Bulletproof cooling system
by: Alittle1, Andromeda451, Cobalt327, Cooling, Crashfarmer, Crashtech, Crosley, Curtis73, Jon, T-bucket23, Techinspector1, Valkyrie5.7
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 How the cooling system works
The cooling system works by absorbing, transporting, and dissipating heat. Therefore, anything that impedes any of those functions can cause overheating:
Heat flows from higher temperature to lower temperature. A car's engine is generally cooled by keeping it in constant contact with a cooler fluid. As soon as that circulation is impeded, temperatures rise. The radiator must also be working efficiently to transfer engine heat to the atmosphere, and the thermostat must be in perfect working order.
The cooling system transports heat from the engine, into the coolant, and out into the atmosphere. If the amount of heat that it picks up from the engine is roughly equal to the amount of heat that it dispenses to the atmosphere, the engine temperature will stay constant.
As long as the coolant is taking on roughly the same amount of heat that it can dissipate, it will be effective.
It's easy to assume that cooler temperatures are better. However, they really just give you more of a buffer before the boiling point is reached. However, a properly functioning cooling system can really operate just fine at any temperature under its boiling point. If a cooling system is taking on more heat than it can dissipate, it will eventually overheat, regardless of the temperature at which the thermostat opens.
This is why using a lower-temperature thermostat often doesn't solve overheating problems. The heat that the coolant takes on must be roughly equal to the heat that it dissipates.
 Overheating and its causes
"Overheating" essentially just means that the cooling system is taking on more heat than it can dissipate. When coolant takes on too much heat, it boils. This means that the water jacket surfaces will be covered with more steam than coolant. Steam can't absorb heat like coolant can, and this exponentially exacerbates the problem: more heat results in more steam, which leads to less effective coolant, and, in turn, more boiling of coolant.
Overheating can be caused by anything that decreases the cooling system’s ability to absorb, transport and dissipate heat. Some causes of overheating are:
- Low coolant level, loss of coolant, or insufficient coolant capacity
- Buildup of deposits that cause poor conduction of heat into the cooling system
- A thermostat that won't open
- Poor airflow through the radiator
- Damaged or worn fan clutch, or broken fan
- Collapsed radiator hose
- Loose or defective water pump impeller
- Defective radiator cap
- Late ignition timing
If the engine overheats when idling, typically the cause is a lack of air flow through the radiator. This can be from a missing of non effective shroud, a too-small fan or a defective fan clutch assembly.
If the engine overheats when cruising, typically this is caused by a too-small radiator/insufficient capacity or by the radiator tubes being occluded or too small.
Lowering the opening temperature of the thermostat is not the cure for these problems. Lowering the thermostat temperature rating only delays overheating. Do check to be sure the thermostat is opening, though. If the thermostat doesn't open, the engine WILL overheat regardless if it's idling or cruising.
 Bulletproof cooling system tips
- Clogging and leaks are two of the most common radiator problems. Bugs, dirt, and debris can block airflow, and limit the radiator's heat-dissipating characteristics. Thus, it's recommended to "back flush" the radiator and cooling system when changing coolant. This helps to clean out deposits, and flushes the remaining coolant from the engine block. You can back flush the radiator by running water through it in the opposite direction of regular flow. Typically, after draining the radiator a t-fitting can be installed in the heater inlet hose. This fitting gets connected to a pressurized water hose, and the system is reverse flushed. Do this until clean water emerges.
- A rough guide is to use a radiator at least as large as the one that was originally used to cool the engine, with the same or more radiator cores. However, it's important to note that additional rows don't add a proportional amount of cooling, i.e. a 3-row radiator does not necessarily offer 50% more cooling than a 2-row radiator. This is because subsequent rows receive warm air from the rows in front of them. However, adding radiator frontal area IS proportional, but this usually causes fitment issues, so additional rows are generally the only viable choice. Also the radiator design and materials can have an effect on the radiator efficiency; a larger radiator is not necessarily a better radiator.
- Oftentimes, the cheapest and most bulletproof way is to use the largest radiator that will fit, along with the fan type and size, and shroud that was designed for the radiator from the factory.
