One of the oldest Mustang gripes we can think of is engine overheating. We can write this problem off as one of the penalties of old car ownership, but classic Mustangs were cursed with overheating issues when they were new. Suffice it to say that Ford's sporty Mustang didn't have a sporty cooling system, especially summertime travel on today's congested city highways.
In the years since, Ford has significantly improved the cooling in new Mustangs with better technology. The aftermarket has also stepped up with larger radiators, high-flow water pumps, heavy-duty fans, and coolant enhancers. However, some Mustang overheating issues can be overcome with common sense and good seat-of-the-pants troubleshooting.
A cooling system's job is to control engine heat. The key to making it work is having the right amount of heat transfer. Internal combustion produces tremendous amounts of heat, along with internal friction and the squeeze of compression. Of all the heat energy generated, 70 to 80 percent is wasted. Only 20 to 30 percent is put to work. The rest is given up to the atmosphere via cooling and exhaust systems.
Classic '65-'78 Mustangs were equipped with marginal cooling systems-radiators too small, tight engine compartments, water pumps with insufficient flow, inadequate cooling fan performance, and dated coolant flow technology. With time and use, Mustang cooling systems become victims of poor mechanical repairs, scale, corrosion, and other forms of deterioration, which only makes the situation worse.
Cooling System Basics
Heat is transferred out of your Mustang's engine in four ways-cooling system, exhaust system, lubrication, and heat conductivity to the atmosphere. Believe it or not, lubricating oil is more important to cooling than coolant because it has more intimate contact with the hottest parts of your engine. How long do you think your engine would run without oil versus the absence of coolant? Oil is more crucial to heat transfer than coolant, not to mention its lubricating value. This means oil must flow across hot parts as swiftly as possible, yet still provide the oil wedge necessary to keep moving parts apart.
Coolant, like oil, must flow through and around hot parts at just the right pace in order to transfer heat effectively. When coolant flows too fast, it doesn't carry away enough heat. If it flows too slowly, heat builds.
Coolant also needs a traffic cop-the thermostat-to control temperature. Engines that run too cool suffer just like engines that run too hot. When an engine doesn't get warm enough, it is grossly inefficient. Oil also doesn't flow as well in a cold engine, which adversely affects lubrication and cooling of key parts. So it goes two ways. Engine operating temperature should be right where Ford specified-around 180 degrees F for older carbureted engines and 195 degrees for later Mustangs with electronic fuel injection.
When everything is working properly, an engine warms up and coolant in the water jackets gets hot. Coolant then reaches operating temperature, which opens the thermostat, allowing hot coolant to flow into the radiator. Heat is transferred to the atmosphere via tubes, which carry coolant, and fins, which conduct and radiate heat to the atmosphere. Air should not flow too quickly across radiator fins because heat transfer then becomes hindered by air turbulence, a phenomenon known as "boundary layer" where air doesn't contact the fins and tubes.
Adequate cooling system pressure is also needed in addition to good air and coolant flow. The more pressure inside the cooling system, the higher the coolant's boiling point. This comes from the use of a good radiator cap, which also happens to be a relief valve rated in pounds-per-square-inch (PSI). It is important to know the appropriate pressure rating for your Mustang. Pressure cap function is little more than a rubber gasket and a pressure rated spring. Classic Mustangs typically call for a 7-12-pound cap, although you can use upwards of 12-16 pounds. However, with a higher pressure cap, you want to be sure of the cooling system integrity, meaning solid hoses, heater core, radiator, water pump, and gaskets.
Another important issue is coolant recovery. As coolant gets hotter, it expands, which means it needs to have a place to go. In the old days, excess coolant simply spewed out the overflow, making it necessary to add coolant from time to time. Coolant recovery systems were conceived both to protect the environment and contain coolant. With a recovery system, the coolant is pushed into a tank. As the engine cools down, lost coolant is sucked back into the cooling system. Although a coolant recovery tank may not be to your liking because it wasn't factory equipment on earlier Mustangs, you can always hide it. The most important issue here is keeping your engine supplied with plenty of coolant.
The engine's thermostat regulates coolant temperature. Without it, there's no coolant temperature control. Your engine will run too cool on the open road (unless you're in Arizona and it's 115 degrees) and overheat promptly in traffic. Overheating should never be blamed on the use of a thermostat. However, your Mustang's thermostat should be the first item checked when there's overheating.
Ford called for 180-degree thermostats in classic Mustangs, with the outside option of a 160-degree unit if you were running alcohol antifreeze. Your classic Mustang should run happily with a 180-degree thermostat if everything else is up to par. This means a good high-capacity three- or four-row core radiator, high-flow water pump, molded hoses with an anti-collapse spring in the lower hose, proper fan/shroud/spacer combination, and correct drive pulley sizes.
