Engines produce a great deal of heat, and the cooling system exists to deal with it.
How liquid cooling works
Cooling does several jobs. It prevents meltdown, stops clearances shrinking and heads off early fatigue, and it keeps the engine inside the temperature band where it runs most efficiently. The exceptions are older air-cooled VWs and Porsches; all other car and truck engines rely on liquid coolant.
Most of the moving parts are surrounded by water jackets, which are empty passages. Coolant moving through them absorbs engine heat, carries it to the radiator and releases it there. The water pump then returns the cooled liquid to the engine, and the loop starts again. Some cars also send coolant to other places.
Cars with a long list of features and multiple batteries can need a high-output alternator, which gets hot. To manage that, Audi, Bentley, BMW and Mercedes have used water-cooled alternators. Those are powerful, but leaks are their weak point. Turbochargers vary: some are cooled by oil alone, while most modern makers go for oil plus coolant. That helps the turbo last, yet it loads the cooling system, which then typically needs to be enhanced. As for the throttle body, any coolant sent there is meant just to heat it, so cold weather cannot freeze it.
Radiators: tanks, cores, passes and fins
A radiator is a heat exchanger: air flowing through it draws heat from the coolant. At its center is a core built from plates and fins, with a tank at either side. Coolant enters and leaves the core by way of the tanks, and the core does the heat transfer. Thousands of thin fins separate the coolant channels, and they add surface area. Four things set quality: how many cores and passes the radiator has, how many fins it has, and what the tanks are made of.
A typical road car has one core, while heavy-duty use frequently needs two. Extra cores make the radiator thicker and give it more area to shed heat. The OE radiator on a non-performance SUV or sedan is single-pass, with Nissens and Mahle Behr units as examples: coolant flows across the core with nothing steering it. Many aftermarket cores instead use two or three passes. A wall or baffle forces the coolant onto a longer route, so it spends more time in the airflow and loses more heat. Koyo and CSF are named as leading names for aftermarket replacements of OE radiators, and their designs frequently use two or three passes.
Fins matter as well: more of them between the plates provides extra surface area and better cooling. Low-cost unbranded replacements frequently economize on fins, since fins are difficult to spot. Unless the car carries a gauge showing detailed temperatures, the resulting higher average temperature is easy to miss, but wear on nearby engine parts speeds up.
Plastic radiator tanks
OE radiator tanks are always plastic. The material is cheap, light and simple to produce, though it is not the most durable choice. Repeated heating and cooling breaks the material down, so cracks show up as the radiator ages, and many OE units have to be replaced somewhere near 100,000 miles. CSF mostly builds with TIG-welded aluminum tanks.
Water pumps
The water pump keeps coolant moving around the system. A mechanical pump is turned by a crankshaft belt through a pulley, so its output rises and falls with engine speed. It is less efficient than the alternative but simple and dependable, and it has been the standard for decades. An electric pump sits away from the engine. Many makers fit it as standard, and it has been the most popular choice on BMW products over the past decade. Because the engine computer can vary its speed, it can also vary coolant flow.
Both types have similar durability, typically somewhere between 60,000 and 80,000 miles, but they wear out differently. In a mechanical pump the bearings wear, which lets the impeller and pulley wobble. Electric pumps can outlast that, yet they are prone to control-module faults. Whichever one fails, you could be left stranded.
Thermostats
The thermostat controls flow because an engine must reach a particular operating temperature. While the engine is cold the thermostat stays shut, so no coolant circulates through the radiator side. Once the coolant has warmed up, the thermostat opens, and the circulation holds the engine within its range. Most thermostats are purely mechanical: a valve that a spring holds closed, connected to a push rod and a cylinder filled with wax, both on the engine side. Warm coolant melts and expands the wax, pushing the rod so the valve opens.
Over the last decade BMW has fitted a so-called characteristic map thermostat. It is an electronically controlled thermostat built into its housing as a single black plastic sphere that has multiple coolant ports. It works with the electric water pump, which runs at variable speed, and together they hold coolant temperature tightly for efficiency. There is more to go wrong with it, but should the electronics fail, it falls back on the wax thermostat inside, meaning you will not be stranded. That wax unit can fail as well. When it fails open, the car is still drivable, but the engine needs far longer, possibly forever, to reach operating temperature. When it fails closed, coolant flow stops and the engine overheats.
