Coolant being heated doesn't create pressure because liquids don't expand very much. And since the container it's in (the radiator, block, & heads) are solids (metals), they also expand at roughly the same rate, so again: no real pressure change. And given that the rubber hoses (even with fiber reinforcments) will stretch fairly easily, they'd absorb whatever minor pressure change was generated by a liquid-only system.
The actual pressure comes from air in the system. It's SUPPOSED to be there. If you took it all out, the system wouldn't build up nearly as much pressure, but you'd spend so much time doing it that it wouldn't be worth the effort. And you couldn't design around it (lower pressure caps, etc.) because you couldn't be sure the system would NEVER get any air in.
But air pockets don't create hot spots because they don't occur against the ONLY things in the system that put off heat: the cylinder walls/head. Those are all sideways or downward surfaces within the coolant system, and air goes to the upward surfaces, away from the cylinders. So not only does air not cause high temp - even IF steam gets generated somehow (which is unlikely considering the pressure & anti-boil/anti-freeze additives), it won't survive for long because it'll immediately rise away from the hot cylinder & cool back to liquid against the upper surfaces of the cooling system, if not by merely passing thru the cooler liquid coolant.
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.
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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), 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.
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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 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. 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.
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