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Thread: Engine Cooling System

  1. #1
    Senior Member Sucked In Steve83's Avatar
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    Default Engine Cooling System

    Everything you never wanted to know about engine cooling
    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, compressed natural gas, alcohol, or diesel 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 with vented systems 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 - the thermostat is actually part of the PCV system. 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. 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 the engine & restrict the air moving through 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 through 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 through liquid fast enough, the liquid must be forced to flow through 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), erosion of the impeller blades (usually due to corrosion or cavitation; both caused by improper coolant blend), bearing seals leaking (they're drained through 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 through 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 through 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 through the upper radiator hose into the top of the radiator & through 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 through the heater core circuit. The outlet for 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. Coolant returning from the heater core 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 coolant flow in the heater core is irrelevant for its function, but some side-outlet heater cores can hold air if flow is reversed.

    . .

    6. As with virtually every substance, coolant (and any trapped air) 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) on this type of system, to allow coolant to return from the unpressurized overflow tank into the pressurized system when the system develops a vacuum (during cooldown). This return of vented coolant from the overflow 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. A failed cap is a more-common cause for collapsed hoses. It is also dependent on the overflow hose being airtight from the radiator neck vent to the bottom of the overflow tank. Also, the tank itself must be able to contain the vented coolant. These stipulations are some of the reasons 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 through 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 effect may be noted if the vehicle is lifted significantly, or if the hood is left open on the safety catch, or if the hood is vented incorrectly for the vehicle's aerodynamic flow.

    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 through 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'. Most of the time, it's preventing BOILING (even in cold weather), so "antiBOIL" would be more-accurate. The antifreeze 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. The bore plugs in most engine castings (necessary to release sand after the casting process) are frequently called freeze plugs, but that's another misnomer. If the coolant freezes hard, they may pop out, but the block & heads will still likely be damaged. Steel being cheaper than brass, most factory-installed plugs are the former. Most aftermarket plugs; the latter, due to its corrosion resistance. Temporarary rubber bore plugs are also available, but they aren't reliable. In some climates, and often for diesels in any climate, some boree 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. 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 to the concentration indicated on the back of the bottle for your climate. Coolant costs $10-15/gal and grocery-store-brand distilled water is generally less than $0.75/gal (thus averaging $6-8/gal). Don't pay $10-18/gal for ready-mix.

    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.

    12. Hoses, Pipes, & Nipples used to connect cooling system components must form airtight, watertight seals, and maintain those seals under a WIDE temperature range (-40 to 250F), pressure ranging from -5 to 20psig, and decades of exposure to coolant, contamination, engine bay fluids & chemicals, battery acid, road salt, air pollution, rodents & insects, and anything else in the vehicle's environment. Steel pipes & cast-iron nipples (like many water pumps) rust, and that scale can lift the hose away; Copper & Aluminum are typically thin, and can be easily abraded, collapsed, or corrode through; brass is more robust, but still susceptible to corrosion or mechanical damage; and the vulcanized rubber of most hoses can swell, harden, crack, split, delaminate from its reinforcing fibers, degrade from acid exposure, burn from being too close to the exhaust system, or slide off the nipple from poor clamping force. Common aerosol gasket adhesive (CopperCoat, etc.) will protect the nipple from some corrosion and help keep the hose in-place. High-quality stainless hose clamps maintain clamping force over a longer period, and a thin coat of silicone grease on the clamp's inner surface will keep it from adhering to or pinching the hose. For some applications, silicone rubber hoses are available, and they generally last longer than the vehicle (making used hoses a viable option). But the best protection for all these components is to simply change the coolant on-schedule. Use high-quality hoses & replacement parts. Doing so will also reduce its tendency to cavitate at the pump impeller, which actually abrades away the steel.

    13. COLOR 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 a quick-reference chart for Service Coolant Usage on this page, along with several other useful PDFs. Many European brands require O.A.T. (organic acid technology) which is a red coolant. Some BMWs (including some Land Rovers) use a blue type. In most cases, common green coolant is the best, and will do everything that needs to be done in any engine, with no side-effects.

    14. Radiator Testing:

    1) The most basic test of the cooling system is the ability to contain pressure. A simple pump with an appropriate adapter is connected in place of the cap while the engine is cool, and the system is pressurized to the cap's rated pressure while checking for leaks that might be small enough to evaporate from a running (hot) engine before detection. Another adapter can be used to test the cap's actual vent pressure. Neither a cap nor a radiator can be reliably repaired, but a leak AWAY from the tanks can be temporarily plugged by ripping out the fins around the leak, cutting the tube(s), & folding/crimping it shut. A leak near the tanks (too close to fold the tube) can't be fixed.

