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Cooling system with a De-gas tank Background I have been interested in the Bongo cooling system for sometime, mainly since I had to replace a split radiator in 2009. It was led by both curiosity and as to why something did not happen with the system as expected to by claims made on the forum and internet. I got involved in a small way, with Steve Widdowson's road to discover the facts about the Mazda Bongo/Ford Freda cooling system, l although recently, event's have put me at arms length from the project. History We all know that the Mazda Bongo/Ford Freda cooling system is unusual by having 2 heaters and with the engine located between the front seats too. Cooling systems with 2 heaters are not unusual in Japan as several other manufacturers vehicles have them as standard equipment. There is often plenty of discussion about the cooling system on the Bongo Fury Forum as well as discussion at Bongo meets as well as fact sheets and now a set of video's. In the main the Mazda Bongo/Ford Freda cooling system is reliable and efficient, but with all cooling systems on vehicles, failures can occur. Sometimes these failures are due to component ageing and sometimes failures are a result of poor maintenance. Component failure is mainly hoses but also water pumps/radiators and other components can fail. Poor maintenance failures are mainly due to lack of level checks and also coolant issues, either incorrect coolant by mixing or ineffective coolants in cold weather situations. Steve Widdowson has produced, with input from several members of Bongo Fury, diagrams of the Mazda Bongo/Ford Freda cooling system including the coolant flow of the system. These diagrams clearly show how the system works and also indicates the way the bypass element of the cooling system does it's job and the importance of the double valve thermostat in such a cooling system. This type of cooling system and thermostat is now common place, with many manufacturers adopting this idea, mainly because of quicker warm up periods which subsequently mean lower engine emissions. We now know quite a bit about the Mazda Bongo/Ford Freda cooling system but up to now we have not looked at the Header/Expansion Tank as we have called it but it is known as “Tank Sub – Radiator” on the parts list description. Now, as the forum members have not really focused on the “Header Tank” much before, other than cutting one up and sharing that the tank has chambers/compartments inside it and up to this point it has not really been recognised what these chambers/compartments are for although a member “The Great Pretender” did suggest that the Mazda Bongo/Ford Freda cooling system was a crude degas type a couple of years ago. With this in mind,along with the Cooling system diagrams, I decided to further research of the de-gas tank and alongside this Steve Widdowson was producing more excellent diagrams with sizes and direction of flow. As I am a drawing numpty, yet a fully fledged spanner man/fabricator I set about the “how it works route” not only the theory, but the practice and using Steve's excellent illustrations. The De-gas cooling system I first researched as much information as possible, from all across the world including from Japan and some Mazda engineers, from degas tanks for cars and descriptions to patents and the theory of how it works in simple terms. The background to this type of system is way back in 1992, and in Japan and was invented by some Toyota engineers and is a 1992 patent, now we can speculate if these inventors defected to Mazda but I guess Mazda and others adopted the Bypass/Degas cooling system after the patent had expired and as a slight tangent, these self same inventors patented other cooling system items after this patent. What I am going to do now is to illustrate the Theory of the system in simple terms rather than the terminology that the Patent shows and then follow that with how it used on the Mazda Bongo/Ford Freda Cooling System. Should anyone wish to see a copy of the patent paperwork then I am happy to supply the information where the document can be downloaded in PDF format The basics of a de-gas system can be seen below. This really only shows the flow of the system which will help everyone start to understand how the system circulates and what it does. As can be seen the Red areas are the hot parts of the flow ans the blue as the cooled part of the coolant flow...... The basics of the de-gas tank. This is a "purge" canister positioned at the highest point in the cooling system. This allows the coolant circuits to be filled more completely than a conventional system. The canister bleeds off the trapped vapours that tend to accumulate in a sealed system as a result of normal thermal cycling. In very basic form the heated coolant is sent out of the top of the engine as normal and when the coolant reaches the top of the radiator a certain amount expands into the de-gas tank to remove air and gases as well as cool the expanded coolant and then the cooled coolant is then returned into the system on the cold side of the cooling system and after the bottom hose. The rest of the coolant passes through the radiator in the normal fashion. To be a full de-gas system there are obviously more components needed than in this flow diagram, you do indeed require a thermostat and bypass circuit with a close by water pump and the mathematical parameters determine the location and design for any particular engine. The main reason behind the design is quicker warm up times and what seems to be important since the mid to late 90's, less exhaust emissions as a result. The system is such that it clears effectively air and gasses that are built up in cooling systems, not only at coolant change times but during normal usage. One bonus too is that complicated engine layouts in vehicles, due to designs of said vehicles, can be utilised. Obviously our Mazda Bongo/Ford Freda spring to mind, but this system has been used widely. Some vehicles that are fitted with this kind of system are the Rover 75, MG TF, Land Rover Discovery TD5 and Lotus Elise but a few. As can be seen not are all specific vehicle types but all have the de-gas cooling system. Now for how this works in the Mazda Bongo/Ford Freda. As can be seen by Steve Widdowson's diagram (below), the coolant is pictured in the same colours as the basic diagram but with the added elements of this kind of systems design added to it. Those elements being, Thermostat, Water pump, Bypass housing plus importantly the front and rear heaters. On a vehicle with this system that has a single heater, the coolant is cold fed from the de-gas tank to the heater, so by default on a Mazda Bongo/Ford Freda, the cold feed is piped to both heaters. The de-gas set up also connects to a bypass housing which has a mathematical formula as to where it needs to be placed in conjunction with the Water pump and this is also fed from the de- gas tank as part of this type of system. Now let us put this all together. The 1st part of any de-gas cooling system comprises of a primary cooling circuit having a radiator to cool the liquid coolant for the engine with a bypass arranged in parallel with the radiator and pump to circulate the coolant around the engine. The Radiator and the bypass and a thermostatic control valve controls the flow of the coolant from engine to the bypass and the radiator. A radiator return line is arranged to deliver coolant from the radiator to the thermostatic control valve(Thermostat) to the pump. The cooling system also comprises of a heater circuit in which a water to air heat exchanger is connected to the engine delivery line by a heater return line and a de-gas circuit, in which the de- gas circuit is connected between the engine delivery line and the pump return line, wherein the bypass and heater supply line are connected to the engine delivery line at a junction at which coolant from the engine can flow to the radiator in a substantially straight line. While the coolant flowing from the engine into the bypass is deviated downwards and around a bend , the heater supply line is connected at an opening on the inside of the bend The Bongo/Freda cooling system of what is called a “closed” system and comprised of a primary cooling circuit that has a radiator for cooling liquid coolant in an engine where a pump (water pump) circulates the coolant from a pump return line, through the engine into an engine delivery line to the radiator and to a bypass which is connected in parallel with the radiator. A thermostatic control valve known usually as a thermostat is connected to the radiator return line and the bypass is returned to the pump. The thermostat controls the flow through the radiator, preventing flow until the engine has reached an appropriate running temperature. However as with cooling systems of this type the thermostat also controls the flow in the bypass The heater circuit supplies coolant cabin heaters and is fed by the supply line from the de-gas tank and a return line from the bypass De-gas circuits include an expansion or de-gas tank, which is connected by the engine delivery line and the pump return line In normal cooling systems, the expansion tank is connected to the engine delivery line where it receives coolant from the engine, which may contain quantities of entrained (To carry (suspended particles, for example) along in a current.) air, hence it is usually connected at a high point in the primary cooling system.

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