- Use a full shroud, with the radiator positioned so that the fan blades are half-in and half-out of the shroud hole, and no more than 1" of clearance between the shroud and the fan blade tips (just enough to prevent interference when the motor rocks on its rubber mounts).
- Fan recommendations: OEM 18 inch, 7-blade steel fan with 2" to 2-3/4" pitch. The pitch of a fan can be measured by laying the fan down on a flat surface and measuring from the flat surface to the edge of the fan blade. Fans that are relatively flat (such as a flex fan) may not move enough air at idle and low engine RPM to cool the engine properly.
- When possible, use a thermostatically controlled fan clutch. While a thermostatically modulated fan clutch is an effective means of operating the cooling system's fan, a worn or defective fan clutch can cause overheating if left undiagnosed. Sometimes they may appear to be OK when cold but they will free-wheel when hot.
- Water pump and crankshaft pulleys sized according to what was on the engine from the factory. On a street motor, shoot for 1.2 to 1.3 times crank speed for pump pulley speed. This is usually true until you get to 3.55 gears and numerically higher, then 1:1 works better. Most 1960s muscle cars are 1:1. Sustained pump speeds over 4200 rpm can cause cavitation. Race vehicles may use a 2.3:1 ratio for a 9000-plus rpm engine.
- On a carburetor-equipped engine, often a 180º thermostat is used, although a little hotter thermostat rating (190º-195º) may make the motor more responsive and add a little fuel mileage. It may also help to burn off some of the by-products of combustion, such as moisture and acids which form and get into the oil. Motors using EFI induction should use the thermostat temperature specified by the factory for that particular motor to prevent false input to the computer and consequent problems. The sensor pill goes toward the motor.
- Use a spiral-wound spring in the bottom radiator hose, to prevent collapse of the hose.
- Use the proper pressure cap for the radiator being used.
- Ensure that there is adequate airflow from the engine compartment to allow the exit of the air drawn into the compartment. In custom applications this might require the removal or surgery of inner fender panels or using spacers to raise the hood of the car up an inch or two at the back.
- Maintain the proper coolant/water mix to prevent freezing up in winter. Water transfers heat better than coolant, but some coolant must be used to prevent freezing. Using a 50/50 mix of coolant/water is a necessity for motors using aluminum parts. Plain water will turn aluminum into oatmeal.
- Before installing the water pump, grasp the impeller with one hand and the drive hub with the other and twist to make sure the impeller is tight on the drive shaft. If the driveshaft is not spinning the impeller, no water is being moved through the motor. This problem can be the source of great frustration and is hard to find unless you know to look for it when installing the pump.
- Although it may not be necessary, the concept of a "water pump conversion disc" can be researched. Flow Kooler originally marketed flat aluminum discs to be riveted to the backside of the stamped steel impeller of the water pump. With an iron impeller, a steel disc could be welded or brazed onto the impeller. Such a disc wouldn't be that difficult to make. Space the water pump backing plate back farther with a couple of gaskets to prevent interference of the rivet heads on the backing plate if riveting a disc to a stamped steel impeller. More info: brazing cast iron, Flow Kooler water pump conversion discs. This disc could make an appreciable difference in the flow of water at engine speeds under 3,000 RPM. On the other hand, Howard Stewart of Stewart Components (the guy with the water pump dyno), says that these discs have little to no effect.
- FlowKoolerestablished a new standard for hi flow water pumps in 2013 with the release of a full line of precision machined billet impeller. The impellers have larger diameters and additional vanes and by reducing the clearance between impeller vanes mating surface create an appreciable improvement in flow rate and block pressure. This pressure increase helps prevent formation of steam pockets and hot spots on the cylinder wall and can also prevent cavitation of the impeller.
 Swapping a core support and matching radiator into a recipient vehicle
There are times when it may be easier, cheaper, or just better to swap in a radiator assembly from a donor vehicle to get better cooling, instead of buying a new radiator, etc.
In doing this swap, you will have to re-install the recipient vehicle's hood latch onto the donor core support in the proper location. Make up a fixture beforehand from scrap metal that bolts to the fender bolts or some other location that will be the same after the core support swap, and will show the proper location for the latch. This is a must-do when doing a frame or clip swap.
 Cadillac radiator swap
Any of the Cadillac Fleetwood or Eldorado’s from '70 to '76 with a 472 or 500 cid engine will work.
