I'm not a chemist & I didn't
stay at a Holiday Inn Express last night. But I wrote this a few months ago, and it relates to this topic. My understanding in item 10 about LL coolant & in item 12 about the bacteria is based on discussions with coolant flush reps, parts store managers, & GM technicians. I don't claim it's a superior explanation to stalag's - it's just what I've heard. I'm adding a link to THIS thread for anywhere else that I post this article.Engine cooling basics
I've noticed a lot of questions about the cooling system, and there seems to be a lot of misunderstanding about this seemingly simple system. Even pros don't always grasp the basic concepts about it, so I'm going to try to lay it out. Some of this may seem overly simplified, but I'm trying to make it readable by anyone.
1. Internal combustion engines produce heat by burning gasoline in air. In fact: every bit of energy produced by the engine ultimately becomes heat (the simplest form of energy). Since an engine block large enough to dissipate this heat would be too heavy, and since it's not practical to direct sufficient airflow past the engine, a denser fluid than air is needed to carry it away so that the metals don't oxidize & the lubricants don't combust. Water was the early obvious choice because it's cheap & plentiful, but its relatively low boiling point made it less effective than needed. So chemicals were added to raise its boiling point (any mixture of liquids has a higher boiling point & lower freezing point than any single component); specifically, ethylene glycol (a poisonous alcohol with a sweet flavor). Certain other chemicals are added to inhibit corrosion, lubricate the water pump seals, make the coolant bitter so animals don't drink it, give it color for identification, etc. Some of these additives are consumed over time, requiring regular replacement of the coolant mixture. Additionally, the system is sealed to create higher-than-ambient pressure, which also raises the boiling point. The main benefits of a higher boiling point are that the coolant can carry MORE heat (energy) at a lower flow rate, and the coolant isn't lost as fast as with a vented system. Some early water-cooled vehicles consumed more water than fuel.
2. But the DISadvantage of a liquid cooling system is that it can prevent the engine from reaching operating temperature. So it needs to be regulated in order to allow the engine to get hot enough to vaporize the fuel, boil contaminants out of the oil, maintain proper clearance in the bearings, etc. The obvious regulator is the thermostat. Its purpose is to restrict flow when the coolant is cold so the engine warms up faster. Virtually all thermostats contain a wax pellet with a calibrated melting point. When the wax melts, it expands, generating a force that overcomes a spring which normally holds the thermostat's valve closed. As the valve opens, coolant rushes past, and the wax may cool, allowing the spring to close the valve again. So the flow will "pulse" as the system warms up. Most t'stats include a weep hole to allow a VERY small flow during warmup so that the engine doesn't overheat before the t'stat gets warm. This weep hole also helps to bleed air from the system, and so should always be installed upward. The other regulator is the cooling fan clutch (or relay/PCM/ECT for electric fans). A thermal fan clutch is designed to absorb heat from the radiator & conduct it to a bimetallic coil which operates a lockup mechanism inside a silicone grease bath. When the coil is cold, the clutch is unlocked, allowing the fan to spin slower than then engine & restrict the air moving thru the radiator. As this airstream heats up (due to the engine warming up the coolant), the mechanism links the fan blades to the fan shaft (usually attached to the water pump), which then boosts the airflow thru the radiator. Again, a "pulse" effect can develop under certain conditions. Some early systems without a fan clutch used a flex fan whose blades created very high flow at low RPM, but then flexed forward into a low-flow angle at higher RPM. These were often unacceptably loud, which led to their blades being irregularly spaced to reduce the drone. This irregular blade spacing was carried over into clutched fans, as well as most others, like alternator fans which were noted for "sirening" at certain speeds.
3. Since heat doesn't flow thru liquid fast enough, the liquid must be forced to flow thru the system from the hot area (the engine block & heads) to the heat exchanger (the radiator). The most common method is a belt-driven centrifugal pump, used for it simplicity of design, & general reliability. Most are simply a stamped steel impeller pressed onto a shaft supported by 2 sealed bearings within a cast housing that includes the water inlet from the radiator. Common failures in the water pump include the impeller slipping on the shaft (reducing the flow to almost nothing), impeller erosion due to abrasion &/or corrosion, bearing seals leaking (they're drained thru a hole drilled into the housing), bearing noise, or shaft damage from some external failure (like belt failure or collision). The water pump may be embedded in the block (Ford 300ci/4.9L & modular V8s), embedded in the timing cover (Land Rover 3.9L/4.0L/4.2L/4.6L), attached to the timing cover (Ford 302ci/5.0L & Ford 351W/5.8L), forward of the timing cover (many GM smallblock V8s), or remote (certain VWs).
