Ford Mustang Cooling System Basics
Understanding how your Mustang’s cooling system works
Your Mustang's cooling system is one of the most basic systems of the internal combustion engine, be it a vintage or a late-model Mustang. The cooling system's function is a simple one; transfer the heat produced by the combustion process to an external heat exchanger, via fluid that is pumped through the engine via a pump, where the heat is radiated to the atmosphere with the help of a fan that pulls air through the heat exchanger. Yeah, that description was a little “shop manual-ish“, but you get the idea. No matter the year or model of Mustang you own, while the components may differ slightly, the cooling system is essentially the same. Whether it is a '66 Mustang with an inline-six or a '09 Mustang GT with a 4.6L Three-Valve, they all have a radiator, a water pump, hoses, a cooling fan, and coolant/anti-freeze.
What happens when you have a problem with your cooling system however? Be it a leaking water pump or perhaps an overheating problem, how do you find out what the problem is? Or perhaps you're building a restomod or modifying your late-model with a supercharger, how does this affect your cooling system? Do you have a radiator of sufficient size to transfer the heat your engine generates efficiently? What about your choice of cooling fan, or even the water pump and cooling hoses? These are all important decisions that need to be made to ensure your cooling system can handle your performance upgrades. Let's start with an overview of how the Mustang's cooling system works to keep your engine running at the proper temperature.
The Heart of the System
The water pump is belt driven on Ford engines and pressurizes the coolant passages of the engine block with coolant where heat from the combustion process transfers to the coolant and is then passed to the cylinder head(s). In a V-8, the coolant path travels to the rear of the block and then flows forward to the intake/thermostat and forced through your upper cooling hose to the radiator core where the hot coolant is cooled by ambient air forced through the fins of the radiator, either by ram air at speed or via a fan that pulls the air through the core. After travelling through the radiator, the water/coolant is returned to the engine via the lower radiator hose to the water pump to repeat the cycle.
The importance of a quality water pump cannot be underestimated. As the core of the cooling system, you shouldn't skimp on your water pump for your Mustang. If your water pump is of questionable history/age, it is a worthwhile investment to install a new high-flow unit. Look for quality hardware like a large bearing shaft, billet steel hub, CNC machined impeller (and not just a universal stamped steel one), and a quality casting. Don't rely on a $29 remanufactured parts store water pump to cool your high compression aluminum headed stroker!
The Cooling Cop
The cooling system's thermostat controls coolant flow through the engine and is essential to how the cooling system works. The thermostat controls the cooling system's flow through the block. When cold, the thermostat is closed and restricts coolant flow, allowing the block to warm up quickly (hot water is needed for cabin heater use, for example). Once the coolant in the engine has reached the appropriate temperature, the thermostat opens and allows the coolant to transfer to the radiator where the heat is dissipated. We highly recommend that all street driven cars use a thermostat of the proper rating. Typical thermostat ranges are from 160-degrees to almost 200-degrees. Modern EFI engines use higher temperatures, typically in the 192-195-degree range, while traditional carbureted engines will normally run just fine with 180-degree thermostats. It is rare these days a 160-degree thermostat is used and using a lower than required thermostat in an attempt to fix a cooling system problem (or running no thermostat at all) is nothing more than a Band-Aid fix. Find the real root of the cooling system issue instead.
When it comes to radiators, there have been huge advancements over the years. From down-flow copper brass radiators to aluminum cross-flow to hybrid radiators made from aluminum and plastic, the radiator is arguably the second most critical cooling system component behind the water pump. The most important factor when considering a radiator for your Mustang is the surface area of the radiator core. Maximize your surface area for the best cooling efficiency. If your core size is smaller than your radiator support opening, you're not taking advantage of the available air flow opening/surface area. Once you've maximized the surface area of the core size, you can then go thicker with the radiator core to further increase cooling. Adding core thickness does not increase the radiator's efficiency as much as maximizing the core's surface area however. A thicker radiator core does have more resistance to airflow than thinner radiator cores, but the difference is typically minimal considering a 4-inch thick radiator core has roughly 10 percent more resistance to airflow than a 2-inch radiator core.
