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. In the Bongo system, the de-gas tank is connected to the radiator at the highest possible available feed from the radiator feed tank, sometimes known as the hot tank as can be seen in the photo below.
The Bongo has a conventional cross-flow type of radiator with a hot tank on one side (top) and a cold tank on the other side (bottom) and connected to the radiator return line, therefore the hot tank is considered to be part of the engine delivery line and the bottom tank is considered to be part of the pump return line.
The operation of the primary cooling circuit is fairly conventional, when the engine is started from cold it is desirable to increase it's temperature as quickly as possible to reduce emissions, increase fuel economy and provide hot coolant for the heater. During initial running of the engine the thermostat is arranged to prevent coolant through the radiator, while flowing through the bypass. Once the engine has been running for a sufficient period for it to have reached normal operating temperature the thermostat starts to allow coolant to flow through the radiator and then the bypass, in the Bongo/Freda system the double valve thermostat does not simply open to close off the bypass but due to it's construction it's normal running is between fully open and closed, dependent on operating conditions. During initial running of the engine, there is no coolant flow through the radiator and there is a small flow to the radiator hot tank (top of radiator) and into the de-gas circuit. So the majority of the flow from the engine is through the bypass and the heater supply lines. When the cooling system is first filled with coolant, air dissolved in the coolant will form a mist of small bubbles which tends to rise towards the top of the engine delivery line, while the denser almost bubble free coolant remains at the bottom.
The path taken by the flow into the bypass means that air bubbles will either remain at the top of the delivery line or will be swept into the bypass. The flow from the engine delivery to the heater will be relatively free from air bubbles. The air entrained coolant flows into the radiator hot tank and into the de-gas circuit. The air can separate in the de-gas tank and coolant relatively free of air is returned to the pump return line through the de-gas return line.
The thermostat is of the kind that can prevent flow from the bypass and all the flow from the engine delivery line is into the heater circuit when the engine is running slowly and cold, which means that the heater circuit will be relatively free of air bubbles
When the engine has warmed enough for the thermostat to allow flow from the radiator through the radiator return line, the flow to the radiator increases so bubbles of air will be swept along and into the radiator hot tank and subsequently into the de-gas tank
When the coolant is at the normal operating temperature then the coolant circulates through the radiator like conventional cooling systems but hot coolant from the hot tank that is passed into the de-gas tank where entrained air is removed from the coolant but the air free coolant then returns via the heater feed circuit.
This obviously makes de-gas tank coolant level reasonably important when the engine is cold, by being filled between the high/low parameters, but explains why there is an large expanse of tank unused when the engine is cold. The coolant expands and rises when it is heated and bubbles will occur in normal running, even if it does not boil.
It is highly possible, that the tank fill height suggested by the venting process in the service manual when changing the coolant,is that the de-gas tank flange is actually the operational height of the coolant on a hot engine, with the rest of the chamber reserved for the de-gas process.
Needless to say with the de-gas process, coolant will be flowing back into the system via the heater radiator and the de-gas tank return, which in itself will make it difficult to monitor the level of the de-gas tank while it is hot and under the circulation/degas process.
Inevitably this area, shows a slight “weakness” of the system, where you could be losing some coolant and be unaware that the loss is occurring, other than by an effective temperature gauge reading unusually high on the scale. This is because the coolant that will be expanded and then “held” with a delay period for de-gas to take place in the tank before being returned to the system.
The only danger too of relying on any temperature gauge is the high temperature that the engine fans “kick” in to cool the engine, albeit effectively.
Therefore you could also suffer some engine damage in this scenario too, should there be a substantial coolant loss, however it is likely that the engine temperature would rise at an unusually faster rate than in normal operation conditions which would give the driver cause to stop the engine as soon as possible. The De-Gas Tank
The basics
If you ever watched a pot of water start to boil you can see bubbles form on the bottom of the pot. This is where the water is turning to steam. Same thing happens inside your engine at local hot spots, in the water jackets around the cylinders. The slots in a de-gas tank work a bit like a spoon in a cup stirring a drink, but in the case of the tank, the coolant is forced through the passages to “squeeze” the air/gas out of the coolant, whereas the spoon is stirring a still liquid to force out any air. In the main, any excess and stagnant air/gases are suspended in the de-gas tank spare expansion space, unless the system pressure is such that the pressure cap opens to expel the excess air/gasses.