4. In almost all, however, the coolant flow path is virtually the same: coolant drains to the bottom of the radiator where it flows out thru the lower radiator hose to the water pump inlet. The pump then forces the coolant into the block, where it flows around the cylinders to the back of the block. Cutouts in the head gasket regulate where & how much coolant enters the head & returns to the front of the engine. Within the head(s) is where the coolant reaches its highest temperature, which is why all coolant sensors are near the head(s). In V engines, the coolant flows into a crossover journal in the intake manifold before diverging; in straight engines, it diverges from the head either thru the t'stat or into the heater outlet. In either case, this is generally where its temperature is detected by both the sensor for the gauge & by the ECT for the PCM (EEC). Some V engines also have a bypass hose which allows coolant to return directly to the water pump. There may also be a small circuit to the throttle body for de-icing, which typically returns to the radiator upper tank. Coolant that exits the t'stat flows thru the upper radiator hose into the top of the radiator & thru the core where heat is radiated into the airstream. The cool (lower) radiator tank may contain the upstream heat exchanger for the automatic transmission, and the lower radiator hose may contain an orifice which diverts some coolant to the engine oil cooler.
The lower radiator hose flows TOWARD the engine.
The upper hose flows AWAY from the engine.
The heater hose connected to the intake manifold or t-stat outlet flows AWAY from the engine.
The heater hose connected to the water pump flows TO the pump.
The little bypass hose on V8s flows TO the pump.
The metal line on the radiator flows TO the radiator.
Hot coolant flows OUT of the head or intake manifold.
5. In most engines, coolant ALWAYS flows thru the heater core circuit. The outlet to the heater core is beside the t'stat, so the t'stat can never restrict flow to the heater core. This serves 2 purposes: it allows an unrestricted failsafe coolant flow (although the heater core isn't nearly large enough to cool the engine if the radiator becomes restricted), and it allows the cabin to receive heat as soon as it becomes available, irrelevant of the radiator temperature, ambient temp, t'stat, fan, or clutch/relay. Even if the coolant level becomes critically low, the heater circuit will still generally have coolant in it since it takes less coolant to sustain flow within its smaller capacity. In some vehicles, a problem has been recognized in which high engine RPM during warmup can result in excessive pressure within the heater core, resulting in rupture. The fix is to retrofit a slight restriction (an orifice plate) into the circuit upstream of the heater core to limit the flow, and thereby, the pressure. Returning coolant is typically routed directly into the water pump. If the heater core fails, it is safe to loop a hose from the outlet directly back to the return indefinitely. It may also be beneficial to occasionally reverse the hoses at the heater core to keep it flushed out. The direction of flow in the heater core is irrelevant.
6. As with virtually every substance, the trapped air in the coolant system expands as it is heated by the engine. Up to a limit, this effect is utilized to create the pressure which increases the boiling point. But excess pressure must be vented, without releasing poisonous coolant onto the ground. So a pressure cap is used either on the radiator for a system with a vented overflow tank, or on the "degas bottle" for a fully-pressurized system. The cap has 3 main functions: a) to seal the pressurized portion of the coolant system up to the target pressure; b) to direct the UNpressurized portion of the vented system into the overflow tank; & c) to allow coolant to return from the unpressurized tank into the pressurized system when the system develops a vacuum (during cooldown). This return of vented coolant is dependent on the radiator hoses being fairly rigid, either because of their rubber compounds being stiff, or because of internal springs which support their shape. Hoses that are too soft (often due to oil contamination or just age) will simply collapse, preventing the return of lost coolant from the unpressurized overflow tank. This is just one reason for the increasing use of a pressurized tank (degas bottle) which is designed to hold a specific air pocket within the pressurized system. The air creates a spring that allows for coolant expansion without the risk of coolant loss due to venting; even to an overflow tank. Both systems ultimately allow failsafe venting to the ground.
7. Another refinement to the liquid-cooling system is the fan shroud. Often misunderstood as dead weight or an unnecessary safety shield, the shroud performs an integral function in hi-performance lightweight cooling systems. It vastly improves the fan's efficiency at moving air, as well as assisting the fan in BLOCKING airflow during warmup. Some fan shrouds also include vent flaps which open at high vehicle speed to allow extra air to flow thru the corners of the radiator not sufficiently served by the fan blades. Equally (if not more) misunderstood is the bumper valance. Not merely a cosmetic addition to reduce approach angle - on some vehicles, it is critical to engine cooling. The air-damming effect it produces at high speeds results in a slight vacuum under the engine bay which dramatically increases airflow through the radiator. Without the bumper valance, air can strike the front suspension & bounce up into the engine bay, blocking the radiator's airstream. This same overheating/undercooling effect may be noted if the vehicle is lifted significantly, or if the hood is left open on the safety catch.