Core thickness has been blamed for decreased cooling many times, with the thought being the air flow through the thicker core was reduced and became fully heat saturated before exiting the core. Sounds great in theory, but usually what is happening is a decrease in coolant flow, not air flow. An older down-flow radiator design like that found in a vintage Mustang usually has a very narrow cooling tube design, which is fine with a stock water pump. When you move to a modern aluminum radiator with wider coolant tubes, there isn't sufficient coolant flow to create turbulence in the tubes with a stock type water pump. You need this turbulence to force the coolant against the outside walls of the coolant tubes for thermal transfer to the tube wall and subsequently to the air passing over these coolant tubes. With the combination of a modern large tube radiator and a stock-flow water pump, the velocity decreases and you don't get the turbulence you need for proper heat transfer. If the radiator core is doubled in thickness, the coolant velocity is halved. This is why we recommend a modern cross-flow design in vintage Mustangs whenever possible (short of a concours car that needs a stock radiator), as it uses wide tubes with less cross-section (thinner cores), which requires less velocity to achieve the needed heat transfer.
One thing many people don't consider when choosing a radiator is fin count. Yes, we want the largest core possible, but the fins between the cooling tubes of the core are what give the radiator surface area and help transfer the heat to the air passing through the core. The higher the fin count, the better the radiator will cool compared to a core with fewer fins. The main concern with higher fin counts is keeping the core clean of dirt and debris, as the higher fin count traps these insulators more easily, decreasing the efficiency of the radiator. Fin count doesn't come into play much unless you having a custom radiator built to your specs for a modified car or custom project but it is something to consider if the radiator manufacturer offers optional core/fin count options.
The last thing to consider when it comes to radiators for vintage Mustangs is whether to retain the original classic style down-flow radiator with tanks on the top and bottom, or to update to a more modern cross-flow radiator. If there's room for a proper cross-flow core design, and the Mustang doesn't need to have correct under hood appearance, we recommend the cross-flow design for the reasons mentioned previously, plus they position the radiator cap on the low pressure side of the system. A high flow pump can create high system pressures, which will force water/coolant past the radiator cap at high rpm, something that's terribly common on down-flow radiators, which is why we also recommend the highest cap rating you can find if you're going to keep your standard down-flow radiator due to space constraints (the stock 13 psi cap doesn't cut it with today's engine rpm capabilities). For late-model owners, a cross-flow radiator is standard in these applications so your main area of concern should be core thickness and fin count.
Caps and Coolant
Speaking of radiator caps, it should be noted that the fill cap needs to be the highest point in your cooling system. This isn't usually a problem with your typical small-block and down-flow radiator or a stock radiator setup, but larger engines (dimensionally) and certain engine placement issues, like a modular engine swap, can sometimes mean the highest point in the system ends up somewhere in the cylinder head or intake manifold versus the radiator cap (you modular and V-6 owners are likely familiar with vent fittings on your intake to help refill an opened cooling system). If the top of the radiator is not the highest point in the system, then you must use a degas tank of some sort and mount it higher than the engine. If you're yanking a modular engine from a wrecked late-model Mustang, do yourself a favor and grab all of the stock cooling hoses and the degas tank from the car and factor them into your build. It will save a lot of headaches down the road trying to figure out hoses, cooling issues, and so forth. The one thing you don't want is trapped air in your system. While trapped air is inevitable in a new build, it is imperative your cap is at the highest point to help extract the trapped air, as the trapped air will always seek the highest point. We've seen people raise one side of a car to move the trapped air out or even add a radiator petcock to a coolant passage in their intake manifold to bleed out the trapped air.
As noted previously, the classic 13 psi cap you'll find as OE on classic Mustangs is not suitable for today's driving environment and should be restricted to “show use“ in our opinion. The higher the rating of your radiator cap, the higher the system pressure, which raises the boiling point of the coolant and increases the ability for the coolant to transfer heat from the engine. Check with your radiator manufacturer and use the highest pressure cap the radiator is rated for. Don't be surprised to hear that performance radiators can often handle a 22-24 psi cap. We highly recommend overflow canisters for vintage Mustangs along with a modern radiator cap. The combination allows for coolant that has expanded into the overflow to be pulled back into the closed cooling system as it cools. This is how all late-model Mustangs are setup.
Water is by far the best “coolant“ you can use in your cooling system. However, water alone does not inhibit corrosion, nor does it have the ability to raise the freezing point of water all by itself. This is why you need some form of cooling system corrosion inhibitor at the least and for cold climates the proper anti-freeze mixture to prevent the system from freezing solid. Believe it or not, a 50/50 mix, as many manufacturers recommend, is actually too high of a ratio for warm climates. If you live in a warmer climate a 70/30 water/coolant mix is better, or like we said, straight water with a conditioner/lubricant/inhibiting agent added to the system is preferred. For those of you who enjoy a good track run, check with the sanctioning body, as many tracks prohibit the use of coolant on the track surface.
