatures can damage a thermostat. The little temperature-sensing element that opens the valve contains wax. If it gets too hot, the wax can be forced out of its housing. This leaves less wax inside the thermostat to push open the valve against spring pressure, increasing the chance that it may not open fully the next time the vehicle is driven.
Related parts that may be needed include a new gasket or O-ring for the thermostat housing, and possibly a new housing if the old one is badly corroded.
Water Pump Woes
The next most troublesome component in the cooling system is the water pump. Most OE water pumps should last upward of 100,000 miles, but they won’t last forever – and some may start to leak or make noise long before the odometer hits six figures.
Most OE water pumps have a hard ceramic seal that prevents coolant from leaking out around the pump shaft. Over time, the seal can wear and start to leak. Coolant neglect and the formation of sediment in the coolant due to corrosion can accelerate seal wear and cause premature pump failure.
The bearing that supports the pump shaft and impeller also can wear as the miles add up. Any wobble that occurs in the shaft will also accelerate seal wear and lead to leaks.
Cooling system sealer products that can plug small radiator and head gasket leaks cannot seal a leaky water pump. If it is leaking coolant, the only fix is to replace the pump.
Another problem that can affect some water pumps is impeller wear. Some late model water pumps have plastic impellers instead of stamped steel impellers. Plastic is lighter and allows a more refined impeller design to reduce drag and improve pumping efficiency.
But plastic also is a soft material that can easily be eroded by sediment in the coolant. It also is vulnerable to cavitation erosion, which occurs when the rotating pump forms small bubbles in the coolant. When the bubbles pop, they create shock waves that slowly chip away at the plastic impeller. Over time, the impeller blades can wear down to almost nothing – causing the engine to run hot and overheat since the pump is not circulating much coolant. Impellers are not available separately as a repair item, so the pump must be replaced as an assembly.
Related items that may be needed when replacing a water pump include a new V-belt or serpentine belt if the old belt is cracked, glazed or making noise. Your customer also may need thread sealer for water pump bolts that thread into open holes and extend into the engine’s cooling jacket.
If the application is a rear-wheel drive vehicle with a mechanical cooling fan, your customer also may need a new fan clutch. The silicone fluid inside a fan clutch that allows it to slip suffers sheer breakdown over time. This slows the speed of the fan, which may lead to engine overheating when moving in slow traffic with the A/C on during hot weather. Any wobble in the fan clutch, or any sign of fluid leakage or excessive slippage would call for a new fan clutch.
Hoses
Hoses also often need replacing. The synthetic rubbers that are used in late model radiator and heater hoses are much longer-lived than the natural and synthetic rubbers commonly used a couple of decades ago. Most late model OE hoses are capable of lasting 10 years or more, and 100,000-plus miles. Even so, older vehicles and high-mileage vehicles may need new hoses to replace old ones that have become hard or are cracked or leaking.
Molded hoses are used on most late model vehicles, so some trimming may be required to make a replacement hose fit correctly. The old clamps also should be replaced with new ones, as old clamps may be weak or corroded.
If one hose has failed and a customer is buying a replacement, you should warn him that the other hoses on his vehicle also may be on the verge of failure. Replacing all of the hoses now can prevent a breakdown later. This includes the upper and lower radiator hoses and the heater hoses.
Expansion Plugs
Also called freeze plugs, these little plugs seal the casting holes in the engine block. They usually are stamped steel and pressed into place. Over time, the plugs may corrode from the inside out, especially if the coolant has never been changed. If a plug is leaking, it needs to be pried out and replaced.
Some replacement plugs have a rubber expansion grommet that makes installation easier. A related item that will be needed with a press fit plug is sealer.
Radiator & Heater Core
Radiators and heater cores usually are trouble-free, but on some vehicles (many Chrysler cars) repeat heater core failures have been blamed on electrolysis corrosion. The problem may be due to coolant neglect or improper grounding of the heater core to the chassis.
Almost all late model vehicles have aluminum radiators. With proper cooling system maintenance, the radiator should last the life of the vehicle. But radiators can fail as a result of internal corrosion, vibration damage that causes cracks and leaks, or physical damage. On radiators that have plastic end tanks, the seal between the radiator core and end tank may develop a leak. The plastic tank also can be damaged by internal erosion caused by sediment or by extreme overheating.
Aluminum radiators can be tricky to fix, though small leaks may be successfully sealed with cooling system sealer or externally applied high-temperature epoxy. A radiator with multiple leaks, physical damage or leaky end tanks usually has to be replaced.
The main point to keep in mind when selling a replacement radiator is that it must have the same width, height and thickness as the original radiator in order to fit properly. Also, it must have the same hose connection locations and fittings. Many radiators have an internal loop of pipe that serves as a cooler for the automatic transmission fluid. So if the vehicle has an automatic transmission with a radiator ATF cooler, the replacement radiator also must have one.
Also recommended is a new radiator cap (unless the cap is on the coolant reservoir tank). Many radiator caps are spring-loaded to maintain a certain pressure in the cooling system. Over time, the spring may weaken, causing a loss of pressure. This can lead to coolant loss and overheating. A bad seal on the underside of the cap can cause the same kind of problems.
Coolant
Replacing cooling system parts almost always requires draining and refilling the cooling system. Today’s long-life coolants are supposed to be good for five years or 150,000 miles. Unfortunately, many people think that means forever. It doesn’t.
After five or more years of service, the corrosion inhibitors usually are worn out and the coolant needs to be replaced. If the coolant is not replaced for preventive maintenance, corrosion can start to eat away at the inside of the cooling system, attacking the radiator, heater core, water pump, engine and even rubber hoses.
Any customer who is buying cooling system parts, therefore, also should be encouraged to buy some fresh antifreeze. Premixed coolant is easier to install and reduces the risk of coolant contamination by dissolved minerals or salt in tap water.
A vehicle owner should use the same type of coolant that was the factory-fill in his cooling system. It doesn’t have to be the same brand, but it should be a long-life organic acid technology (OAT) type of coolant that meets OE requirements or is a “universal” (all makes/all models) type of product. The less expensive and shorter-lived “green” coolant, which is okay for older cars, should not be used in late model cooling systems that specify a long-life or hybrid coolant.
Other Parts
In servicing cooling systems, a number of other related parts may need attention, including an automatic tensioner for a serpentine belt if the old tensioner is not maintaining proper tension or is making noise.
A coolant sensor may have to be replaced if it is defective, not reading accurately or is leaking. A bad coolant sensor can prevent the PCM from going into closed loop, upsetting the engine’s fuel and emissions calibrations. A quick way to check a coolant sensor is to use a scan tool to compare the coolant temperature reading against the ambient air temperature sensor reading when the engine is cold. Both should show the same temperature. If not, one of the sensors is off.
A bad fan relay or a burned out fan motor can prevent the radiator fan from coming on when extra cooling is needed. On most vehicles, the fan(s) should come on when the A/C is turned on. If the fan circuit is not working, the engine may overheat when the A/C is on, or when the vehicle is creeping slowly in traffic. The relay for the fan circuit usually is located in the engine compartment, and is relatively inexpensive and simple to replace once you find it.