The Bongo/Freda Tank The degas tank, like all de-gas tanks,comprises of 2 parts a top and a bottom, they have matched chambers on both “halves”. Slots are moulded in the the tank to allow predetermined coolant flow and the finished assembly is bonded together and the point of this fusion of both parts form the flange around the de-gas tank The chambers within the de-gas tank are designed to both enhance structural stiffness of the tank and the slots are there to create a vortex, to both dissolve air in the coolant, and the reverse, the removal of dissolved air from the coolant, the sizes and flow are calculated by a formula based upon the cooling system capacity and one of the tank's chambers (in the diagram it is chamber x)collects the stagnant coolant after de-aeration prior to recirculation through the tank and back into the cooling system. In order to allow the air and gasses to escape into the degas tank, it is normal to provide the degas tank with a number of chambers or compartments which are connected in series through windows or openings formed in the ribs separating adjacent chambers so as to allow the coolant to flow between the chambers. System pressure, is also an important factor when designing against cavitation, This is a common finding on vehicles which use de-gas bottle air compression to regulate system pressure.
The pressure cap for the Bongo system is 1.1 bar or 15 psi, which is the default maximum pressure pressure for this kind of cooling system
Any excess air/gasses from the coolant, not able to be accommodated by the de-gas tank, and this will also be under pressure, is expelled to the open air via the cap vent hose. Summing up As is well known, in this type of cooling system, the degas tank fulfils three important chores. 1) it serves to compensate for volume changes resulting from the temperature changes of the coolant. 2) it collects the air present in the cooling system
3) it prevents cavitation (Cavitation is the formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapour pressure - see notes) in front of the water pump, in order to achieve these functions, the degas tank is installed in parallel with the main coolant circuit of the engine cooling system and the highest point of the main coolant circuit is connected to the inlet tube of the degas tank while the outlet tube of the degas tank is connected ahead of the water pump These 3 reasons are probably why this type of cooling system was chosen for the Mazda Bongo/Ford Freda due to the design and layout if the vehicle, especially the location of the engine/transmission unit. As for maintenance of the cooling system, this will mean routine coolant changes plus any other items that require replacement. Visual checks for leaks and rubber hose condition is important as well as the condition of the radiator hot and cold tanks, as this seems to be a common failure point The coolant used in Japan is similar to the old G12 type Long Life Coolant but without the silicates and this type of coolant in Japan is available in several colours, unlike coolants in the UK which are Blue,Red & Green. Many Bongo owners prefer to use a good G12+ type 5 yr coolant, typically red in colour, for the Bongo/Freda cooling system and it is also good practice to change other components, like thermostats for instance, in the cooling system while the coolant is drained. 2 yr Blue or Green G11 coolants are also acceptable BUT do not mix the types. When the cooling system is refilled, the system needs air removed from it, so the coolant has a full flow. The factory method of bleeding, as translated in the Service manual and in pictorial form on the internet by Team MHO, is the default and shows that after you have successfully filled the system, you need to warm it up for appropriately 10 minutes to allow the thermostat to begin to open, check the bottom hose for some warmth, then accelerate the engine for up to 6 minutes at 2,500 rpm..... Then if air bubbles are cleared, you put the bung back into the hose and then continue to accelerate and rest the engine per the cycles recommended. This allows the engine to purge the rest of the air out of the system and into the de-gas tank for processing. We all know that there are several methods of bleeding the air from the Bongo/Freda, that Bongo Fury members use, and in essence they all work and are all fine to use. The key element of the successful removal of air from the cooling system is to get as much air out when you refill the system Then after that to allow the thermostat to open and allow the rest of any trapped air to evacuate the system, before replacing the hose plug, same principle applies to all of the known methods, Factory or otherwise, using whatever method you wish to employ. The important thing to remember is, that the Mazda Bongo/Ford Freda cooling system, continuously self bleeds entrained air/gas from the system, every time the engine runs during the hot/cold cycles and at differing ambient temperatures and running conditions.
