Ford Mustang Cooling System Basics
Understanding how your Mustang’s cooling system works
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.