Wednesday, January 30, 2008

Under hood Maintenance: Coolant

Question: How often should I change my antifreeze?

Answer: For "ordinary" antifreeze, the vehicle manufacturers generally recommend coolant changes every two to three years or 30,000 miles. Others say it's not a bad idea to change the coolant every year for maximum corrosion protection -- especially in vehicles that have aluminum heads, blocks or radiators. But such recommendations may soon be obsolete. Several antifreeze suppliers have just recently introduced "long life" antifreeze formulations that claim to provide protection for four years or 50,000 miles.

General Motors just introduced a new five year, 100,000 mile antifreeze in its 1996 cars and light trucks. The new coolant is called "Dex-Cool" and is dyed orange to distinguish it from ordinary antifreeze (which is green).

CAUTION: These new long life coolants provide extended life only when used in a clean system mixed with water. If mixed with ordinary antifreeze and/or old coolant in a system, the corrosion protection is reduced to that of normal antifreeze (2 to 3 years and 30,000 miles).


The life of the antifreeze depends on it's ability to inhibit corrosion. Silicates, phosphates and/or borates are used as corrosion inhibitors to keep the solution alkaline. As long as the antifreeze remains so, corrosion is held in check and there's no need to change the coolant. But as the corrosion inhibiting chemicals are used up over time, electrolytic corrosion starts to eat away at the metal inside the engine and radiator. Aluminum is especially vulnerable to corrosion and can turn to Swiss cheese rather quickly when conditions are right. Solder bloom can also form in copper\brass radiators causing leaks and restrictions. So changing the coolant periodically as preventative maintenance is a good way to prevent costly repairs.

The basic idea is to change the coolant before the corrosion inhibitors reach dangerously low levels. Following the OEM change recommendations is usually good enough to keep corrosion in check, but it may not always be the case. That's why more frequent changes may be recommended to minimize the risk of corrosion in bimetal engines and aluminum radiators.


One way to find out if it's time to change the antifreeze is to test it. Several suppliers make special antifreeze test strips that react to the pH (alkalinity) of the coolant and change color. If the test strip indicates a marginal or bad condition, the coolant should be changed.


Reverse flushing is the best way to change the coolant because draining alone can leave as much as 30 to 50% of the old coolant in the engine block. Reverse flushing also helps dislodge deposits and scale which can interfere with good heat transfer.

The concentration of antifreeze in the coolant also needs to be checked prior to the onset of cold weather. A 50/50 mixture of antifreeze and water is recommended and will protect against freezing down to -34 degrees F and boilover protection to 263 degrees F.

For maximum protection, up to a 70% mixture of antifreeze can be used for freezing protection to -84 degrees F.

CAUTION: Do not use more than 70% antifreeze, and never run straight water in the cooling system because it offers no corrosion, freezing or boilover protection.


I've heard about a new "environmentally safe" nonpoisonous antifreeze. What is it?

It's propylene glycol (PG) antifreeze, sold under the "Sierra" brand name. Every other brand of antifreeze contains ethylene glycol (EG).
Antifreeze made with propylene glycol is being marketed as a "safer" alternative to ordinary antifreeze. Though it is by no means safe to drink, it is significantly less toxic than ordinary ethylene glycol antifreeze -- which may be a important difference to pet owners and parents of small children. PG also has an unpleasant taste which discourages further sampling by thirsty animals and toddlers. Safety is an important issue with coolants because of the frequency of spills, leaks and improper disposal.

According to one supplier of PG-based antifreeze, over 3,000 people in the U.S. were treated for ingesting antifreeze in 1991 (the latest year for which figures were available). Eight of them died. Had the antifreeze they ingested contained PG instead of EG, the consequences may not have been so dire.

Because of its significant safety advantages, PG coolants represent far less risk to wildlife in case of spills, leaks, or careless disposal. Because of this it can be claimed that PG coolants have an environmental benefit. However, both PG and EG are biodegradable and both may pick up lead or other heavy metals once they've been used in a cooling system. Both types of coolants, after being used, should be disposed of properly and in compliance with local regulations.

Though some auto makers were initially cautious about using PG when it was first introduced, GM has now said that propylene glycol may be used in GM vehicles without voiding the manufacturer's warranty coverage and will perform adequately under most vehicle operating conditions. Most vehicle manufacturers, however, don't currently use PG as a factory-fill antifreeze because of its higher cost (about $1 more per gallon at retail).