 Examples of donor vehicles
- 1976 Cadillac Fleetwood or Eldorado. For example: 1976 Cadillac Fleetwood 8.2 liter V8 radiator.
- Mid-70's Chevrolet truck with a 454. For example: 1975 Chevrolet C20 Pickup - 7.4 liter V8 radiator, 4-row capacity upgrade (and, same radiator in aluminum: here).
 Swap procedure
Call around and find a boneyard that still has the fan, shroud and core support. You'll be using a new radiator and viscous drive fan clutch to bulletproof your installation. Make yourself a memo of the exact year and model the pieces came from so you can match up the parts.
You may or may not have to alter the fan clutch hub where it bolts to the water pump/pulley. Usually, the holes are slotted so you can make it work. If not, some minor surgery on the hub with a rat-tail file will do the trick. With the motor in the vehicle and finalized for position, bolt the fan clutch and fan to the water pump. Mount the Cadillac radiator and shroud to the Cadillac core support.
The Cadillac core support will probably be longer side to side than the stock one in the recipient vehicle. Retain the outer pieces of the recipient vehicle support where it bolts into the body and cut the middle part of the recipient vehicle support out with a reciprocating saw, leaving a few inches on each side. Then, measure the opening between the two stubs that are still bolted to the recipient vehicle and cut the Cadillac support to fit into this opening. It's better to leave a little more sheet metal on the Cadillac support until you determine the correct position of the fan where it engages the shroud opening.
Then, position the Cadillac support with radiator and shroud attached up to the fan, equalizing the distance between the fan blade tips and the inner circumference of the shroud all around. Move the shroud around the fan until you have the fan blades halfway in and halfway out of the shroud opening. You may need to tilt the top of the radiator/shroud back a little at the top to match the fan angle if the motor sits in the recipient vehicle with a rearward tilt. If you need a little more front to rear clearance for mounting the support, you can position the fan blades a little further inside the shroud, as long as the fan clutch is at least 1" from the radiator core. A little further out should be avoided if possible.
With that accomplished, simply attach the middle piece of the Cadillac support to the stubs of the recipient vehicle support. Use whatever pieces of sheet metal or whatever that you have to in order to make the connection. The Cadillac support may end up sitting forward of the stubs or a little behind them or it might fall exactly into place and you'll have very little welding to do to stitch the Cad support and the stubs together. Just use your head and figure out how to connect the sheet metal, then MIG it in place.
Now, you will have a radiator that will cool anything and you still have the stock attachment of the stubs to the recipient vehicle so you can use simple hand tools to disassemble it later if you have to; it'll all come out as one piece.
This swap may not be for everyone, you will have to judge that for yourself. Consideration should be given to the weight of the system when at full capacity, this could mean as much as 50 extra pounds on the front end.
 Directing air flow
Moving the air through the radiator is one of the most important points of engine cooling and heat dispersal. In order for cooling to take place, the air MUST move through the radiator fins, and, by way of convection, the cooler air will remove the heat from the engine coolant to the outside air flow. For this to happen, the frontal area of the vehicle must be clear and the entering air must not be blocked.
The radiator fins must be clean, clear and unblocked by mechanical damage, i.e. folded over fins, plugged by bugs and dirt, etc. The air must pass THROUGH the radiator, NOT over or around it. Seal up the hood-to-radiator support air spaces with sheet metal, foam rubber or rubber sheeting.
The fan shroud should contain at least 50% of the fan blade as viewed from the side (blades half in/half out of the shroud looking across the radiator shroud), and the edges should be sealed to the contours of the radiator for maximum suction by the fan. Hot exhausted air should have an escape route out of the engine compartment. If it doesn't, make louvers or outside air scoops.
Direct air to pass over the engine and exhaust manifolds or headers, then out the bottom and sides away from the passenger compartment. If possible, don't remove the rubber skirts from the inner wheel wells over the suspension- they are there for a reason. The reason is, air flow is smoothed over the A-arms and is not turbulent or 'ragged' so it will flow out of the engine compartment faster. The rubber skirt also keeps road grime, stones and debris from entering the engine compartment and radiator area.