8. Possibly the latest refinement to the liquid cooling system is the electric cooling fan motor. More controllable than the thermal clutch, the e-fan allows designers to instantly control the airflow thru the radiator & condenser through the PCM's programming. Using any number of relays & resistors to produce any number of speeds (similar to the HVAC blower motor), engine temperature can be much more precisely regulated, at the cost of slightly higher complexity & weight, with slightly lower efficiency (due to the mechanical/electrical/mechanical conversion of energy). E-fan vehicles require a noticeably larger alternator, and some require failsafe cooling programming in the PCM to protect the engine from fan motor failure. E-fans also have an attraction for off-roading since they allow the driver to turn off the fan before fording deep water, thereby reducing the chance of engine or radiator damage. A common misconception is that the e-fan is somehow more fuel-efficient, but it is inherently LESS so.
9. In typical American fashion, coolant is most often referred to by a misnomer: 'antifreeze'. That characteristic is as much a side-effect as a desirable one. But it IS desirable because water alone would freeze in many climates where vehicles are used, and even WITH antifreeze, this danger is still a cause for concern. So much so that every liquid-cooled engine incorporates freeze plugs to reduce the risk of engine damage resulting from water's peculiar characteristic of expanding when freezing. Ice is so strong that it will crack a mountain of the hardest stone, so a cast iron block doesn't stand a chance without carefully-designed & -positioned freeze plugs to relieve the stress. Steel being cheaper than brass, most factory-installed plugs are the former. Most aftermarket plugs; the latter, due to its corrosion resistance. Temporarary rubber freeze plugs are also available. In some climates, and often for diesels in any climate, some freeze plugs are replaced by a block heater; most often with a common plug for 110VAC household power routed to the grille so that it can be plugged into an extension cord overnight.
10. Other than collision, the most common cause for coolant leaks & blockages is corrosion
. Corrosion is a natural effect of pure metals & alloys being exposed to water, which naturally absorbs oxygen & CO2. It is also caused by dissimilar metals (iron, steel, aluminum, etc.) being in contact with an electrolyte (water with ions), called "Galvanic action". Both of these act continually in varying degrees to eat away at most metal components exposed to the coolant. Pump impeller blades, radiator cores, heater cores, steel pipe nipples, & thermostat housings are susceptible. The results of unchecked corrosion are leaks in the affected parts (usually the thin steel & soft aluminum ones go first) & sedimentation in the radiator, blocking the lower tubes. To combat their effects, various compounds are blended with the coolant. But they don't last forever, especially when the vehicle is NOT operated (stored/abandoned). So regardless of mileage, COOLANT MUST BE CHANGED REGULARLY
. And despite its intentionally-misleading name, long-life coolant must be changed on the SAME schedule, if not sooner. The "long-life
" terminology only applies to its antifreeze/antiboil characteristics; its corrosion-inhibitors are consumed even faster than standard coolant, making it "short-life
" coolant. Another marketing ploy is "ready-mix" coolant, which has gained much popularity over the typical concentrated (half-&-half) coolant previously available. A quick comparison of price (often higher for a gallon of ready-mix than for concentrate) shows that a vehicle requiring 2 gallons of coolant will cost more than twice as much to fill using ready-mix as with concentrate + distilled water.
There's a sucker born every minute - don't be one. Buy only normal-life concentrated coolant
, and mix it yourself with distilled water.
11. If you have a leak, don't waste time or contaminate your cooling system with any "trick fixes
" like cracking a raw egg or dumping pepper into the radiator. They don't usually work for long (if at all), and they cause problems later after the leaking part is replaced. Just START by replacing the leaky part, and you'll save money, time, & sweat. If you absolutely have to use a temporary fix, use Bar's Stop-Leak, which is a neutralized sawdust tablet.
When GM introduced its ill-fated (like so many other GM innovations) Dex-Cool coolant, it chose to distinguish its product (thankfully) by using an orange dye, instead of the common green. Both colors are intended to be detectable by UV light for tracing leaks, but Dex-Cool's formula failed for 2 reasons: 1) it contains a compound that is apparently very nutritious for certain bacteria, & 2) the tap water used at many GM factories for coolant mixing contained those bacteria. The resulting slime from the flourishing bacteria created an effective glue, which blocked up the coolant passages in the radiators & heater cores, causing mass overheating for several years. The problem has since been eliminated, but the color remained, causing more confusion. Ford went to a yellow dye (also UV-detectable) to distinguish its bittering agent (& a few other chemical changes), and now some aftermarket coolants contain other colors in an attempt to indicate compatibility with certain OE coolants. The typical result is simply MORE confusion, and the only remedy is to carefully read the labelling
, since no standard has yet emerged. Ford offers these quick-reference charts as PDFs:http://www.fcsdchemicalsandlubricants.com/dealer/quickref/scuc.pdfhttp://www.fcsdchemicalsandlubricants.com/dealer/quickref/ethylene.pdf
For more info, read these captions: