By John Bellah
An engine can overheat for a multitude of reasons. Some reasons are ridiculously simple while others can be complex, requiring major repairs or even complete replacement of the engine block or cylinder heads. In any event, overheating problems need to be immediately tracked down and corrected to avoid major repairs.
Combustion temperatures in an internal-combustion engine can exceed 3,000 degrees Fahrenheit, which can quickly melt vital components and cause catastrophic engine damage if the heat is not removed. Crankcase temperatures in excess of 250 degrees will cause engine oil to break down into a varnish and eventually cease lubrication. Remember, engine oil has a part in cooling the engine.
Some engines are air-cooled, such as the early Volkswagens, Porsche and that fine American icon, the Corvair. Most other engines are liquid-cooled. For purposes of this article, we’ll discuss liquid-cooled engines. In a liquid-cooled engine, the coolant (in some cases, straight water, and others an anti-freeze mixture) is circulated through passages in the engine block by the water pump, to be cooled by the radiator and directed back to the engine. Sounds simple, but it isn’t quite so.
We’ll go backwards from the radiator, which is a series of tubes with fins attached. Hot water circulates through the tubes and the fins dissipate the heat into the atmosphere. After years of service, tanks and tubes can develop leaks or the tubes can become clogged with sediment. If the damage is not extreme, a radiator shop may be able to clear the tubes by a process called “rodding-out” the radiator core and repairing any leaks. In extreme cases, the radiator may need replacement. On newer cars with plastic tanks and aluminum cores, replacement is the only option if the tank develops a leak. It is unusual, but occasionally on an older car, the cooling fins can become separated from the tubes, also requiring replacement of the core or the whole radiator.
In some cases, the radiator simply doesn’t have enough capacity for a number of reasons. Some vehicles had barely enough cooling capacity when they were new, or the wrong radiator was installed somewhere along the way. Add years of sediment build-up, air conditioning and high ambient temperatures, and things can heat up fast. An example is when the Audi was exported to the United States in the early 1970s. Most Southern California owners equipped their cars with air conditioning and soon overheating complaints rolled in. Requests to increase cooling system capacity fell on deaf ears as the Audi performed well in Germany’s colder climate.
The Audi factory eventually sent a team of engineers on a routine visit to Southern California. To prove a point, the Audi distributor loaded an air-conditioned Audi sedan with the visiting engineers and they headed for the Mojave Desert. When the engineers returned to their homeland, the cooling systems were quickly redesigned and capacity increased. Today, almost all the automotive manufacturers, domestic and foreign, conduct hot testing in Death Valley to ensure adequate cooling system performance.
Radiator clogs and leaks
A radiator that is not leaking but perhaps internally clogged or has separated fins will show progressively higher temperatures when operated at highway speeds. If this is the case, it’s time to head for the radiator shop. If the radiator is under capacity, the shop can supply or build a new unit with more rows of coils (hence the terms two-row, three-row and four-row radiators). Standard radiators are usually one- and two-row units, with high-performance or extra-cooling units falling into the category of three- and four-row units. In some cases, such as adding the heavy-duty cooling option or air conditioning, the manufacturer will fit a wider unit to the vehicle. Thus, when ordering a radiator, it’s wise to measure the radiator’s dimensions.
In order for a radiator to function properly, air must pass through it. Usually, that isn’t a problem at speed as air is rammed through the fins, keeping the coolant cool as air flows through the radiator. The fan, usually engine-driven, pulls cooling air through the radiator at slow speed. In some instances, there is a fluid-filled clutch that either disengages or slows the fan down at high speeds as the fan usually isn’t necessary. These fan clutches can leak fluid, making them useless. If this is the case, the fan clutch should be replaced. Proper storage of a fan clutch is also important; if stored upside down, the fluid can drain out down the fan clutch’s shaft and make the clutch useless.
Flex fans are another method to ensuring adequate cooling at low speeds while eliminating excessive fan roar at high speeds. Reduced noise results as the fan flattens at high speeds, thus decreasing its pitch. Flex fans are either Original Equipment (OE) or can be purchased as an aftermarket accessory.