If you have an auxiliary transmission cooler in front of the radiator and also have a high stall torque converter, the fluid temperature in the transmission cooler will pre-heat the incoming air to the radiator. If this is the case, there may be a benefit to relocating the transmission cooler to another location.
Hood removal will cause buffeting of air in the engine compartment and result in uneven pressure and airflow through the radiator. Removal of side panels on three piece hoods will exhaust air better in some cases. Louvers in hoods and side panels are a godsend to ventilating hot air from the engine compartment. Fade-away fenders of the '30s and '40s remove a lot of air from the engine compartment.
All air in front of the radiator cradle is positive flow, air after the radiator cradle is negative flow in a driving automobile. In order for that arrangement to continue, air must be evacuated from the engine compartment. When the vehicle is at a standstill, the engine fan provides flow from positive to negative and cools the engine compartment. The fan should have the ability to push/pull enough air to keep the temperature within operating range of 160º to 210º.
If you are at the drags and need to cool down between rounds, run your electric water pump and fan along with a portable squirrel cage fan to bring down the temperature. On the return road turn the heater and fan on high if so equipped.
 Water pumps: electric vs. mechanical
Electric water pumps are constant flow pumps that push ‘X’ amount of gallons of water per minute, no matter what the rpm of the engine is.
Mechanical water pumps are variable speed pumps which have decreased flow at idle and increased flow at higher speeds within their limitations. The limitations are mainly RPM induced cavitation. Mechanical pumps will only operate while the engine is running, but with electric pumps, operation is user-selectable.
On a SBC for instance, the coolant flow of the OEM mechanical water pump is around 10-12 gall./min. per 1000 RPM.
The "usual" drive ratio of a SBC mechanical water pump is between 1:1 and 1.3:1, over-driven.
The location of the pump is important. Some vehicles have been built with high mounted water pumps which have been a disaster. In the case of a vehicle which has its engine mounted north-south, the water pump will often become the highest component in the cooling circuit when the vehicle is ascending a steep hill. Engine designers ought to study “steam trapping” and “air venting” as it is a very important topic even for things that do not run on steam.
With a transverse engine vehicle it is possible to have the water pump behind the engine block. With this arrangement steep hills will not starve the pump of water.
 Serpentine cross-flow radiators
Most cars from 1960 and up used cross flow radiators. One of the reasons was a lower hoodline, and two, more cooling area was required to cool the larger engines. Cross flow radiators had a tank with an inlet/outlet placed on either side. The water entered on one side and passed through the core of the rad, was cooled by the air flow and the heat escaped through convection to the outside air. Engineers found that the longer the liquid was exposed to the cooling flow of air through the radiator core, the more heat could be extracted from the flowing water. They slowed down the water travel by increasing the size of the water pump pulleys, but that had its limitations. They also added more rows of core, but that too had limitations.
Road course racers found a way to keep cooling to a simple easy form. To do this, they pulled the tanks off the radiators that they were using and placed baffle plates in the tank covers. The baffles were placed so as to divide the radiator core section into three distinct areas. Water would enter the upper radiator inlet on the right side and would flow across the top section of the radiator to the left side, a baffle plate located 2/3 of the way down the tank caused the coolant to flow across the radiator to the right side to the right radiator tank. The coolant couldn't rise upwards because a baffle plate located 1/3 of the way down stopped it and forced it to head down lower in the right tank, where it again was drawn across the radiator core to the lower left tank outlet and out to the engine. This serpentine course that the coolant took allowed the coolant to be cooled THREE TIMES by the cooling air flow coming through the core area. "Excellent idea!" you say, “Why don't they do that to all cars today?” In a closed course environment, the theory works, but in real everyday life the average auto might never warm up to operating temperature during the daily commute.
 Does radiator tube size matter?
The automotive radiator is essentially just another name for a heat exchanger, whereby combustion temperatures are transferred to the cooling system of the engine block and taken outside the block via flexible radiator hoses to be exposed to the cooling force of air through the radiator core, thus reducing the temperature of the coolant before returning it to the engine block. There are two restrictions in the system. One is the thermostat, which restricts flow and holds heat in the engine until warmed up, and the other is the radiator core tubes. The radiator tubes have to be of sufficient size so as to allow the coolant to flow through in an unrestricted manner, but also able to 'scrub off' BTU's or heat; which is based on the shape of the tube and the convection of heat away from the coolant to the outside air. A wide flat tube will expose more surface area to the outside flow of air than a narrow tube. The reason for this is more surface area is exposed to cooling. Look at the pictures located below and see why that is.
