Jim Smart
August 24, 2011

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 mustang.monthly@sorc.com. 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

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.

Overheating Damage

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.

Photo Gallery

View Photo Gallery

Photo Gallery

View Photo Gallery

Photo Gallery

View Photo Gallery

Photo Gallery

View Photo Gallery

Photo Gallery

View Photo Gallery

Photo Gallery

View Photo Gallery

Photo Gallery

View Photo Gallery

Photo Gallery

View Photo Gallery

Photo Gallery

View Photo Gallery

Photo Gallery

View Photo Gallery