When mixed with water (50/50 ratio), ordinary ethylene glycol antifreeze provides freezing protection to -34 degrees F. and boilover protection to 263 degrees F.. By comparison, propylene glycol provides freezing protection down to -27 degrees F. in a 50/50 mixture and boilover protection to 257 degrees F.. Though it might be argued that PG provides a few degrees less protection than EG, the difference can be easily offset by using a slightly higher concentration of PG in the coolant mix.

In terms of thermal efficiency (heat transfer), both types of antifreezes perform about the same (though EG has a marginal edge). Corrosion protection is about the same as long as the coolant is properly formulated with inhibitors.


Regardless of the type of antifreeze you use, it should be disposed of properly. In many areas, it is okay to flush used coolant down the toilet (sanitary sewer) as long as the amount does not exceed a few gallons. But it should not be poured down a floor drain or into a storm sewer.

Both types of antifreeze are biodegradable but take some time to break down. Dumping used antifreeze into a storm sewer, ditch, creek or on the ground can contaminate ground water and kill plants and fish. What's more, used antifreeze picks up lead from solder in copper/brass radiators. Lead is a toxic heavy metal that can also cause pollution problems of its own.

Some areas prohibit ANY dumping of used coolant (sanitary or storm sewers). They also may not accept used antifreeze in a sealed container for landfill collection because eventually the container will leak its contents into the ground causing possible ground water contamination.

So how do you get rid of the stuff? You can take it to a local collection center that accepts used antifreeze for disposal or recycling, you can pay to have it disposed of as a hazardous waste (yeah, right) -- or you can take your vehicle to a garage or service facility that has a coolant recycling machine. The latter is the best choice because it eliminates the disposal problem altogether.

Coolant recycling machines work their magic by a variety of means. Some use filtration while others use a distillation process to remove the harmful contaminants from the old antifreeze. Corrosion inhibiting chemicals are then added to restore the coolant's corrosion protection. The auto makers have all approved coolant recycling as an effective means of eliminating coolant disposal problems, and each publishes a list of machines that meet their specifications. Recycled coolant must meet minimum standards of purity before it can be reused.


What are the most common causes of engine overheating?

The thermostat, which is usually located in a housing where the upper radiator hose connects to the engine, controls the operating temperature of the engine. It does this by blocking the flow of coolant from the engine to the radiator until the engine reaches a certain temperature (usually 190 to 195 degrees F.). When this temperature is reached, the thermostat opens and allows coolant to circulate from the engine to the radiator.

If the thermostat fails to open, which can happen due to mechanical failure or if a steam pocket forms under the thermostat due to incomplete filling of the cooling system or coolant loss, no coolant will circulate between the engine and radiator, and the engine will quickly overheat.

You can check for this condition by carefully touching the upper radiator hose when the engine is first started and is warming up. If the upper radiator hose does not become hot to the touch within several minutes after starting the engine, it means the thermostat is probably defective and needs to be replaced.

CAUTION: The replacement thermostat should always have the same temperature rating as the original. Do not substitute a colder or hotter thermostat on any vehicle that has computerized engine controls as engine operating temperature affects the operation of the fuel, ignition and emissions control systems.


On rear wheel drive vehicles with belt-driven cooling fan, a "fan clutch" is often used to improve fuel economy. The clutch is a viscous-coupling filled with silicone oil. The clutch allows the fan to slip at high speed, which reduces the parasitic horsepower drag on the engine. If the clutch slips too much, however, the fan may not turn fast enough to keep the engine cool.

The silicone fluid inside the clutch breaks down over time and can leak out due to wear, too. If you see oily streaks radiating outward on the clutch (and/or the fan can be spun by hand with little or no resistance when the engine is off), it means the clutch is bad and needs to be replaced. Any play or wobble in the fan due to wear in the clutch also signals the need for a new clutch.


On most front-wheel drive cars, the fan that cools the radiator is driven by an electric motor. A temperature switch or coolant sensor on the engine cycles the fan on and off as additional cooling is needed. If the temperature switch or coolant sensor (or the relay that routes power to the fan motor is bad), the fan won't come on when it is needed and the engine will overheat. Likewise, if the fan motor itself is bad, the fan won't work.