Coolant
Early coolant was simply distilled water. The trouble is that water freezes, cracking the iron or aluminum around it, and its boiling point is low compared with engine operating temperature. Modern coolant is an even split of water and antifreeze (50/50). Antifreeze is a chemical concentrate, mostly glycol, that pushes the boiling point up and the freezing point down, and it also holds lubricants meant for the pump and the thermostat. The remaining ingredients are corrosion inhibitors and dyes, which come in blue, green, orange or red. The inhibitors largely determine the right mixture for a particular engine, and they depend on the materials in that engine.
Hoses, cap and expansion tank
Hoses are rubber reinforced with cord. They connect the radiator, the engine and the pump, and they let the engine move. Age is nearly the only thing that goes wrong: across tens of thousands of road miles of heating and cooling, the rubber degrades and eventually splits. Because swapping a hose means opening up the system, it makes sense to put new hoses on whenever the water pump or radiator is being done. Look them over whenever the hood is up, and look at nearby plastic parts too; some probably need replacing.
As the system heats up it builds pressure, which helps: it pushes out air pockets that would create hot spots, and it lifts the coolant's boiling point as well. The cap vents surplus pressure in a controlled way, and a faulty cap that fails to hold pressure can lead to overheating. Most modern European cars have an expansion tank so that coolant has somewhere to go as pressure rises. The tanks are all plastic and crack with age, just as other plastic parts do; this is a common failure on VW and BMW models and on Porsche 996/986-generation cars. Caught early, it is no major problem. If missed, the tank can split open while you are driving, and the car cannot be driven on without engine damage.
Never open the cap hot
The cap is marked with a warning not to open it when hot. Doing so can throw out a plume of scalding coolant and steam at nearly 300 F, around 149 C.
Replacement intervals
How often to replace a part depends on the part itself and on the make, the model, the era and the engine. Manufacturers may describe coolant as a lifetime fill, though that claim is mostly about looking greener. A flush and refill is advised roughly every three years, or once the car reaches 50,000 miles, as protection against electrolysis once the antifreeze additives are worn. When the antifreeze wears out, a small electrical charge looks for the easiest route, grounds through aluminum or iron parts and corrodes them.
Plan on replacing the water pump together with the thermostat somewhere between 60,000 and 80,000 miles. One of the two often fails first, but the two are so closely linked that replacing both together always makes sense. You cannot see a thermostat failing. A mechanical water pump frequently has a weep hole, and a very small coolant leak there is the signal that replacement is near.
Radiators and expansion tanks are fairly tough and can run past five years if they are treated well, but plastic is unpredictable, so think about replacing them at around five years or at 100,000 miles, whichever arrives first. Hoses depend on the individual case, but they cost little, so change them when doing bigger jobs.
Cooling upgrades
Original-equipment parts do fine for daily commuting. A few models offer optional packages that add coolers or strengthen the system, yet plastic stays, because it is inexpensive, light and helps handling. Cars built for racing use none of it in the cooling system. Metal parts weigh slightly more but last far longer. CSF, an OE supplier that also has an aftermarket side, uses aluminum tanks. Its B-Tube Technology gives the coolant tubes a shape that is stronger and thinner, and it delivers 15% more thermal efficiency than the conventional shape. The package also includes CNC-machined aluminum fittings and a drain plug in race style.
A race thermostat opens at a temperature below the industry standard of 180 F, around 82 C, so keeping the system cool is easier. That suits cars that are tracked regularly, and it works especially well on sports cars from Porsche, whose radiators are small relative to their coolant volume and clog easily. Fixes aimed at one make are popular too, for example, an aluminum flange exists for N5X and N2X engines from BMW. The stock plastic fitting there is called the Mickey Mouse flange, and it has a reputation for turning so brittle that it crumbles. The aluminum part is an upgrade only in a technical sense, and it is essential for those engines.
Silicone hoses, with do88 and Mishimoto given as examples, tolerate heat much better than rubber and outlast it in extreme use like track driving. The catch is that silicone is porous enough to let coolant evaporate through it slowly, which makes it a fit for a dedicated track car, or for owners who check the coolant level regularly. For a look at the range, see Mishimoto.
Key takeaways
- Liquid coolant carries heat from the engine to the radiator, and the pump sends it back.
- Radiator quality depends on its tanks, cores, passes and fins.
- Plastic tanks age, so plenty of OE radiators get replaced at about 100,000 miles.
- Water pumps and thermostats are due between 60,000 and 80,000 miles, ideally as a pair.
- Whatever a lifetime fill claim says, flushing coolant roughly every 50,000 miles or three years is advised.
- Never open the cap on a hot system.
- Metal parts and silicone hoses come with trade-offs, and race thermostats suit regularly tracked cars.
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General information only. Always check the specification for your own vehicle.