What To Check For
When your Mustang overheats, it's easy to fear the worst. However, most of the time, it's a simple problem. Remember, automotive cooling systems aren't that complex, even on the new '12 Mustangs, because basic principles haven't changed much in 48 years. Start your troubleshooting with the easiest elements first. Is the engine really overheating or do you have a faulty temperature gauge or sender? Open the hood and check for signs of overheating, like the aroma of antifreeze and the rumbling of boiling coolant. Wait for the engine to cool down before removing the radiator cap. Remember, when the cap is removed from a hot engine, cooling system pressure plummets quickly to barometric pressure and hot coolant goes immediately past its boiling point, roaring out of the radiator and possibly scalding you in the process.
Once your engine has cooled down, use a shop cloth or towel to carefully turn the radiator cap to its first detent to relieve any remaining pressure before removing it completely. With the radiator cap off, start the engine, run it at a fast idle (1,500 rpm), and watch coolant flow as the engine warms. If the thermostat is working normally, it should open at its calibrated temperature and you should observe aggressive coolant flow just inside the filler. If coolant flow is sluggish and the engine begins to overheat, the thermostat is the first suspect. Replace the thermostat.
Another often overlooked issue is pulley sizing. You want the water pump turning at just the right speed for your engine's cooling needs. Street corner logic says the faster you turn the water pump, the better it cools. But this isn't true. If a water pump spins too fast, coolant moves too quickly to efficiently transfer heat. You also run the risk of blowing radiator hoses. Always use the correct crank and water pump pulley sizes, as specified by Ford in the Master Parts Catalog.
Another big overheating culprit is clogged cooling system passages, typically caused by an excessive amount of corrosion on everything from radiator tubes to the water jackets to water pump. Corrosion prevention is your best ally against overheating. This means complete cooling system flushes every two years whether your Mustang is driven or not, or the use of a non-aqueous coolant. Although a non-aqueous coolant is very expensive, it never has to be replaced.
Corrosion issues occur mostly where dissimilar metals get together, such as an aluminum intake manifold and iron cylinder head or aluminum heads on an iron block. Clogging also comes from the excessive use of gasket sealer. When you use too much sealer at cooling passages, it oozes into water jackets, breaks off, and clogs passages. We've seen radiator tubes blocked with sealant.
Have you ever had an overheating problem and a reason we haven't addressed here? We would like to hear from you; send an email to firstname.lastname@example.org. We would like to know both the problem and your solution. MM
Reasons Engines Overheat
- Thermostat won't open (thermo- stats do not "stick")
- Thermostat missing entirely
- Collapsed lower radiator hose (at highway speeds)
- Clogged radiator tubes
- Radiator fins blocked with bugs and other debris
- Wrong fan or improper fan installation
- Water jackets blocked with debris (remove freeze plugs and check)
- Blown head gasket (chronic boil-over)
- Head gaskets on backwards
- Water pump passages clogged
- Water pump impeller corroded
- Water jackets blocked with corrosion or foreign objects
- Crushed exhaust pipe or header tube
- Incorrect ignition timing
- Incorrect valve timing
- Improper clearances on new engine (too tight)
- Improper piston compression/deck height (compression too high)
- Valve lash set too tight (also rough running)
Overheat prevention is strictly a matter of preventative maintenance. Flush your Mustang's cooling system every two years whether it's driven or not. Use a coolant filter to help keep the radiator clear. If you're concours showing, swap the upper hose coolant filter out for show purposes. Check the coolant filter whenever you check oil.
Here are some other tips:
- Install a coolant recovery system.
- Install block drain petcocks or brass drain plugs for easy service. When you flush, completely drain the block. Be environmentally responsible when you dispose of coolant.
- Use a coolant enhancer like Water Wetter to improve coolant surface tension.
- We will get arguments on this one-you can run 100 percent ethylene glycol without consequence. Although straight water is the most effective coolant out there with the best heat transfer, 100 percent antifreeze will keep your Mustang's cooling system corrosion free. And yes, it will run a pinch hotter, but not if you have a healthy cooling system.
- If you're going to run antifreeze and water, go with a 50/50 mix of antifreeze and water. Distilled water only.
- Run a good corrosion inhibitor as well as water pump lubricant, both of which are also included in most antifreeze.
- If your budget allows, run Evans Non-Aqueous Coolant, which means never having to flush the system or change coolant again.
- Replace all cooling system hoses every 4-5 years or every other flush.
- Inspect cooling fan for damage and proper operation every time you check oil.
- Check belts and hoses every time you check oil.