Use caution if using a used fan from a junk yard as they may be unbalanced, cracked, bent or otherwise be damaged. A flying fan blade can be potentially fatal if it lets loose.
A fan is virtually useless if the cooling air is routed around the radiator instead of through it. Many manufacturers installed a fan shroud or piece of rubber weather-stripping to route the air through the radiator. Unfortunately, shrouds are easily broken in collisions or when engine mounts fail or are discarded during engine repairs. The shroud is there to direct air through the radiator, not to protect one’s fingers from the fan. If the shroud or weatherstrip around the radiator is removed, an engine may overheat at low speeds. Some owners fit aftermarket electric fans to ensure an engine stays cool at low speeds.
Of water pumps and belts
Most cars have water pumps, save for those with older thermo-siphon systems. Water pumps may appear to be functional on the outside, when in reality, they should be replaced. Cavitation along with grit and sediment in the coolant can deteriorate the impeller blades, impairing cooling ability. Other water pump issues can be a worn bearing or a leaking unit. The water pump should be inspected for leakage and the shaft checked for excessive looseness at every service interval. A loose shaft or defective bearing can lead to a broken water pump, causing considerable damage to the radiator, shroud and fan.
A slipping fan belt can cause loss of efficiency of the water pump and generating unit. Keep the drive belts tight, but don’t overtighten them otherwise premature water pump and alternator/generator wear can result. When in doubt, consult a repair of service manual for proper tension.
Pumps and pressure
If water pump replacement is indicated and the vehicle in question has overheating issues, a high-capacity pump from a later model or air conditioned application might be substituted. Several aftermarket manufacturers such as Edelbrock offer high-capacity water pumps for popular V-8 engines.
The cooling systems on modern vehicles are pressurized. This is a good thing as pressure will raise the boiling-point of the coolant, thus less coolant is required. That means less weight on the front end. Normal pressures run about 13 psi, but if the condition of a radiator is marginal, a 7 psi cap may be substituted.
A common “shade-tree” cure-all for overheating is removal of the thermostat. A word of advice — do not remove the thermostat! While many consider the thermostat to be in the “useless junk” category, it is there for a reason. The thermostat has very little to do with controlling upper temperatures.
Occasionally, a thermostat may stick in the closed position, causing severe overheating. If that is the case, it should be replaced. In reality, the thermostat restricts coolant flow until the engine warms up to ensure better low temperature driveability and reasonable fuel mileage. Complete removal of the thermostat will cause premature engine wear, crankcase sludge build-up, poor fuel mileage and may still cause overheating problems as the coolant will flow through the radiator so rapidly the radiator won’t be able to properly cool the engine.
During the 1970s, coolant temperatures were raised to reduce emissions and to improve fuel mileage. In some cases, thermostat ratings up to 205 degrees were not uncommon. If there are no cooling system issues, stick with the higher rating recommended by the manufacturer. Driveability under cold conditions and fuel mileage will improve. If the cooling system is marginal, sometimes going with a 180 degree unit may overcome minor problems on a temporary basis. An infrared non-contact thermometer is a useful tool to determine thermostat values or if a vehicle is overheating.
The combination of higher operating temperatures and pressurized cooling systems can mean serious personal injury if the radiator cap is suddenly pulled off a hot engine. Use caution as the burns can be severe. If the engine has overheated, allow it to cool down. Spraying water on the outside of the radiator core will help cool things down. Don’t quickly pour cold water into a hot radiator while the engine is running as a cracked block can result.
Another “out of sight, out of mind” item is the manifold heat-control valve, otherwise known as a heat-riser. If this unit sticks, continually directing exhaust heat to the carburetor, local overheating may occur, which may not show up on the temperature gauge. These problems may show up as vapor lock, carburetor flooding or pre-ignition. The heat riser should be periodically checked and lubricated with a special lubrication intended for heat-riser applications.
