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Fox Body Mustang Cooling Issues, Problems & Solutions

Are you having cooling issues & problems with your Fox Body Mustang? Check out our solutions guide.

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  • Fox Body Mustang Cooling Issues, Problems & Solutions

Why is my Fox Body Mustang overheating?

If you find yourself asking this question, you are not alone. We answer this question regularly. We are going to look at all of the reasons that lead to Fox Body Mustangs overheating. The most common reasons for Fox Body Mustangs to overheat include coolant issues, worn-out radiator hoses and radiators, and missing radiator air deflectors. In this tech guide, we will also share some insight on when it is best to upgrade your cooling system components versus simply replacing them with a factory-style replacement part. The first part we will look at is the radiator. This is a part that you are better off upgrading rather than just replacing.


Let me just come right out and say it. If your Fox Body Mustang has any engine modifications at all, the stock radiator is already over capacity. It is barely adequate for a bone stock engine. Every Fox Body on the road really needs a larger radiator. Especially if you don’t know the maintenance history of your cooling system. We carry a complete line of radiators to fit just about any build or project.


If your Mustang is overheating, inspect the front of the car! This is an easy fix. Check the A/C condenser and the radiator for visible external blockage such as grass, mud, inspects, plastic bags, cardboard, etc. Scrubbing with a soft bristle brush or cleaning with a power washer will do the trick.

Another form of blockage can come from inside your radiator. Pop the cap and look in the radiator. If you see significant scaling on the core tubes, you probably need to remove the radiator and have it professionally cleaned or simply replace it as the coolant cannot flow through a sealed-off passage. This is one of the most overlooked reasons for overheating issues in Fox Body Mustangs.


It's no secret that factory radiator hoses tend to wear and tear over time. A weak radiator hose can be a ticking time bomb that can leave you stranded on the side of the road. A leak in your cooling system can eventually cause your motor to overheat over time which will lead to more costly repairs.

Many of our radiator hoses are available in different colors and finishes to also allow you to dress up your underhood as an added benefit. Don't forget to pick up new clamps as these can also wear out over time.


The factory cooling fan set up on your Fox Body Mustang is more than adequate to handle the everyday demand of a full-bolt on Mustang – especially if you have added an aftermarket radiator. When it comes to maintenance on your factory fan, make sure the shroud is in place and fully intact. Inspect all the fan blades for cracks and damage. Do not forget to check the fan clutch for signs of leaking or damage. Any signs of residue on the fan clutch mean it needs to be replaced.

The fan and fan clutch setup does have its limitations. It does rob some power from your engine because it is driven by your engine’s accessory system. For many, upgrading to an electric fan setup will free up this horsepower, give more room for turbo kits and provide more cooling airflow. One of the most popular and best-performing electric fan upgrades is the Contour Electric Fan Upgrade.

When it comes to maintenance for your Fox Body electric fan system, make sure it is actually coming on at the desired temperature. All of the common aftermarket fan upgrades have an adjustable fan controller that does need to be properly set after installation. Also, if it is wired properly, it will come on full time when the A/C is turned on, regardless of temp.

NOTE: Proper installation of an electric fan should include an upgraded alternator. A popular upgrade is a 130 Amp Alternator. Check out our complete line of alternator upgrades here: Fox Body Alternators.


The lower radiator air deflector is probably one of the most overlooked causes of overheating in most Fox Body Mustangs. If this is missing, you'll typically get overheating issues while cruising. Having this deflector in place allows proper airflow distribution through the radiator at cruising speeds when the clutch fan is typically not doing much work.

Luckily this is an easy fix! Check out our installation video here: Fox Body Air Deflector Install Video

Your Fox Body also has two-side radiator deflectors as well. These are equally important to ensure the airflow is properly going through your radiator. Just as with the lower deflector, these can be commonly destroyed or missing and this can lead to overheating issues as well. It is a good practice to replace these any time you replace the radiator. Watch our installation video on these here: 5.0Resto Radiator Side Air Deflector Installation


Coolant is a critical part of your Fox Body's cooling system. There are many components and factors that go into keeping your coolant operating at an optimal level. We will look at everything from your thermostat to coolant levels in the information below.


An old thermostat is prone to sticking, and if it sticks shut, you'll of course overheat under all conditions. Having your thermostat stick open can also cause cooling issues in your Fox Body Mustang. If it stays open, this will allow the coolant to pass through the radiator too quickly and not dissipate enough heat, thereby causing an overheating condition.

A common misunderstanding is that having a lower-degree thermostat will improve your engine cooling. This is not always the case. For any lower-degree thermostats that open up at 180 degrees or below, you can run into the case that the thermostat always stays open and this does not give the coolant time to sit in the radiator and cool down. Be very mindful that lower-degree thermostats do not equal better engine cooling.

PRO TIP: If engines do to reach optimal engine temps, you can have Air Fuel issues or cause the factory ECU to not go into a proper closed loop.

Low Coolant

Having a low coolant level of course means a diminished cooling capacity due to a lack of volume as well as a lack of flow. If your coolant is low, you obviously have a leak. Fix the leak and refill the coolant. Check after a few drive cycles and top off again as needed.

Age/Condition of coolant

Old, nasty, brown coolant cannot transfer heat well and therefore does not allow proper engine cooling. This also means the engine block and radiator are coated in the nice slime of decomposing ethylene-glycol. A full flush is needed before replacing the coolant. Another issue with old coolant is that it develops an electrical charge over time. This charge causes electrolysis which is the #1 cause of cooling system corrosion and heater core failure.

Don't forget to add coolant additives such as Royal Purple Purple Ice when refilling your cooling system. This synthetic additive further reduces the chances of overheating by reducing the surface tension of the radiator fluids and improves heat transfer.

Blown Head Gasket

This one is typically associated with the cooling system constantly "boiling over" because of compression escaping into the cooling system and the cylinder/cylinders affected will typically exhibit a miss. This failure is most prevalent in power-adder cars.


If your coolant matches the description above, it is possible that the impeller on your water pump has corroded away and is no longer properly moving coolant through the system. This is not a very common failure and the pump tends to develop a leak before this takes place. If you have replaced and checked everything above, this should be the next step in your overheating evaluation.

Installing the wrong water pump is another major cause of overheating in Fox Body Mustangs. 5.0L Mustangs using a serpentine drive belt need a "reverse" rotation water pump. This is because the back (slick, non-ribbed) side of the belt is what makes contact with the pulley spinning the pump in the opposite direction as the rest of the pulleys. If a standard rotation pump is installed in a serpentine application, coolant will be flowing backward through the system and proper cooling cannot be achieved.

Another common misconception is that a "high flow" water pump can improve the cooling capabilities. Before moving to a setup like this, make sure your cooling system is completely restored and can handle the high volume. You would always consider a larger aluminum radiator and make sure the system is properly flushed before installing one of these pumps.

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