What causes an Automobile Engine to Overheat
The spark ignition internal combustion engine is, at best, 30% efficient. That means that approximately 1/3 of the heating value of gasoline is used to power the vehicle and the remaining 2/3 is released into the atmosphere. The energy lost in the conversion of chemical energy into mechanical energy enters the atmosphere via the exhaust system and the cooling system. In addition to the heat released from the burning of the fuel mixture, friction created by the moving parts of the engine must also be addressed. The cooling system has a formidable task and must be in good working condition to properly function.
The cooling system consists of the radiator, radiator cap, recovery tank, water pump, cooling fan, thermostat, hoses, water jacket and coolant. Following the function of the cooling system will expose potential sources of trouble and overheating.
When the engine is started and the coolant is cold the thermostat remains closed trapping the coolant within the water jacket in the engine. This is intended to speed the engine warm up process to reduce wear, improve cold performance and reduce pollution levels due to the rich fuel mixture necessary for cold engine operation. As the engine warms up and the coolant temperature increases the thermostat will cycle open and close to permit some water from the radiator to enter the cooling jacket and expel some hot coolant to the radiator. In this manner the thermostat permits the engine to reach the proper operating temperature and then maintains that temperature. The thermostat is a favorite candidate for replacement when overheating is experienced. The typical failure mode is a sticking thermostat caused by corrosion on the operating components or failure of the heat sensing bulb which opens and closes the thermostat. Replacement is the solution.
Early cooling systems did not make use of a pump but instead used a thermo-syphon system. In his book, “The Legendary Model T Ford” by Tom Collins, Mr. Collins describes how the Model T originally had a water pump but Henry deleted that as a cost reduction and switched to a thermo-syphon system. The system functions based upon the physics that a hot fluid, air or liquid, rises. The hot water in the water jacket would rise and flow into the radiator where it was cooled. As it cooled it would sink to the bottom of the radiator and thence into the engine water jacket. The system worked under moderate use conditions but a water pump was necessary and available in the vast Model T aftermarket, when more severe use was planned.
In today’s engines the water pump is required and is typically driven via a belt system off of the crank shaft. Electrically driven pumps have also been used as well as pumps driven off of the cam shaft. The pump is not a positive displacement pump but rather an “impeller” type device. This is necessary because some amount of “slippage” is necessary when the thermostat is closed.
Water pumps are generally dependable. Overzealous mechanics occasionally shorten water pump life by over-tightening the drive belt and thereby overloading the water pump bearings. The bearing begins to fail which damages the seal around the shaft and water begins to leak out of the breather hole in the water pump casting. The water pump usually signals it is going to fail in advance with this breather hole leakage. An antifreeze puddle under the engine is a warning sign the owner ignores at his peril.
The aforementioned drive belt is used to drive the water pump and cooling fan. Often it also typically drives the power steering pump and alternator. On occasion the cooling fan may be driven by an electric motor controlled by a thermostatically controlled switch. Failure of the drive belt will lead to overheating in short order. Failure of the cooling fan may not become immediately evident if the driver is cruising on the highway. However when stopped the fan becomes more important and the failure of the fan painfully evident. The drive belt is one of those components that require occasional replacement. Inspection of the belt for nicks, fraying or cracking should be done at each oil change.
The radiator is another key element of the system that deserves attention. Leaves, debris or other foreign matter blocking the air inlet is an invitation for overheating. Air flow through the radiator is critical to a satisfactorily operating system. Likewise, damaged fins or radiator tubes pinched shut also bode ill for the owner.
While the radiator is under discussion this would be a good time to discuss corrosion. Because the tubes in the radiator are so small the tendency to become blocked by corrosion or other matter floating in the cooling system exists. The engine is used as part of the automobile electrical circuit, providing ground for the starting and ignition systems. The presence of an electrical current, dissimilar metals and a coolant moving through the water jacket begs for the creation of corrosion. Corrosion can be largely prevented with the proper additives in the cooling system. Failure to maintain a level of fresh additives invites corrosion. Corrosion can block the radiator passages, passages in the water jacket, thermostat and hoses. Corrosion clogged radiator tubes will eliminate part of the radiator from the cooling system, leading to overheating.
The radiator pressure cap is an important part of a functioning system. As the cooling system heats up the coolant expands. This increased volume of coolant must be accounted for, and is, by the radiator cap and recovery tank. When the system pressure exceeds the radiator cap pressure setting, the cap permits coolant to pass to the recovery tank. When the engine is off and the coolant cools the pressure in the system drops below atmospheric. The radiator cap again yields and permits the coolant in the recovery tank to return to the radiator. This maintains the closed cooling system. However, if the hose from the radiator cap to the recovery tank leaks, coolant is lost instead of being saved in the reserve tank. This loss of coolant means a reduction in the volume of coolant and raises the potential for overheating due to lack of coolant. If the radiator cap is not up to the job and releases coolant at lower pressure than by design, coolant loss is again the result.
The connection from the engine to the radiator is made via the radiator hoses. Although reasonably durable, the hose material is subject to deterioration due to heat, coolant, oil and gasoline. A hose leak can lead to overheating, and overheating caused by other failures can accelerate the deterioration of the hoses for a future problem.
As a total system, the coolant flows through the engine absorbing heat and then passing through the radiator expelling the heat to the atmosphere. The system performance depends upon the adequate volume of coolant circulating at the correct rate, unimpeded. The system is designed to cope with the heat generated by the engine operating normally. The final source of overheating to be discussed is the engine itself. An engine that is not operating efficiently, one with incorrect ignition timing, fuel system too lean, leaking exhaust or failing head gasket generates excessive heat. Add a cooling system not up to par on a hot day in traffic, and an overheated engine is the result.