The main caveat is of course is that should any large pockets if air form in the Cylinder Head and Engine Block they sometimes can only be evacuated from the system via the air bleed tube. Remember that the Bongo/Freda has a “closed” cooling system, but when a component fails like a hose/radiator etc. and allows the system to be “open” it can cause these large air pockets to form, often in the cylinder head and cause overheating of the engine.
Engines fitted with a Ford Ranger Head, with no “Bleed” hose...... The following information was provided by an owner with a Ford Ranger head fitted of how he “bled” the Bongo cooling system without the vent hose.....
My 1997 Mazda Freda (only one in the world) out of the blue split a top tank on the radiator the other week
I replaced the radiator no problem. Then decided to do something different.
I filled the coolant without any "bleeding".It took a while, indeed well over half an hour, to see what the beast would do all on its own, without all our best Fury! consternations.
So I filled it as one would fill a Ford Mondeo (purely random example).
Bit of squeezing of top hoses, you know the usual mechanic's decades-old drill. Make sure top tank is full. Rad cap on, yellow cap still off.
Then I just let Freda idle away to her heart's content. All I did was keep an eye on the X-tank as she contentedly idled her way to the ecstasy of her idea of Freda Heaven - 86 degrees and pure WL-T bliss.
You need to know too that my X-tank is spotless inside. Sure the plastic is slightly yellowed, but there is not even the hint of redness, so you can see with beautiful clarity into my X-tank (much under-rated importance on Fury!).
So as bubbles rise up through the X-pipe and into the X-tank, I am able see every individual one of those gorgeous little pearls rising up the left side of the tank (left side as I'm watching it).
I topped up the tank with coolant mix perhaps six or eight times till things were settled. By this time stat was open.
Went for a small drive.
Went back to examine X-tank.
Remembering that I have no "hose-to-nowhere"(Bleed hose- for clarity),(engine off) I carefully removed, the narrow head/turbo coolant pipe, looking for escape of air, preventing entry of new air. There was every indication to me that my coolant filling had been complete. I know you will be in no way surprised by any of this.
The self-bleeding system had done its job, and with all the knowledge I have accumulated of this vehicle, I can say with as much confidence as one can ever say about our marque,that there was no longer any air in the cooling system.
Over the next couple of outings, did the usual pre- and post- drive inspections, and all is well, and my highly responsive temp gauge tells me that the by now well- familiar readings are completely where they always were.
Notes on Cavitation:- 1)The physical process of cavitation inception is similar to boiling. The major difference between the two is the thermodynamic paths that precede the formation of the vapour. Boiling occurs when the local vapor pressure of the liquid rises above its local ambient pressure and sufficient energy is present to cause the phase change to a gas. Cavitation inception occurs when the local pressure falls sufficiently far below the saturated vapour pressure, a value given by the tensile strength of the liquid. In order for cavitation inception to occur, the cavitation "bubbles" generally need a surface on which they can nucleate. This surface can be provided by the sides of a container, by impurities in the liquid, or by small undissolved micro bubbles within the liquid. It is generally accepted that hydrophobic surfaces stabilize small bubbles. These pre-existing bubbles start to grow unbounded when they are exposed to a pressure below the threshold pressure, termed Blake's threshold. The vapor pressure here differs from the meteorological definition of vapour pressure, which describes the partial pressure of water in the atmosphere at some value less than 100% saturation. Vapour pressure as relating to cavitation refers to the vapour pressure in equilibrium conditions and can therefore be more accurately defined as the equilibrium (or saturated) vapour pressure. 2)Cavitation is, in many cases, an undesirable occurrence. In devices such as pumps, cavitation causes a great deal of noise, damage to components, vibrations, and a loss of efficiency. When the cavitation bubbles collapse, they force energetic liquid into very small volumes, thereby creating spots of high temperature and emitting shock waves, the latter of which are a source of noise. After a surface is initially affected by cavitation, it tends to erode at an accelerating pace. The cavitation pits increase the turbulence of the fluid flow and create crevasses that act as nucleation sites for additional cavitation bubbles. The pits also increase the components' surface area and leave behind residual stresses. This makes the surface more prone to stress corrosion.