    2) Over time, sediments & debris can collect in the radiator, potentially blocking its core tubes. My method for checking is to remove the fan, shroud, & clutch (but NOT the belt - be sure the WP pulley is secure) from the cool engine, wet the radiator fins thoroughly, and start the engine. As it warms, the t'stat will open, allowing a sudden rush of hot water into the radiator. A fog will rise from the fins as the water evaporates off the tubes that are NOT blocked, and they'll dry instantly. Any tubes that remain visibly damp (usually at the bottom) are not flowing. If more than 1/4 of the radiator stays wet, I'd either backflush & retest, or just replace it. Old brass radiators used to be rodded out, but modern Aluminum cores aren't robust enough to tolerate that procedure reliably.

    15. MYTHCONCEPTIONS: The worst one (IMO) is that the thermostat is supposed to "slow the coolant down".
    Heat transfer is driven by temperature gradient (difference), and the gradient in the heads is around 1,000F (between the combustion chamber & the coolant). The gradient in the radiator is typically in the 100-150F range, but never more than 220 (for a fully-warmed-up engine in an arctic climate). So the slower the coolant flow, the hotter it gets, because it'll be picking up heat faster in the engine than it can get rid of in the radiator. If it was supposed to move slowly, there would be no need for a pump pushing it around. Fast-moving coolant cools better, and transfers heat faster. So the myth that removing a thermostat will cause the engine to overheat is absurd. Engines overheat because:
    1) there isn't enough coolant in the system (low leaks, air pockets)
    2) the coolant isn't moving fast enough (belt, impeller, blockage)
    3) the coolant is boiling at too low a temperature (weak mix)
    4) there isn't enough pressure in the system to keep the coolant from boiling (cap, high leaks)
    5) the radiator can't exchange enough heat out of the coolant (paint, blockage)
    6) the fan isn't moving enough air for the radiator to work at the ambient temperature (low speed in hot weather, fan clutch/motor, lack of a shroud, mud, leaves, flattened fins)
    7) the engine is running badly, causing it to produce too much heat (lean mix, advanced timing, overloading)
    If your engine was running hot, and you removed the t'stat and it ran HOTTER, it's because the real problem is still there, and without the t'stat, the pressure inside the heads is lower, allowing the coolant to boil more easily.

    A common misconception (misnomer) is that coolant spraying out or the sound of boiling means the engine has overheated. Not necessarily. "Overheating" refers to the temperature at which the engine becomes permanently damaged. An engine can get VERY hot, lose coolant or boil the coolant, and NOT be overheated. If the coolant isn't strong enough, or if the cap isn't holding pressure, or if the system contains too much air, the coolant can boil, causing a pressure spike that can spray coolant out. But boiling actually carries a LOT of heat away from the engine, so it's a form of protection from overheating.

    Another common mistake is to turn off an (apparently) overheated engine and immediately refill the cooling system before restarting it. That can destroy an engine; even one that hasn't overheated yet. The thermal shock of starting a hot engine (whose thermostat is wide-open) and pumping a radiator-full of cold water through it can warp the heads or crack the block. Assuming the pressure has ALREADY VENTED, there are 2 correct procedures for cooling a hot engine:
    1) With the engine off, open the hood & allow the engine to cool down by ambient air. This takes the longest, which is why it's the safest - there's no sudden temperature change to warp the metal. (Aluminum is ~13x more susceptible to warping than cast iron or steel, but cast iron is ~3x more susceptible to cracking.)
    2) With the engine off, collect some replacement water or coolant. Start the engine and SLOWLY pour in the cold liquid, allowing it to mix with the hot liquid still in the system. (If the system is dry, stop, and use procedure 1.) If the thermostat closes before the radiator is full (no flow in the radiator), shut the engine off until it reopens. Spare coolant is sometimes stored in the engine bay (especially in vehicles with known coolant leaks) so that it will be hot enough to pour in immediately.

    The last one is that popping the top on a hot coolant system can cause it to explode. THAT'S TRUE! The sudden drop in pressure can allow all the hot coolant in the hot engine (about 3.5 gal) to boil (vaporize) almost instantly, pushing the hot coolant in the radiator & hoses out the radiator neck, where it bounces off the open hood and onto the sucker who pulled the cap off. It has been known to cause blindness, in addition to life-threatening burns. If you MUST remove the cap from a hot system, use a HEAVY water-resistant cloth (thick plastic bag first, then folded towel on top) to block any spray, and prevent liquid or steam from touching your skin. Remember steam is invisible and hot enough to instantly remove flesh; the fog you can see is much cooler, and not nearly as dangerous, although it suggests the presence of steam. But it's always a better idea to just let it cool off.

    Searchability: cooling antifreeze freeze boil overheat

  2. #2
    Senior Member Sucked In THE BOSS's Avatar
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    Nov 2006
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    Default Re: Engine Cooling System

    good info for some who doesnt know. Also on My 90 7.3L I.D.I it takes a thermostate with a rubber gasket that covers the bell and not the flange if you look it up in the Stant books they are wrong,and the part stores dont have the right listing either and wont work properly after a year w/stant stats my engine would barely heat up during winter months and after about an hour of drive time but no heat if left idling. the required thermostat is still availible thru FORD but dont know for how long I ordered 4 extras.will post part# later
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