- Corrosion issues occur mostly where dissimilar metals get together, such as an aluminum intake manifold and iron cylinder head or aluminum heads on an iron block.
It's easy to dismiss engine overheating as a pesky problem, but depending upon the severity of your overheat, it can cause severe engine damage. If you're unaware of an overheat in progress, damage can go over the top before you discover the problem and shut down. If your engine's coolant temperature is 260 degrees and you're boiling over, that means your oil temperature is well over 300 degrees. Conventional engine oil begins to break down at 260 degrees; synthetic survives until 300 degrees. When an engine overheats, oil overheats and cooks, which breaks down the lubricating value. Overheating also drives combustion temperatures skyward, which causes detonation (pinging or spark knock) under acceleration, causing piston and ring damage. If you can hear pinging or spark knock under acceleration when it wasn't happening before, check your temperature gauge.
Spark knock (detonation) broke...
Spark knock (detonation) broke the ring and ring land on this piston, damaging the cylinder wall as well. This happens under hard acceleration
Extreme combustion temperatures...
Extreme combustion temperatures melted this spark plug firing tip.
Oil breakdown from an overheat...
Oil breakdown from an overheat turns moving parts into scrap metal in no time. When oil breaks down, it cannot lubricate. Instead, it cooks and cokes into gooey black carbon.
This illustration shows the...
This illustration shows the basic cooling system function in a vintage Mustang engine. Heat is transferred to coolant in the water jackets. When coolant temperature reaches the thermostat’s rated opening temperature of either 160 or 180 degrees F, the thermostat opens to release hot coolant into the radiator. Coolant from the radiator then flows into the engine’s water jackets. Engine cooling is nothing more than heat transfer to coolant to the radiator to the incoming air stream. The thermostat controls the heat transfer to keep engine temperature constant.
The radiator cap is a pressure...
The radiator cap is a pressure relief valve that both contains coolant and relieves pressure when its rating is reached. Coolant under pressure raises the boiling point. A 50/50 mix of ethylene glycol (antifreeze) and water raises the boiling point even higher.
With the radiator cap off,...
With the radiator cap off, you can expect a boil-over at approximately 220-230 degrees, depending upon elevation. A 16-pound cap will raise the boiling point of a 50/50 mix to roughly 270 degrees, although you’d never want your engine to get that hot.
The thermostat regulates coolant...
The thermostat regulates coolant temperature by starting and stopping coolant flow between the engine and radiator. Thermostats are very simple—spring pressure versus wax module (also known as the “pill”) expansion. Although the term “stuck thermostat” is common, there’s no such thing. If the wax module fails (leaks or bursts), the thermostat will not open. Temperature rating is determined by spring pressure. With a 195-degree thermostat, more heat is required to overcome the higher spring pressure, as compared to a 160 or 180 degree thermostat.
When water pumps fail, it’s...
When water pumps fail, it’s normally shaft seal failure that leaks coolant all over the driveway. Water pumps also lose effectiveness over time because impeller blades corrode. Passages can become clogged from corrosion and sometimes excessive amounts of gasket sealer that have found their way into water jackets. This hurts pumping efficiency. Always opt for a high-flow water pump in either a cast-iron stock replacement or aluminum aftermarket, as offered by Edelbrock (shown here).
Fan type and configuration...
Fan type and configuration causes more overheating problems than you might imagine. Your Mustang’s cooling fan is needed most at idle with the vehicle sitting still. Basic Mustang cooling system technology in 1965 utilized this two-piece X-fan without a shroud, very ineffective in hot weather.
Installing a shroud around...
Installing a shroud around the X-fan improves air velocity across the radiator. Fan placement is everything, however. The fan must be half way into the shroud for it to be effective.
The thermostatic clutch fan...
The thermostatic clutch fan is the most efficient cooling fan. There are lots of blades for improved cooling. And because the thermostatic fan clutch works only when needed, it doesn’t rob power. Don’t waste your money on a fan clutch that doesn’t have the thermostatic feature. Always use a shroud with a clutch fan—again, halfway into the shroud.
Here’s a thermostatic clutch...
Here’s a thermostatic clutch fan with the large 24-inch radiator, which is what you want in a ’67-’70 Mustang with air conditioning for optimum cooling capacity. Go with a three- or four-row core.
Always use an anti-collapse...
Always use an anti-collapse spring in your Mustang’s lower radiator hose. The spring keeps the lower radiator hose from collapsing at higher engine rpm on the open highway. At high rpm, the water pump is drawing coolant faster than the radiator can provide, which causes the lower hose to collapse and reduce coolant flow to the engine.
No need to search salvage...
No need to search salvage yards for original clutch fan components because Flex-A-Lite (available from Summit Racing Equipment) has you covered.