Moving the Air
Having the biggest radiator you can fit in your engine bay and a high-flow water pump won't mean much without adequate airflow. Essentially, you have two avenues: electric cooling fans and belt driven fans. Each category has its good and bad points. For most, the traditional belt driven fan is what we're used to in the vintage Mustang engine bay, though Ford did use a belt-driven fan up until 1994 when the Mustang finally went with an electric fan from the factory. The fan bolts to the water pump's hub and is driven by a pulley and belt combination. We've seen fans installed backwards, we've seen the wrong pulley ratio used (salvage yard pulley swap for example), and we've seen belt routings that do not put enough belt surface on the pulley, causing slippage. Of course, you can have too small a fan, or a fan with not enough blades to cool as well.
When using a belt driven fan, we recommend the “A/C“ style fan with deep curved blades to really pull the air through the radiator core instead of the stock four-blade fan most vintage Mustangs came with (again unless you have to run the stock parts for show use). If you're using a belt-driven fan, then use a shroud, no excuses. Air flow will take the path of least resistance, and that will mean around the radiator instead of through it. Ensure the fan blade pitch is half into the shroud and half out by changing or adding a fan spacer as required to ensure the air flow is drawn through the radiator instead of around it.
As we just stated, the Mustang has used an electric fan setup with multiple speeds since 1994 (that's nearly two decades now) so electric fans certainly aren't just for race cars or for added towing insurance anymore. Electric fans allow for more room in the engine bay and offer better control over your cooling system, as they are an “on demand' cooling device, only running when the thermostatic switch in the coolant stream deems the fan necessary (low speed driving, long periods of idling, etc.). In some instances, like modular V-8 swaps, they are mandatory since there's no way to mount a belt driven fan to the engine's water pump hub. There are essentially only two ways to mount an electric fan, either as a pusher or a puller; meaning on the engine side of the radiator pulling air through, or on the grille side of the radiator pushing air through. An electric fan is going to be more efficient as a puller, but if you must use a pusher type, ensure it has adequate core coverage and moves enough air for your application.
When it comes to fan coverage, you want as much of the radiator core covered by the fan as possible, but a minimum that should be considered is 70 percent. If a shroud is available for your electric fan(s) package, by all means use it. The shroud not only makes the fan more efficient, as it is pulling air through the entire core, but the shroud usually makes the installation of the fan easier since the shroud reaches the mounting edges of the radiator itself. While we've all done it, the last thing you should use for any sort of long term fan mounting solution are those plastic “through the core“ zip tie affairs. The weight of the fan, coupled with the vibration of it in use, can cause the fan to wear/cut through the radiator's cooling tubes when mounted in this manner. At the least use solid mounting ears/straps and when at all possible a shroud (1⁄4-inch deep at a minimum) is the best solution.
There's a fairly common misconception that S-shaped blades outflow straight blades on an electric fan. Truth of the matter is, more often than not the S-blade fan has a different motor on it which increases the airflow cfm, so we're not comparing apples to apples here. According to engineers we spoke with at SPAL, straight blade fans are usually the more efficient of the two styles if the motors are the same, however they do have a blade pitch that is slightly noisier than the S-blade style. No matter the size of the fan or the type of blade, it is going to make some noise. When you move air you create noise. When looking at electric fans, be aware of cheap models that cut corners. On large diameter fans, you'll find a support ring to stabilize the blades so they don't flex and cut into your radiator core. Also look for glass reinforced plastic for the fan body and blades. This increases the stiffness of the unit as a whole and prevents blade breakage.
Lastly, a quality fan will often have an IP68 rating for dust and water intrusion. Many low dollar fans aren't rated as such and driving in rain can severely shorten their lifespan to a matter of months. Many electric fans come without any wiring or controls, leaving it up to the installer to determine how to control the fan. We recommend controlling an electric fan via a thermostatic switch in the engine. Fan wiring should be sized properly for the amp draw of the fan motor, and due to the rather high amp draws of the typical fan, you should always use a relay to allow direct connection to the battery (properly fused) so that the thermostatic switch turns the relay off and on for fan control.
Keeping Your Cool
Now that you have some education and background on your Mustang's cooling system and its core components, we can delve further into the typical cooling system issues that plague Mustang owners, what to look for, and how to fix these issues. We'll work our way backwards in model years, starting with the latest Mustangs. The '05 and up Mustang cooling system is a reliable setup that really only needs some attention if you plan extended track use of your Mustang. As Ford has upped the power on these cars, you'll notice larger grille openings, larger radiators, and even model specific electric cooling fans. That's great news if you own a '13 GT 500, but what if you have a '08 GT with a supercharger? Upgrading the radiator is going to be your best bet, along with ensuring adequate airflow. In really hot climates, consider grille opening changes for the maximum airflow. There's not much in the aftermarket in the way of modular engine water pumps, but the stock unit works well.
