Cooling system service: Keep systems functioning at peak performance

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Cooling system service: Keep systems functioning at peak performance

Weber is president of Virginia-based Write Stuff. He is an award-winning freelance automotive and technical writer and photographer with over two decades of journalism experience. He is an ASE-certified Master Automobile Technician, and has worked on automobiles, trucks and small engines. He is a member of the Society of Automotive Engineers (SAE) and numerous other automotive trade associations. He has worked as an auto service technician, a shop manager and a regional manager for an automotive service franchise operation.

The automobile is slowly evolving into an appliance, at least in the minds of many motorists. Much like a refrigerator or microwave, owners tend to use the equipment and ignore service until something breaks.

Motor vehicles use less than a third of the energy from combustion to propel them. Roughly, another third of the heat must be carried away by the cooling system while frictional losses and exhaust account for the remainder.

To keep the system functioning properly and to prevent problems, the coolant (the mixture of water and antifreeze) must be replaced on schedule. In many car owners’ minds, that schedule is 100,000 or 150,000 miles because that is what the salesman touted. They forget that there is a time interval. People who do not drive very far may forget that the coolant must be routinely replaced and never do it. In reality, most vehicle owners likely ignore the importance of coolant replacement altogether, unless the system leaks or the engine overheats.

Neither appearance nor the freeze protection are measures of the coolant’s protective qualities. The additives that prevent corrosion and lubricate the water pump may be depleted. According to industry experts, over 75% of water pump failures are due to worn-out or contaminated coolant.

Signs of trouble

Usually, the first sign of a water pump problem is coolant seeping from the weep hole on the bottom. Corrosion or debris can wear at the shaft seal. However, keep in mind that a small amount of initial seepage is completely normal. In fact, it may take a couple hours of running before a replacement water pump’s seal stops weeping, so don’t rush to send it back as defective.

If you must replace a water pump, the first step is not belt removal. It is system flushing. Get rid of any junk in the cooling system that could damage the replacement pump.

Before ordering the new pump from your jobber, determine if it is a standard rotation or reverse rotation pump. How? If the pump is driven from the front of the serpentine belt (the grooved side) it is standard rotation. If the pump is driven by the back of the belt, it is reverse rotation. They are not interchangeable.

After installing a new water pump, spin its shaft to make sure it rotates freely before installing the belt. Speaking of the belt, now is a good time to replace it along with its tensioner (for serpentine designs). If the water pump is driven by the timing belt, you could save your customer plenty of money for labor by replacing the timing belt now rather than later.

Always replace the coolant when replacing the water pump. That doesn’t mean a simple top-off — drain, flush and refill with a fresh mix.

As you know, the thermostat regulates the engine’s operating temperature. If it fails by staying open, the engine may run too cool. The heater output will probably be inadequate. If it fails by not opening, the engine will overheat.

Some people think that an engine that runs cool is a good thing. It isn’t. Cool running can contribute to oil sludging and reduced fuel economy. The engine management system uses coolant temperature as one of its vital inputs for fuel mixture enrichment. An engine running slightly cooler than normal may not set a trouble code or check engine light.

Thermostats can become sluggish or weaken over time. They are inexpensive and it makes sense to ensure your repairs by replacing them when other major cooling system service is performed.

Coolant hoses are often taken for granted. However, they should be visually and manually inspected at the very least. Check them for heat, oil and abrasion damage. Squeeze them to check for mushiness. Remember: Hoses degrade internally, so a visual inspection of only the outer surfaces isn’t adequate.

Electro-chemical damage is public enemy number one. Automotive engines are a metallurgical mélange. There may be a cast iron block, aluminum head(s), copper/brass radiator and so on.

Electrical charges can be created and carried through the coolant hoses, especially if there are any loose or missing grounds in the engine compartment.

The hose degrades from the inside out as the inner tube beneath the reinforcement layer fails. When this happens, coolant seeps into the reinforcement and then, as pressure builds, creates a tiny hole in the cover creating a leak. Ultimately, the hose will burst and it usually happens while driving. The best case scenario is an inconvenienced motorist. The worst case is a damaged engine.

Squeeze the hoses with a finger and thumb near the end — close to the pipe to which it connects. Feel for gaps or soft spots. Next, squeeze near the middle of the hose’s length. If it feels firmer, electro-chemical degradation has begun. It starts at the ends of the hose.

Replacement hoses that are resistant to electro-chemical damage are readily available in the aftermarket.