 Aluminum radiators
If you are designing/redesigning a cooling system for your car, the utilization of the aluminum radiator is the best overall product on the market for the dollar. This is not to say that the radiators made from copper and brass are not good, and if you have one that works, don't go out and change for the sake of change. But, the choice of aluminum media will outperform their copper counterparts quite easily even though copper is a better conductor of heat. What is a lesser conductor of heat (aluminum) makes up with more surface area available for heat exchange. A 1.25" two-row aluminum radiator will cool just about anything up to 450 HP. If designed correctly, it will outperform most 4 or 5 row copper brethren. Not only does aluminum offer a great deal more surface area for cooling, but also has a more rigid structure, making for a less likely leaky situation. Also to the credit of this technology and the fact that more modern cars are implementing aluminum, more and more vendors are competing in this product line making for very attractive pricing.
 Sacrificial anode in aluminum radiators
When running an aluminum radiator or any aluminum parts in contact with the water jacket, make sure to run a sacrificial anode. Aluminum is prone to electrolysis and corrosion and in many cases cannot be repaired. Usually zinc, magnesium or a combination of the two can be used as a consumable "edible" part to prevent the electro-displacement of the aluminum radiator. This may save the aluminum core, in the short run.
Sometimes the key to a bulletproof system is what you put in it and how. You can't just open the radiator cap and dump anything in it. A 50/50 mix of water and ethylene glycol antifreeze boils at 225º if there is no pressure like when the radiator cap is open or defective. But if the cooling system is sealed and will hold pressure, a radiator cap rated at 15 psi will increase the boiling point of a 50/50 coolant mix to 265º. If the ratio of antifreeze to water is increased to 70/30 (the maximum recommended), the boiling point under 15 psi of pressure increases to 276º.
Water is the basis of coolant in most systems. Can the water that you start with have a detrimental effect on your cooling system? Yes! Water is not necessarily clean and free from contaminants. Water can contain acid, alkaline, foreign matter, etc. These contaminants can combine with the metal within the cooling system and contribute to plugging or slowing down the flow within the system. Today, you will find about 10 different antifreeze products and about 30 different additives for your cooling system. WHY do you need them? Good question!
Engineers have come up with cooling system recommendations based on their extensive research and their recommendations and warnings should be heeded. Your OEM dealer and manufacturer usually recommend using a 50/50 mix of antifreeze and water. This is to protect and prolong the life of cooling system metals and seals. Using unapproved additives or incorrect chemicals or ratios, etc., could be harmful to the cooling system and could also result in needless repair and expense.
When you are refilling your cooling system, the radiator cap is open, and you pour directly into the system until it is full. "Full" means a level one inch less than the cap height. The engine should be warmed up and running at a fast idle of 1000 to 1200 rpm's. The engine is run until you can see movement in the radiator and a slight steam rises from the open cap outlet. If you have a gauge, verify that the temperature is at operating temperate of 160º to 195º. The cap is placed on the radiator outlet and turned until tight with the arrows aligned to point at the overflow outlet. The overflow bottle should be within its limits, which is usually marked on the container walls.
When you are adding to the system, do not open the radiator cap. Instead, add directly to the reservoir tank.
If you have ever watched a pot of water start to boil on the stove, you will know that tiny air bubbles start to rise from the bottom of the pot as the heat is raised. Adding a water 'conditioner' like WaterWetter to your coolant will break the surface tension and provide a greater contact area for the coolant. A wetting additive can supposedly decrease the temperature up to 20º F.
On a closed system with an overflow bottle, that the system should be filled to the top when it is at operating temperature. One of the advantages of this type of system is to reduce oxidation by eliminating all air from the system. Hence it then becomes a closed system. Why leave a head of air in the top of the radiator when you don't have to? Entrained air is sometimes difficult to get out of the system. The secret in the system lies in the overflow bottle. Coolant expands with heat, thus increasing the total water volume mass. When the pressure increase exceeds the radiator cap's capacity, the coolant passes from the radiator to the overflow bottle which is vented to the atmosphere. When the mass is cooled due to the air flow through the radiator or turning off the engine, the radiator cap will allow the coolant to be syphoned from the overflow tank back into the radiator/cooling system, thus keeping a set amount of mass present in the cooling system.
