Part 4: Cooling Systems

In previous articles, we have explored the different components that make up the vehicle cooling system. In this week’s edition, we continue by focussing on two components that we all hope we never have to replace. Not because of the cost of the parts, usually a mere few Rand. But because of their inaccessibly, the labour costs may run into several thousand. Sit down before reading any further. 
2.6 Bypass System 
This is a passage that allows the coolant to bypass the radiator and return directly back to the engine. Some engines use a rubber hose, or a fixed steel tube. In other engines, there is a cast in passage built into the water pump or front housing. In any case, when the thermostat is closed, coolant is directed to this bypass and channelled back to the water pump, which sends the coolant back into the engine without being cooled by the radiator. 
2.7 Freeze Plugs/Welch Plugs 


When an engine block is manufactured, a special sand is moulded to the shape of the coolant passages in the engine block. This sand sculpture is positioned inside a mould and molten iron or aluminium is poured to form the engine block. When the casting is cooled, the sand is loosened and removed through holes in the engine block casting, leaving the passages that the coolant flows through. Obviously, if we don’t plug up these holes, the coolant will pour right out. 


Plugging these holes is the job of the freeze-out plug. These plugs are steel discs or cups that are press fit in the holes in the side of the engine block. They normally last the life of the engine with no problems.The legend has it that the colloquial name “freeze plugs” harks back to the early days, when many people used plain water in their engines, usually after replacing a burst hose or other cooling system repair—“It is summer and I will replace the water with antifreeze when the weather starts turning”.Needless to say, people are forgetful and many a motor suffered the fate of the water freezing inside the block. Often, when this happened, the pressure of the water freezing and expanding forced the freeze-out plugs to pop out, relieving the pressure and saving the engine block from cracking (although, just as often, the engine cracked anyway). 


Another reason for these plugs to fail was the fact that they were made of steel and would easily rust through if the vehicle owner was careless about maintaining the cooling system. Antifreeze has rust inhibitors in the formula to prevent this from happening, but those chemicals would lose their effect after 3 years, which is why antifreeze needs to be changed periodically. The fact that some people left plain water in their engines greatly accelerated the rusting of these freeze plugs. 


When a freeze plug becomes so rusty that it perforates, you’ll have a coolant leak that must be repaired by replacing the rusted-out freeze plug with a new one. This job ranges from fairly easy to extremely difficult depending on the location of the affected freeze plug. These components are located on the sides of the engine, usually 3 or 4 per side. There are also freeze plugs on the back of the engine on some models and also on the heads. 


As long as you are good about maintaining the cooling system, you need never worry about these plugs failing on modern vehicles.If you do ever need a freeze plug, just type Welsh Plug into start my car and you will find our full selection on offer. 


2.7 Head Gaskets and Intake Manifold Gaskets 



All internal combustion engines have an engine block and one- or two-cylinder heads. The mating surfaces where the block and head meet are machined flat for a close, precise fit, but no amount of careful machining will allow them to be completely water-tight or be able to hold back combustion gases from escaping past the mating surfaces. 


In order to seal the block to the heads, we use a head gasket, which has several things it needs to seal against. The main thing is the combustion pressure on each cylinder. Oil and coolant must easily flow between block and head, and it is the job of the head gasket to keep these fluids from leaking out or into the combustion chamber, or into each other for that matter. 


A typical head gasket is usually made of a soft sheet metal that is stamped with ridges that surround all leak points. When the head is placed on the block, the head gasket is sandwiched between them. Many bolts, called head bolts, are screwed in and tightened down, causing the head gasket to crush and form a tight seal between the block and head. 


Head gaskets usually fail if the engine overheats for a sustained period of time causing the cylinder head to warp and release pressure on the head gasket. This is most common on engines with cast aluminium heads, which are now on just about all modern engines.Once coolant or combustion gases leak past the head gasket, the gasket material is usually damaged to a point where it will no longer hold the seal. This causes leaks in several possible areas. 


For example: 

• Combustion gases could leak into the coolant passages causing excessive pressure in the cooling system 

• Coolant could leak into the combustion chamber causing it to escape through the exhaust system, often causing a white cloud of smoke from the tailpipe 

• Other problems such as oil mixing with the coolant or being burned out of the exhaust are also possible. 


Some engines are more susceptible to head gasket failure than others. The best advice is, if the engine shows signs of overheating, find a place to pull over and shut the engine off as quickly as possible. 


Like the Welch plug, head gaskets themselves are relatively cheap, but it is the labour that’s the killer. A typical head gasket replacement is a several hour job where the top part of the engine must be completely disassembled. These jobs can easily run up several thousand Rands. Unfortunately, in the process of opening the engine, more damage may be found and the repair costs will climb. 


A head gasket replacement begins with the diagnosis that the head gasket has failed. There is no way for a technician to know for certain whether there is additional damage to the cylinder head or other components without first disassembling the engine. All they know is that fluid and/or combustion is not being contained. 


One way to tell if a head gasket has failed is through a combustion leak test on the radiator. This is a chemical test that determines if there are combustion gases in the engine coolant. Another way is to remove the spark plugs and crank the engine while watching for water spray from one or more spark plug holes. 


As always, if you need a particular part reach out to us at Start My Car. Every day, we source many unlisted parts for our customers. Get in touch and we will do our utmost to assist you.