Engine heat can damage the hose cover, especially if the hose is routed near a hot component like an exhaust manifold which also can damage the reinforcement layer. Look for hard, glossy areas on the hose surface. Replace marginal hoses and reroute them away from hot zones or wrap them in a heat-resistant sleeve if that is not possible.

A hose that rubs against a component can suffer abrasion damage. It is obvious when this happens. If the damage is minimal, reroute the hose and protect it from rubbing.


Oil can soften and weaken the rubber. Sometimes the oil leak is internal, such as from a transmission cooler or head gasket allowing oil into the coolant and coolant into the oil. Replace any damaged or weak hoses and repair the cause of the oil leak.

Leaking hoses are often easy to spot, but there are numerous other places that coolant leaks can develop. These include:

• Radiator

• Pressure cap

• Radiator drain

• Block drain

• Freeze plug

• Coolant temperature sensor

• Heater core

• Gaskets (intake, head, thermostat, etc.)

• Quick-connect hose coupling

• Water pump

Leak detection

The traditional method for leak detection is a pressure tester used to simulate the pressure in an operating engine, but with the engine off and the coolant at a reasonable, safe temperature.

Before opening the pressure cap to connect your tester, squeeze the upper radiator hose. If it is firm or hard, allow the system to cool for a while.

Connect your pressure tester and pump it up to the high end of the normal operating pressure range. Keep an eye on the needle. If it shows a pressure drop, there is a leak somewhere. External leaks make themselves known, but locating them may take some extra work due to various components or panels hiding their location. This is where tracer dye and a UV light may make the job easier. Don’t forget to check the foot wells inside the car in the event of a heater core leak.

By the way, a sweet or somewhat cloying odor inside the vehicle is a clue that there is a heater core leak. Another clue is a smeary cabin-side windshield surface.

When it comes to internal coolant leaks, you must be more of a detective. Check the engine oil dipstick. A milky goo near the top is an indication of coolant in the crankcase. If the leak is substantial, globs of goo may come out the oil pan drain hole when the plug is removed.

Oil can get into the coolant, as we mentioned earlier. Look for droplets of oil floating on top of the coolant in the radiator or overflow jug or reservoir. Red oil usually means there is transmission fluid getting into the coolant. This is most often a result of a transmission cooler (located inside the radiator) leaking.

A trans cooler leak can damage both the cooling system and the automatic transmission, so both should be serviced. Steam from the exhaust is another obvious indication of an internal leak, often from a head gasket or intake manifold or plenum gasket. If you suspect a head gasket, pull the spark plugs. The cylinder with the cleanest plug is where the problem exists. You will want to follow up with a cylinder compression test.

Whenever there has been oil in the cooling system, replace all rubber parts and flush the system to remove traces of oil prior to refilling with fresh coolant. Don’t forget to replace the pressure cap.

Speaking of caps, use the pressure tester along with the adapter to check the point at which the cap’s release valve opens. A pressurized cap should hold pressure below its rating, usually found on the cap itself. Pressure should hold for at least a full minute.

The cap is actually a two-way valve. Coolant expands when hot so the cap allows the coolant to flow into the reservoir. As the engine cools, a partial vacuum develops and the coolant is drawn from the reservoir back into the radiator. If the cap fails or the reservoir runs dry, air can enter the cooling system.

Air in the system is a problem. Liquids (coolants) do a much better job of transferring heat than gasses (air). Air in the system can block coolant from travelling through the engine and may prevent the thermostat from opening. Air in the system is also a major contributor to internal corrosion, especially in the radiator, heater core and water pump, and potentially the water jacket surfaces in the block. If it is severe enough, water jacket corrosion can lead to pinholes that allow coolant to enter the cylinder(s).

Bleeding the system

Always bleed the cooling system during refilling. If you use a flush and fill machine, this is a snap, but if you use the old-fashioned bucket method you have a few extra steps.

Lower hood lines mean lower radiator locations. On many modern vehicles, a pressure tank (and pressure cap) sits at a high location under the hood so that air can migrate out of the system. Otherwise, you may find a bleeder valve that must be opened during refilling. It may be on the thermostat housing or in the heater circuit. Add coolant until all the air is purged and only straight coolant flows from the bleeder.