 Radiator shroud
Radiator shrouds are devices that control the exiting air from the radiator and direct it to a rearward sucking electric or mechanical fan. Shrouds can be made from plastic, fiberglass, or metal. They cover the rear portion of the radiator. They allow air passing through the radiator to be ducted to the fan which directs air flow over the engine and out of the engine bay. In most cases, the mechanical fan will be inserted approximately 1/3 to 1/2 of the depth of the fan blades into the shroud, and no more than 3/4" from the tip of the blades to the edge of the shroud.
 Radiator cap
A radiator cap is important to the proper operation of the entire cooling system. The cap is designed to hold pressure in the system. Under pressure the coolant boils at a higher temperature than when non pressurized (approximately 3º per 1 psi) so a 10 psi cap will add 30º to the boiling point of the coolant. Check the cap gasket for cracks and other damage. If the cap pressure spring is badly rusted replace the cap, observing the pressure stamped on the old unit, acquire one with the same pressure rating. During inspection make sure that the radiator filler neck (where the cap lives) is clean and the sealing surfaces are undamaged.
A radiator with a nicked or damaged sealing surface can be repaired by employing a metal filled epoxy to fill the imperfections. Recommended steps:
- Sand the sealing surface to bright metal
- Clean the sealing area with water
- Dry thoroughly
- Mix the metal filled epoxy following manufacturer’s directions
- Apply a thin bead on the sealing surface
- Using a plastic implement smooth the bead covering damage and imperfections. Make sure the epoxy is smooth and even.
- Let harden, install cap.
Note: This technique has been used on GM, VW and Porsche radiators to good effect.
The thermostat has two important jobs to perform; to accelerate engine warm-up and to regulate the engine's operating temperature. A quality thermostat ensures excellent fuel economy, reduces engine wear, diminishes emissions and blow-by, improves cold weather drivability, provides adequate heater output, and deters overheating. This is accomplished by blocking the circulation of coolant between the engine and radiator until the engine has reached its predetermined temperature. The thermostat then opens as required in response to changes in coolant temperature to keep the engine's temperature within the desired operating range.
Thermostats have a “rated” temperature such as 180º or 195º. This is the temperature the thermostat will start to open, give or take a few degrees.
Usually located within a metal or plastic housing where the upper radiator hose connects to the engine, most of today’s thermostats utilize the "reverse poppet" design, which opens against the flow of the coolant. Thermostats have a wax filled copper housing or cup called a "heat motor" that pushes the thermostat open against spring pressure.
As the engine's coolant warms up, the increase in heat causes the wax to melt and expand. The wax pushes against a piston inside a rubber boot. This forces the piston outward to open the thermostat. Within 3º or 4º of the thermostat preset/rated temperature which is usually marked on the thermostat, the thermostat begins to unseat so coolant can start to circulate between the engine and radiator. It continues to open until engine cooling requirements are satisfied. It is fully open about 15º-20º above its rated temperature. If the temperature of the circulating coolant begins to drop, the wax element contracts, allowing spring tension to close the thermostat, thus decreasing coolant flow through the radiator.
On some applications, the thermostat performs an additional function. It closes off a bypass circuit inside the engine when it opens the radiator circuit. The bypass circuit circulates coolant inside the engine so that hot spots can’t form when the radiator circuit is closed.
Many thermostats have a “jiggle pin” that allows trapped air in the cooling system to pass through the thermostat and be removed from the system. If a Stant thermostat does not have a jiggle pin, it will have a "bleed notch” or other method of removing air from the system.
 Thermostat failures
There is no such thing as a thermostat that will fail in a “safe” position. All thermostats will fail in either a closed or open position. One brand claims it fails in a safe position, but it simply locks itself open when it is a full stroke open position. It will not spring open if it fails in a closed position.