The system needs to be diagnosed to determine where the problem is so the correct component can be replaced.


Leaks in radiator or heater hoses, the water pump, radiator, heater core or engine freeze plugs can allow coolant to escape. No engine can tolerate the loss of coolant for very long, so it usually overheats as soon as a leak develops.

A visual inspection of the cooling system and engine will usually reveal where the coolant is going.

Leaks in hoses can only be fixed by replacing the hose. Leaks in the water pump also require replacing the pump. But leaks in a radiator, heater hose or freeze plug may sometimes respond to a sealer added to the cooling system.


If no leaks are apparent, the radiator cap should be pressure tested to make sure it is holding the specified pressure. If the spring inside the cap is weak (or the cap is the wrong one for the application), the engine will lose coolant out the overflow tube every time it gets hot.


If there are no visible coolant leaks, but the engine is using coolant, there may be a crack in the cylinder head or block, or a leaky head gasket that is allowing coolant to escape into the combustion chamber or crankcase.

See related question #16 for more information.


In some instances a severe exhaust restriction can produce enough backpressure to cause an engine to overheat. The most likely cause of the blockage would be a plugged catalytic converter or a crushed or damaged pipe. Checking intake vacuum and/or exhaust backpressure can diagnose this kind of problem.


In a high mileage engine, the impeller that pumps the coolant through the engine inside the water pump may be so badly corroded that the blades are loose or eaten away. If such is the case, the pump must be replaced.

Most pump failures, however, occur at the pump shaft bearing and seal. After tens of thousands of miles of operation, the bearing and seal wear out. Coolant starts to leak out past the shaft seal, which may cause the engine to overheat due to the loss of coolant. A sealer additive will not stop this kind of leak. Replacing the water pump is the only cure.

CAUTION: A leaky water pump should be replaced without delay, not only to reduce the risk of engine overheating but to prevent catastrophic pump failure. If the shaft breaks on a rear-wheel drive vehicle, the fan may go forward and chew into the radiator ruining the radiator.


Will a radiator stop leak additive really stop a coolant leak?

Yes, but only if the leak is in a component that normally responds to such a product.
Leaks that can often be sealed with an additive include radiator and heater core pinholes (but not cracks), seepage leaks around engine freeze plugs, thermostat, manifold and head gaskets, and porosity leaks in aluminum heads or blocks.

Leaks that do not usually respond to additives include leaks in hoses, the water pump (around the shaft), or the radiator cap. Large cracks, cracks along outside corners or where hoses attach to the end tanks on the radiator are also very difficult to seal.

Even if a leak is sealed by an additive, no manufacturer will guarantee that such a seal will last. Repairing or replacing the leaky component, therefore, is the best cure -- and the only one that will guarantee lasting results.


My cooling system keeps losing coolant, but I don't see any leaks. Where is it going?

You probably have an "internal" coolant leak inside your engine. The coolant is escaping into the combustion chamber or crankcase through cracks in the cylinder head or block, or through a leaky head gasket.
In rare instances, coolant may also leak into the automatic transmission fluid cooler if one is located inside the radiator. But usually when automatic transmission fluid leaks into the coolant it means the line is leaking.

Pressure testing the cooling system is necessary to diagnose an internal leak. A "cylinder leak-down test" can tell a mechanic if the coolant leak is in the combustion chamber. But to pinpoint an internal leak, it is usually necessary to remove the head(s) from the engine. The head may then be pressure tested and/or checked for cracks using special equipment.

Minor internal leaks can sometimes be temporarily sealed by adding a sealer to the cooling system. But large leaks or ones that do not respond to a sealer will have to be fixed.

If the problem is a cracked head or block, repairs may or may not be possible depending on the nature of the crack. Cracks in aluminum can often be repaired by welding while those in cast iron can be fixed by pinning the damaged area. But some cracks may be so bad that they are beyond repair or in a location that makes repair impossible. In such cases, the head or block must be replaced.

If a leaky head gasket is the culprit, replacing the gasket may only temporarily cure the problem if the head or block is warped. The mating surfaces on both the head and block should be checked for flatness and resurfaced if necessary to restore flatness for a proper seal.

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