Another trick is to use your pressure tester to force air out of the system. Add as much coolant as possible then attach your tester and pressurize the system. Next, open a bleeder or loosen a clamp at the highest point of coolant flow. When only coolant comes out, close the bleeder or tighten the clamp, then add the necessary additional coolant in order to top the system off. Run the engine until the thermostat open and if the coolant level drops, add more

Radiators and fans

Finally, a few words about radiators and fans. Radiators either leak or suffer from clogging. A leaking radiator, if not obviously dripping coolant, usually shows a buildup of white stuff near the leak where coolant has escaped and dried. Check the areas around necks and tanks.

Radiator restrictions, both internal and external, are another problem. Corrosion and debris can clog the radiator’s tubes. The simplest way to check is by using an infrared thermometer to scan the radiator from top to bottom or side to side. Cold spots indicate a restriction.

Bugs, leaves, plastic bags and sand (picked up onto the radiator during driving) preventing air from flowing across the fins cause external restrictions. Bent fins can also be a problem in terms of reduced thermal release. Clean the stuff from the front of the radiator. Replace the radiator if it has severe internal restrictions or leaks.

Most late model vehicles, except some trucks, feature electric cooling fans. They require less mechanical energy than belt-driven fans, they are quieter and they can be controlled by the vehicle’s management computer.

The electric fan(s) should run when the engine gets hot and whenever the air conditioning is running. If a fan does not run, check its electrical circuit including any relays. Then scan the management system to see if the fan is getting a command to run.

Do not overlook the possibility that the coolant temperature sensor (CTS), its connection or wiring could be the source of the problem. If the fan runs as soon as the ignition is switched on (with a cool engine), there is a problem with the control circuit.


On belt-driven fans, inspect the clutch for streaks indicating leaking silicone oil. Try spinning it by hand with the engine off. A degree of resistance should be felt. If it spins freely, the clutch is shot. If the fan wobbles, the clutch must be replaced. In addition, if the belt tension is inadequate, replace the belt and/or its tensioner.

Flushing/filling the cooling system

There are two ways to drain and fill a cooling system: the shade tree method using a garden hose and the professional method using professional-grade equipment.

There are two kinds of professional equipment: the flush and fill tool, and the flush and fill machine.

Modern vehicles have smaller cooling systems; operate at higher temperatures (195 degrees Fahrenheit) and relatively higher pressures (15 psi) than in the past.

The secret to a successful coolant flush is to get all of the old coolant out of the system. Draining the radiator only removes less than a third of the coolant. The rest stays behind in the engine, hoses and heater core. Draining and filling the radiator is like washing only one sock, probably worse.

Installing a flushing tee in a heater hose helps move the coolant, but it must still exit through the radiator petcock or the lower radiator hose after it has been removed. Some radiators today don’t even feature petcocks. Opening a block drain helps significantly, but good luck with that.

Most block drains (on those blocks that feature drains) have never been opened and are corroded shut. Even popping out a freeze plug is better than nothing, but it still allows collected debris to linger in the bottom of the block.

Flushing tools are simple to operate, run on shop compressed air and are relatively inexpensive. They may be the choice for an average shop that does an average number of cooling system services. The tool and adapters usually fit in a carry case.

These tools use shop air to create a vacuum, which sucks the old coolant out through the radiator neck. Many have adapters to allow coolant withdrawal from the heater core via a tee or other sources by changing adapters on the gun. They do the job quickly.

As the old coolant is removed, the new coolant is drawn in by the vacuum.

On the down side the engine must be running to keep the thermostat open. There is also the risk of running the system dry temporarily.

Freestanding flushing machines are also simple to operate. Some use shop air, electric power or both. They are bulkier than the flushing tools having their own cabinets on wheels. Many have tanks to both recover the old coolant and to introduce the new stuff. The major advantage of this equipment is its ability to reverse flush the cooling system. Reverse flushing forces the thermostat open, therefore the engine usually need not be running.

Back-flushing removes more debris than either the drain-and-fill or the forward-flushing process. Most machines are simple to operate and don’t usually require the technician to baby-sit the equipment the whole time.

Perhaps the major advantage to using professional equipment is the ability to fill the cooling system without leaving air pockets. Air in long-life, organic acid technology (OAT) coolants spells trouble.

When you are ready to buy new equipment, consider how often you shop performs cooling system service. Figure your return on investment.

Consider the ease of use and the amount of training required to use the equipment your are considering. Consider the marketing advantages to your customers. Consider the thoroughness of the service.   ‚óŹ

Want more tech articles? To read the entire May/June 2014 issue of Auto Service Professional, see our digital version by clicking here.

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