A thermostat fails “open” if the return spring breaks or debris prevents the thermostat valve from fully seating or closing. In this instance the thermostat allows continuous coolant flow to the radiator; therefore, the engine will be over cooled. The tangible effects are poor warm up and heater performance, increased engine emissions and reduced fuel economy. For these reasons, an engine should never be operated without a thermostat in place, even in extreme temperatures.
A thermostat will fail “closed” if the wax element has been damaged by overheating (from loss of coolant, a defective electric cooling fan or fan clutch) or corrosion (from not changing the anti-freeze often enough). This failure prevents the flow of coolant to the radiator; therefore, the engine will be overheated. The tangible effects are a boil over, the inability to operate the vehicle, and the likelihood of severe engine damage. For these reasons alone, when an engine overheats, it’s a good idea to replace the thermostat whether it caused the problem or not.
 Replacement thermostats
The temperature rating of a replacement thermostat must be the correct one for the application because of the adverse effects the wrong thermostat can have on drivability, engine performance and emissions.
The temperature rating specified by the car manufacturer is especially important in many 1981 and newer vehicles because the on-board computer monitors coolant temperature through a coolant sensor to control fuel enrichment, spark timing and operation of the EGR valve. Even on vehicles without computers, thermal vacuum switches that react to a specific coolant temperature are often used to open and close various vacuum circuits that regulate fuel enrichment, timing and EGR. If a colder thermostat is installed, the coolant may never get hot enough to trigger the appropriate control functions or to allow a computer system to go into “closed loop”. Too hot a thermostat can also interfere with the proper operation of engine controls, and increase the engine’s operating temperature to the point where it may experience detonation (spark knock).
 Thermostat checks
One way to determine if the thermostat is doing its job is to feel the upper radiator hose after starting a cold engine. The hose should not feel hot until the engine has warmed up. If the hose starts to feel hot after only a couple of minutes, the thermostat may be stuck open or not closing completely. Once the engine is warm, the hose should feel hot as coolant circulates between the engine and radiator. If the hose does not feel hot, the thermostat may be stuck shut, blocking the flow of coolant.
A thermostat can be tested by suspending it, using a string through the valve, in a bucket of boiling 50/50 coolant and water. If the thermostat is working it will fall off the string as it starts to open after being in the hot/boiling coolant for a few minutes. When removed and allowed to cool, the thermostat should close.
 Replacement tips
- Don't overlook the water outlet covering the thermostat. Check for cracks, broken flanges, internal pitting and corrosion, and erosion at the hose neck (a real problem with most aluminum housings). The gasket surface must be flat and free from warping or deep scratches.
- Scrape the mating surfaces on the thermostat housing and engine to remove all traces of old gasket material. Use care on aluminum because the soft metal can be easily scratched.
- Temporarily stuffing a clean rag into the thermostat opening on the engine while the housing is removed helps keep debris out of the cooling system.
- Install the new thermostat so the copper heat sensing element is toward the engine. If installed upside down, it won’t open.
- Torque the thermostat housing bolts evenly and to the manufacturer's recommendations.
- To insure air has been removed from the cooling system after replacing a thermostat, be sure to run the engine a few minutes, let it cool, and refill the antifreeze as needed.
NOTE: The old type of thermostat used metal bellows filled with a liquid. The condensed liquid would "suck" the bellows closed. This type of thermostat always fails in the open position which is extremely convenient as one does not have to buy a new cylinder head or engine. Nowadays this type is very difficult to obtain.
 SBC 400 cooling
From a Chevy High Performance article:
This 1003 high- performance Fel-Pro head gasket (below) features larger 7/16-inch coolant passages (a) that will produce greater coolant flow to prevent excessive heat buildup between the center cylinders. The high-performance Fel-Pro gaskets also reduce coolant flow in the indicated areas (b) to help redirect the coolant between the center cylinders. If necessary, you may have to drill a 7/16-inch hole (c) in the block to increase coolant flow between the center cylinders. Only do this when the engine is completely disassembled to prevent iron drill chips from damaging the engine.
 Ford Motor Company
 See also
- Crankshaft Coalition wiki pages
- Thermostats explained
- Make a fiberglass fan shroud
- DIY junkyard electrical fan controller
- Electric cooling fan installations
- Relay application guide- Bosch relays