What Exactly is the Coolant System?

What Exactly is the Coolant System?

Automotive engines get hot because of the explosions that take place in the combustion chambers and friction between the moving parts. The cooling system’s job is to keep the engine at ideal operating temperature. If the engine runs too cool it will operate inefficiently, waste fuel, and wear prematurely. If the engine runs too hot, parts may melt or distort, ruining the engine.

The engine has a water jacket that surrounds the combustion chambers and cylinders that is filled with a 50/50 mixture of ethylene glycol and water known as coolant. The coolant absorbs the heat created by the engine, then the water pump pumps it to the radiator to be cooled down.

The thermostat controls the flow of coolant from the engine to the radiator. When the engine is too cool, the thermostat blocks the flow to warm the engine up. If the engine is too warm the thermostat opens to allow flow to cool the engine down. The radiator is two tanks connected by many thin tubes.

The tubes have thin fins soldered to them that help to dissipate heat. Coolant flows from one tank to the other getting cooler as it travels down the tube. Cool air blowing across the radiator helps it to remove heat from the coolant. Normally the movement of the car as you are driving provides the air flow, but when the car slows down there is a cooling fan to provide air flow across the radiator. There are two common types of radiator fans: mechanical and electric. Mechanical fans are bolted directly to the waterpump pully or more commonly to a thermostatic fan clutch that lets the fan freewheel when the coolant temperature is low. Electric radiator fans use an electric fan motor to spin the fan blades.

The motor is controlled by a thermostatic switch that gives the fan motor power once a preset coolant temperature is reached, then turns the fan off once the coolant temperature drops below a preset temperature. The cooling system is pressurized to increase the boiling point of the coolant. The pressure is regulated by the radiator cap. When the pressure rises above system’s rating (usually about 15 PSI) the radiator cap releases some of the pressure into the overflow tank.

The overflow tank’s level increases as the engine warms up and the coolant expands. When the engine cools down and the coolant contracts, coolant is sucked from the overflow tank back into the radiator.

How often should I replace my coolant?

Are long life coolants better?

What is electrolysis?

Should I rod out my radiator or just replace it?

Why does my car overheat only in traffic?

Why does my car overheat only on the freeway?

Why do you use OE (original equipment) thermostats?

When should I replace my hoses?

When should I pull over?


Q: How often should I replace my coolant?

A: We believe coolant should be replaced every 2 years or 30,000 miles regardless of whether the coolant is one of the many “long life” varieties. On some Toyota models Toyota recommends the first service at 45,000 miles and then every 30.000 miles thereafter and on some new Honda models, Honda is recommending the first change at 110,000 miles. You, of course, can make up your own mind, but we strongly urge you go with our recommendation and replace the coolant every every 2 years or 30,000 miles anyway. Coolant serves 3 purposes: it increases the boiling point, decreases the freezing point, and it protects the engine from rust and erosion (electrolysis). All three of these properties are diminished as the coolant ages. Even if you don’t drive in especially cold or hot climates it’s very important to change the coolant regularly because rust and erosion can damage an engine so badly that it can not be rebuilt.

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Q: Are long life coolants better?

A: We recommend using the manufacturer’s Genuine coolant on most cars, but on some older cars, it can be hard to justify the expense. There are currently too many different types of coolant, which is a pain for us. One coolant for older Hondas, one for newer Hondas, one for older Toyotas, one for newer Toyota, one for older Mazdas, one for newer Mazdas, and yet another coolant for Subarus.

Now Nissan has joined the club and introduced a long life coolant as well. All of these coolants seem to be very similar. Honda and Nissan coolants have the ingredients printed on the bottle, and they are the same, but one is pre-mixed and blue, while the other is full strength and green.

Toyota has two of the same ingredients, but also has two extra ingredients. Do the extra ingredient do something, or are they just a stabilizer for the pink dye? Who knows? Subaru does not have it’s ingredients on their lable, but claims there coolant has a different formula than any other coolant. Are they different, or are they pretty much the same? A Chevron rep once told me *all* Japanese manufacuturer’s coolant were the same — “silicate free”.

There’s no way to know if he’s right, and even if there were, it probably would be more trouble to argue the case than to just use the OE coolant. So we stock 6 different coolants just to cover the limited makes and models we repair.

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Q: What is electrolysis?