Specify a thermostatic fan...
Specify a thermostatic fan clutch along with the appropriate five-, six-, or seven-blade fan, depending upon your appliocation.
When properly positioned in...
When properly positioned in a fan shroud, the Flex-A-Lite flex fan does an excellent job, plus it’s quiet. Be sure to use the correct fan spacer for your application. The Flex-A-Lite flex fan is more efficient that any factory X-fan, multi-blade fan, or flex fan.
Proper water pump and crank...
Proper water pump and crank pulley sizing is important to cooling. You need the correct pulley ratio, meaning crank pulley to water pump sizing. All you have to do is refer to the Ford Master Parts Catalog for the correct part numbers and pulley sizes for your Mustang’s engine.
Aftermarket pulley systems,...
Aftermarket pulley systems, typically offered in good-looking billet aluminum, are engineered to get your water pump turning at the correct speed. Water pumps typically turn at a slower speed than the engine crankshaft.
Corrosion issues lead to overheating...
Corrosion issues lead to overheating because it reduces the size of cooling passages. Corrosion blocks coolant flow between the intake manifold and heads, steam holes between blocks and heads, radiator tubes, etc., causing engines to retain excessive heat.
If you’re experiencing overheating...
If you’re experiencing overheating issues with a rebuilt engine or even a new crate engine, make sure there’s no debris in the water jackets. Marvin McAfee of MCE Engines tells us that just because it’s new doesn’t mean the water jackets are debris free. We’ve seen freeze plugs carelessly knocked inside water jackets and left there, which hinders coolant flow and causes overheating. Many chronic overheating problems are caused by foreign objects in water jackets, like chunks of slag from the casting process, nuts, bolts, sealer, gasket material, and more.
New engine with chronic overheating?...
New engine with chronic overheating? Did you install the head gaskets properly? It’s possible to install cylinder head gaskets backwards, which seals off coolant flow to the rear of Ford engines.
This is an infrared temperature...
This is an infrared temperature sensor, which enables you to determine a dead cylinder from a live one based on exhaust manifold or header tube temperature. It can also tell you a lot about coolant temperature. Check upper and lower radiator hose temperature. If you find the upper hose considerably hotter than the lower, your radiator could be blocked. Harbor Freight can set you up with its Cen-Tech Infrared Thermometer, which is a great troubleshooting tool for engine tuning and cooling problems.
Exhaust systems are rarely...
Exhaust systems are rarely the cause of overheating. However, contact with speed bumps can crush pipes and header collectors, plus catalytic converters on later models can become clogged. Exhaust system impedance not only can cause engine overheating, but serious engine damage as well because the exhaust system is also a huge remover of engine heat. Greatest threat is exhaust valve failure from exhaust system restriction.
Do not use steel block plugs....
Do not use steel block plugs. Use either brass petcocks or plugs for easy service. When you flush, drain the block.
According to Mark Jeffrey...
According to Mark Jeffrey from Trans Am Racing, you can run 100 percent ethylene glycol in your Mustang’s cooling system without consequence. This virtually eliminates corrosion though your engine will run a little hotter. Antifreeze doesn’t transfer heat as well as straight water; however, it is effective if your cooling system is healthy.
Coolant enhancers like Hy-Per...
Coolant enhancers like Hy-Per Lube assist cooling by improving coolant surface tension, which puts coolant in more contact with hot surfaces inside water jackets for better heat transfer.
Cooling system filters keep...
Cooling system filters keep unwanted iron particles and other debris out of your Mustang’s radiator. The downside is the fact that you have to check them often because they can get clogged, especially with a new engine. Coolant filters are available from Gano Auto Coolant Filter Company and GS Performance. Gano has a variety of coolant filters available for Mustangs, including a cool see-through type.
Coolant recovery systems come...
Coolant recovery systems come in many forms. The stealthier you can make it in a vintage Mustang, the better. They can be hidden in fenderwells, behind the grille, and alongside your radiator. Or they can be painted a stealthy black to be virtually invisible. On late-model Mustangs, Ford made coolant recovery an integral part of the cooling system.
Another source of high engine...
Another source of high engine temperature is incorrect engine tuning, including improper ignition and valve timing, improper carburetor jetting, and a fuel line that’s too small. It’s not always the cooling system. Fuel and ignition systems can make an engine run hotter.
Evans Non-Aqueous engine coolant...
Evans Non-Aqueous engine coolant completely eliminates cooling system flushes and corrosion. It also raises the boiling point to 375 degrees or higher. In short, it will not boil. It is also good down to –40 degrees F. All water must be eliminated from the cooling system because you must have 100 percent Evans coolant.