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Never open any part of the plumbing system while the engine is hot. The sudden drop in pressure can cause the coolant to boil, and it can seriously scald you.


The plumbing in a car tends to be a relatively simple system, serving one basic purpose - to move heat around the car. The main purpose of the plumbing is to carry away the intense heat generated by the combustion in the cylinders. Other applications include heating parts of the engine for cold weather, and heating passengers.

The information here is based on my knowledge of BMC A Series engines, but should be similar for most water-cooled models.


The block and head of an engine are cast with integral waterways, through which the coolant flows. I say coolant, because it should normally be a combination of water and antifreeze. Even in summer, antifreeze is used, because it contains rust inhibitors to keep the inside of the engine healthy.

We will start at the water pump. Usually driven by the fan belt or serpentine belt, this pumps coolant from the radiator into the bottom of the engine block. The coolant rises through the block, picking up heat and exits from the top of the cylinder head.

The coolant now goes into the top of the radiator where a combination of fins and fans remove the heat to the surrounding air. By the time the coolant reaches the bottom of the radiator, it is quite cool, and is returned to the pump and recycled.

Thermostat and Bypass

Most engines work best at a specific temperature. It is beneficial to get to this working temperature as quickly as possible. For this reason, a thermostat and bypass are provided. Typically, this means that if the coolant is below the optimum temperature, then it is fed straight back to the pump on leaving the top of the cylinder head. By bypassing the radiator, the engine warms up more quickly.

On reaching the correct temperature, the thermostat opens up, and coolant is then directed towards the radiator. Optimum temperature is maintained.

Passenger Heater

The passenger heater is often provided by a heater matrix and fan. Some of the hot coolant from the cylinder head can pass through a tap (controlled from the inside of the car), and through the heater matrix. This is a small radiator which transfers heat from the coolant to the air blowing through it and into the passenger compartment.

On leaving the matrix, the coolant is relatively cooler, and may be returned to the water pump, rather than having to go through the radiator.


On vehicles prone to carb icing, a heated inlet manifold may be provided. This is often plumbed into the passenger heating circuit, since it is not required in warm weather. In cold weather, the driver turns up the heat in the cabin, and at the same time, the inlet manifold and therefore carb are warmed.


Problems are invariably caused by blockages or leaks. Symptoms can be similar for both, but there will often be other indications to tell you which has occured.

As an emergency measure to prevent overheating, it is often possible to put the cabin heater on full heat with the fan on full blow. This takes a significant amount of heat away from the engine, but expect sauna-like conditions. Wind your windows down if it's summer!

If the temperature needle goes into the red, and the above measure doesn't work, then pull over where safe; stop the engine, open the hood and wait. Call your breakdown service if you have one, or nurse the vehicle to your destination in short steps of driving and cooling. Watch the temperature needle very carefully all the time.


Of the two major problems with the plumbing system, blockages are the harder to spot. Symptoms include overheating and lack of temperature to the passenger compartment. The level of coolant in the radiator will not be seen to drop, but other symptoms should be apparant.

If the thermostat fails to open, then the coolant will always bypass the radiator, leading to overheating. Likewise if waterways are blocked, only certain parts of the engine will overheat. If these are not the parts with a temperature sender, then unfortunately a lot of damage may be caused before any symptoms are seen.

If the blockage is in the heater circuit, then the cabin heater will not work. This should be quite simple to spot, although may go unnoticed until autumn.

To remedy blockage, the thermostat can be tested in a pot of hot water. Use a thermometer, and check that the thermostat opens and closes at the correct temperature.

If the blockage is not due to the thermostat, then rubber hoses may be replaced, or the system may be back-flushed. This means that it is drained, and then water is hosed into the system against the direction it normally flows. This often dislodges pieces of rust or rubber which may be causing the block.


Leaks often occur in the join between various components of the plumbing system; or may be caused by a hose or radiator becoming punctured. The usual symptom of a leak is overheating, and the level in the radiator dropping over a period of a few days. However, some spectacular bursts can occur, filling the engine bay and surroundings with steam. If this occurs, keep calm, pull over safely and wait for the steam to subside before opening the hood.

The sign of a slow leak is the buildup of white deposits around the join of a hose. Usually in this case, the hose may be replaced, or just attached more tightly. A leak in the radiator may be harder to spot, but the coolant level will still drop over time.

If the leak is present in the heater matrix, or any sealing rings related to the heater, it may be found that the passenger compartment smells of antifreeze, or the windows steam up for no apparant reason.

Another cause of a leak - particularly if the weather is very cold - may be your freeze plug. If you have not mixed antifreeze with water to create your coolant, this plug may have popped out, and the water drained away on thawing.


Most car plumbing jobs are relatively straightforward, and may be performed by a competent beginner. Emergency repairs may be carried out by simply wrapping duct tape around a bad hose or joint, but proper repairs should be made as soon as practically possible.

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