A: Electrolysis is the the erosion of metal in the engine caused by an electrical current generated by the dissimilar metals in the engine when the coolant PH drops to 7 or below. It’s much the like the science experiment using zinc plate, copper plate, and lemon many of us did in school. Aluminum in the engine acts as an anode and is transferred to the steel parts of the engine through the acidic coolant. When aluminum parts like the cylinder head erode, the cooling passages get wider and often encroach into the head gasket sealing area. The headgasket will quickly fail if it is not firmly sandwiched between the head and the block. When a headgasket fails due to head or block erosion, replacing the failed gasket will not solve the problem, the eroded part must be replaced, frequently at prohibitive expense. The cost of replacing the coolant every 30,000 miles over 100,000 miles of driving is about $150. The cost of replacing and eroded head or block will certainly be over $2500. It’s another case of an ounce of prevention equaling a pound of cure.

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Q: Should I rod out my radiator or just replace it?

A: Rodding out a radiator is the process of removing the radiator tanks and pushing rods through the radiator core to clean it. After the core is clean the tanks are soldered back on. This can be a good repair but most of the time it makes more sense to just replace the radiator. New high quality radiators have become so inexpensive that it’s getting hard to recommend repairing an old one. Most of the time the cost of a replacement radiator is only 30% more than a repair.

A new radiator is almost always available the same day. A new radiator will usually carry a better warranty. And you should also consider that the metal on your old radiator is going to be at least somewhat fatigued from years of service and is probably more likely to develop leaks. If you are planning to keep your car for a long time it usually makes more sense to buy a new radiator. If you are planning to keep your car a short time it rodding out the radiator might be a good option.

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Q: Why does my car overheat only in traffic?

A: A car that overheats when it’s moving slowly but cools down at higher speeds usually has a problem with the radiator fan. When the car is sitting still or moving slowly there is little or no air flow over the radiator. Without air to cool it, the radiator does a poor job of dissipating heat. All modern cars have cooling fans to help push or pull air through the radiator fins at low speeds. When these fans stop working the car can overheat.

You can check to see if your fan is working by letting the car idle while watching the temperature gauge. You should hear the cooling fan come on somewhere between 1/2 and 3/4 of the gauge travel. If it has not come on by 3/4 gauge, then something is wrong with the fan system. You can drive the car in to the shop instead of towing but avoid stop and go traffic and watch the gauge carefully.

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Q: Why does my car overheat only on the freeway?

A: A car that overheats at high speeds usually has a problem with its coolant flow. Flow can be reduced by a partially plugged radiator or a thermostat that does not open all the way (a thermostat that does not open at all usually causes the car to overheat all the time).

If the radiator is easy to touch it can be checked for cold spots, which would indicate lack of flow in the area that is cold. On most of the cars we work on you can’t see the radiator, much less touch it. If the radiator can not be checked for cold spots, we can remove it and send it out to the radiator shop for flow testing, or we can replace the thermostat and see if it cures the problem. It’s usually cheaper to replace the thermostat than to test the radiator, so that’s what we usually do.

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Q: Why do you use OE (original equipment) thermostats?

A: Because they are far superior in quality to any aftermarket brand that we know of. If you look at a Honda or Toyota thermostat compared to a Stant the difference is immediately apparent. The OE part has twice the metal of the aftermarket part. The OE thermostat will be sturdy and obviously well constructed whereas the aftermarket thermostat will look flimsy.

Some earlier experiences with aftermarket thermostats proved to us that the difference is more than just their looks. We’ve had aftermarket thermostat that were bad right out of the box. With rebuilt engines costing between $2500 and $6000 it makes a lot of sense to pay an extra $4.00 for a good thermostat.

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Q: When should I replace my hoses?

A: Well if you ask Goodyear (a hose manufacturer), they’ll say replace hoses every 3 years or 36,000 miles. If you ask us, that’s a waste of money. When should you replace the hoses? Well, if any of the hoses show signs of deterioration including cracking when squeezed, oil contamination, swelling, excessive softness, etc., then the hoses should be replaced. If your car has made it to 10 years old and the hoses have never been replaced, you may want to consider replacing the hoses even if they seem OK. We’ve seen 20 year old original hoses that are still holding pressure. We’ve also seen 5 – 10 year old hoses, that look OK from the outside, begin to leak from a “pinhole”.

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Q: When should I pull over?

A: When the temperature gauge gets to 3/4 you should start to monitor it frequently. If the gauge climbs into the red zone or suddenly drops to below half you should pull over as soon as possible. Engines can be damaged by driving only a short distance while overheating. After pulling over, either call a tow truck or wait for a half hour or so and check the coolant level in the overflow container and the radiator. Squeeze the upper radiator hose first to see if there is still pressure in the system. You can be badly burned by opening a radiator cap when a car is overheating, so be patient and wait for it to cool. If the coolant is low, add water, or coolant if you have it. Once you’ve checked or added water start driving towards your repair shop (Art’s we hope). Watch the temperature gauge carefully.

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