For the '94-'04 Mustang, you can find several aftermarket radiator options to enhance cooling or to use as a service replacement part for an extra margin of cooling safety. One thing to be aware is of the older 4.6L Two-Valve plastic intake manifold. It is hard to believe there are still some out there, but we just saw one being used in a custom build recently. If your '96-'00 Mustang GT does not have the improved intake manifold with an aluminum coolant crossover passage at the front of the intake, we recommend changing the stock all-plastic intake to the improved version ASAP. You can actually find this intake at most Mustang vendor's websites and even through your favorite auto parts store via Dorman's product line. All '94 to current Mustangs also use aluminum cross-flow radiators with plastic end tanks and external degas/fill tanks. These radiators are much more fragile than the earlier all-brass radiators so care must be taken when removing hoses and you should be extra vigilante with your cooling system maintenance/flushes. All '94 to current Mustangs also use a two-speed electric cooling fan controlled by relays and the PCM. There are some electric fan upgrade kits on the market, just make sure what you're buying is better than the stock setup, as it works quite well.
As we move back to the Fox era Mustangs, the cooling issues really start to be more prevalent. You have a combination of older hardware with more mileage on them, multiple ownership paths with little-to-no maintenance records, and quite often more performance modifications such as aftermarket cylinder heads, high-lift camshaft, and more. Fox Mustangs were Ford's first use of reverse-rotation water pumps with a serpentine belt-drive system. These pumps actually spin backwards from the typical water pump found on a V-belt application. Getting the wrong pump for the application can mean big cooling issues. For a Fox owner this isn't much of a problem, but as more and more of these engines find their way into vintage Mustangs, you have to be aware of the application. You can't simply bolt your V-belt pulleys back onto a 5.0L engine and have it cool properly. You either need to retain the serpentine belt-drive gear, or switch to a standard rotation water pump with the proper inlet orientation.
Getting back to the Fox Mustang though, one of the biggest issues is again simply age. We see a lot of Fox Mustang cooling issues coming from stock parts past their service life (hoses, fittings, gaskets) and the original two-core radiator was marginal at best at keeping the stock 5.0L cool. Add high ambient temperatures, the A/C condenser shedding a lot of heat, and a slipping fan clutch and you have a recipe for overheating. The stock plastic fan on the Fox is notorious for cracking around the base/hub and the fan clutch will often be shot, with telltale oil seepage from the center of the clutch and a buildup of oily dirt around the clutch spring. The Fox era cooling systems also used a traditional radiator cap with an overflow bottle. A simple pressure test of the cooling system will show if your cap is not holding pressure or if any other part of the system is leaking.
Now this brings us to the vintage Mustangs we all love to admire but sometimes hate to drive. You know who we're talking to—those owners who can't drive on the highway with their Mustang club to an event for fear of overheating. Or maybe you're that one Mustang at the cruise night that leaves a nice little green trail to their parking spot and carries a gallon of water in their trunk so that they can “top off“ to get home. These scenarios sound familiar? They don't have to be if you're willing to lose some of the stock appearance and pry open your wallet. Yes, fixing your cooling system isn't going to be free; there's no magic fix. However, there's a lot to be said for being able to simply pull into a show or cruise night, park your car, and enjoy the evening without worrying about how you're going to get home.
Probably one of the biggest issues with vintage Mustang cooling systems is the fact that they sit a lot and don't see much use. Coolant has a lifespan and you need to flush out and refill your cooling system to maintain its ability to protect your cooling system and shed heat. For a vintage Mustang that sits a lot and is only driven a few times a month it isn't a lot to ask to flush the system annually. The stock down-flow radiator in the vintage Mustang isn't the best radiator design either, and while it is hard to fit a cross-flow design into a '65-'66 Mustang, later years you can often fit one. The old 13 psi radiator cap you got from your favorite Mustang vendor isn't helping much either. Upgrade to a 16 psi or greater cap with a recovery system and you'll see a lot of your cooling issues go away (and no more green trail). The stock four-blade fan might have worked in 1965 (actually, we doubt it) but with today's traffic and marginal cooling, a higher blade count with a deeper pitch really helps. Couple said fan upgrade with a fan shroud for even better thermal transfer. Check out our photo captions for more fixes and upgrade tips.