Some owners run straight water in their cooling systems. That may be fine for a vehicle designed to use water as a coolant, such as a Model T. If this is the case, rust inhibitor should be added. Most engines from the 1950s should run on a 50/50 mixture of water and ethylene glycol coolant. This mixture provides a higher boiling point and prevents corrosion. If the tap water in your locale is somewhat hard, add distilled water to the coolant.
Old coolant can cause internal rust and corrosion. Keep that in mind when purchasing that “barn find” 1963 Corvette. Electrolysis is another issue, and the damage can be extensive over time. Electrolysis is the result of interaction between the various metals inside the engine — steel, cast-iron, lead, aluminum, bronze, brass and copper — which eventually damages head gaskets, heater cores, water-distribution tubes, radiators and freeze plugs. Internal combustion leaks will also accelerate internal corrosion. To avoid this, coolant should be changed periodically and old coolant should be properly disposed. (Local repair or radiator shops will usually accept used coolant for recycling.) A coolant change should include not only draining the radiator, but also flushing the block and heater core to prevent the old coolant from contaminating the new.
There are two types of coolant on the market: Standard (green) and long life (orange). They are not compatible, so don’t mix them. Standard coolant has about a two-year service life and the long-life — (sometimes called “Dex-Cool”) has a five-year service life.
Blocking the dam
Leaks can come about from different areas and can often be tricky to locate, especially if the leak is at the head or block and quickly evaporates. A repair shop can quickly pressure test a cooling system to determine leaks. Steam appearing to come from the defroster and wetness accumulating around the passenger floor area indicates a leaking heater core. Stop-leak compounds usually don’t work on these problems, which require repair or replacement of the heater core. To accomplish this, removal of the dashboard is usually required — a tough job. On some cars, the heater core can be accessed from outside the firewall, usually making the repair easier.
The core of the matter
Core holes, commonly known as “freeze” plugs, are thin metal circular discs pressed into the engine block and cylinder heads. In most cases, these items are manufactured of brass because brass doesn’t corrode easily. Unfortunately, some shops use steel freeze plugs. Steel freeze plugs work, but if the owner is lax in changing coolant, the steel plugs will rust-through. If one freeze plug begins to leak, the others will quickly follow.
Many freeze plugs are located in tricky areas and may require removal of the cylinder heads, engine or transmission to access them. Temporary fixes are rubber freeze plugs and epoxy, which have been used with varying results.
A corroded water distribution tube is another “out of sight, out of mind” item that is sometimes found on older inline engines. These are sheet-metal tubes intended to direct coolant to different areas within the engine. Failure will cause localized hot spots to develop, resulting in eventual engine failure if not corrected.
Leaks on the inside
Internal engine leaks from leaking cylinder head gaskets and cracks in the head or block are serious problems. Initial indications would be large amounts of coolant spewing from the filler cap at operating temperatures. This is from combustion gasses entering the cooling system. A radiator shop or repair shop can quickly test for combustion leaks. Unfortunately, if combustion gasses are present, it could be a head gasket; the worst-case scenario is a cracked block. Another indication of an internal problem is water in the crankcase, which will usually accumulate as a milky substance at the tops of valve covers, oil breather caps, or PCV valves.
Lean fuel mixtures and late ignition timing will also make an engine overheat. The distributor vacuum advance unit is another often-overlooked device. If neglected, the distributor vacuum advance will cause overheating, burned valves and eventual engine failure.
Sometimes commercial sealers are successful in curing these problems, but again, the cure is only temporary. Loading the cooling system with numerous applications of stop-leak will eventually clog the cooling system.
If major repairs are indicated or it is rebuild time, the machine shop may be your friend. A quality machinist will inspect and Magnaflux the heads and engine block for cracks before performing any work. Some cracks can be repaired, or in some cases, a cracked cylinder can be re-sleeved. While at the machine shop, the block and heads should be hot-tanked, which will remove accumulated casting sand, rust, corrosion and sediment from the bottom of the block.
On an older engine, consider installing hardened valve seats to run on today’s unleaded fuels. And by all means, specify brass freeze plugs!