Why Does My BMW Expansion Tank Blow Up? Introduction For a decade and a half it’s been a real issue with the e46 chassis engines but other chassis have seen their fair share of this phenomenon. Lately, around BMW forums (one in particular) the issue of sourcing why this happens has gotten quite hysterical with folks rigging up pressure gauges and LED’s to monitor the internal pressures of the cooling system. Users are quoting all types of scientific formulas and hypothesis trying to find common criteria and making defacto interpretations from individual experiments. It really is quite funny. But why do BMW Expansion Tanks (ET’s for the BMW enthusiast) blow up? The reason is actually very simple – they are overfilled and have too much coolant in the cooling system with no room to expand. Yes it’s that simple. I’ll bet most reading this paper think they fully understand the BMW cooling system in their car but do you? So you know it’s a “closed cooling system”? However do you know what that actually infers….”closed”? I’d hazard that many do not, especially given the flurry of users replacing their stock ET caps with lower pressure rating release ET caps. Many who replaced their caps believe that pressure will be released (to atmosphere) like an old Open cooling system should pressures build up in excess within the cooling system. It will not. The PSI rating (2 Bar, 1.6 bar, 1.2 bar etc.) is the rated point of relief for coolant to enter the Expansion tank – not vent to atmosphere. If you thought that at some point you might see coolant or steam spitting out from the ET pressure cap, acting in some form of relief mode then you do not understand this cooling system and need to read on. However, if you think about how modern BMW cooling systems are built as a closed system with no real way to release pressure other than breaking, it follows logically that if the expansion tank is filled to the point that the coolant cannot expand anymore and pressure increases due to heat, then at some point the expansion tank will break. – Mike Miller BMW Closed Pressurized Cooling System Overview In automotive cooling systems there are basically two types of designs: Open - if it exchanges matter and energy with atmosphere Closed - if it exchanges energy but not matter to atmosphere From a classical physics perspective there are actually three types (Isolated - if it does not exchange neither energy nor matter) but since we are talking automotive cooling systems we’ll just need to focus on two. An Open system might be something you see in a 1940’s Ford where the radiator cap would typically vent to atmosphere to relieve system pressures in the cooling system. A closed system, used on modern BMW’s exchanges energy through heat transfer but does not exchange matter as a normal function. Modern BMW cooling systems are therefore termed closed systems – meaning they will dissipate heat but not vent matter (coolant or steam) to atmosphere for normal operation. Closed cooling systems thus operate by managing internal system pressures. For improved energy dynamics modern BMW engines produce a lot of heat while operating. All engines are designed to work efficiently within a certain temperature range, normally 185-205° F (or 85-96° C). The cooling Why Does My BMW Expansion Tank Blow Up? system is designed to keep the engine within this temperature range for maximum performance and fuel efficiency. Closed cooling systems are designed to do all of the following: Allow the engine to warm up to its ideal operating temperature quickly. Circulate coolant through the engine to maintain that ideal temperature. Maintain coolant level and pressure by managing coolant volume fluctuations to and from the expansion tank as the coolant heats up and cools down. Permit increased coolant temperatures to maximize engine efficiency Reduce or eliminate venting to atmosphere of harmful gases and liquids. The coolant in your system should be a 50/50 mix of water and antifreeze. Water is what pulls the heat out of the engine, but due to its high freezing temperature, low boiling point, and the fact that it allows corrosion to develop within the system, we need to add antifreeze. Antifreeze provides a lower freezing temperature, a higher boiling temperature, and inhibitors to prevent corrosion. The water/antifreeze mixture (50/50) provides us with a freezing point of -35° F (-37° C) and a boiling point of 223° F (106° C). The boiling point for the cooling system can also be increased by increasing the pressure of the liquid (coolant) in the closed system. The coolant will expand as it heats increasing coolant system pressure, this increased pressure can be used to benefit the efficiency of the cooling system by increasing the boiling point of the coolant. 2 | P a g e Why Does My BMW Expansion Tank Blow Up? While our coolant mixture gives us some wiggle room for running our engine, warmer weather may prove to be an issue. This is where the “pressure” of our cooling system comes into play. When we increase the pressure on a liquid, you raise its boiling point. Science tells us the boiling point is raised 3° F for every 1 psi. Our coolant’s boiling temperature of 223° F (106° C) is when it is under 1 psi, which is why most cooling systems are designed to operate at 15 or 18 psi. As long as the coolant remains in liquid form it can do its job and transfer heat to the radiator or heat exchanger so it can be dissipated. Coolant that is boiling cannot transfer as much heat and overheating is likely to occur if the coolant turns to a gaseous state. As the coolant heats up, it begins to expand and will continue to expand as its temperature increases thus creating additional pressure in the closed cooling system. By allowing this pressure to increase, the boiling point of the coolant is increased to around 257° F (125° C) at 15 psi. This gives us better performance from our cooling system as it can absorb additional heat from the engine without boiling. However this increased pressure and heat must be managed effectively. What happens when the pressure exceeds the system’s psi rating? As you may have 3 | P a g e Why Does My BMW Expansion Tank Blow Up? guessed at this point, regulating the pressure within the cooling system is the job of the pressure cap with an important role provided by the expansion tank it is attached to. Expansion Tank & Pressure Cap What’s Different than Older Cars - On older vehicles without expansion tanks the primary mechanism for managing coolant pressure was the radiator cap. It had two roles. The first was to maintain a level of pressure inside the cooling system to increase boiling temperatures as described earlier. The second function was to vent surpassed coolant pressures to atmosphere. In these older coolant systems the coolant that surpassed the rad cap PSI would vent externally – typically into the engine bay, and normally as steam then boiling coolant. Owners typically carried a jug of pre-mixed coolant or water in the trunk and would top off after the engine cooled on the side of the road. This approach was not optimal as it did not provide the adequate management of coolant system pressures without inconvenience to the operator. Purpose of the Expansion Tank The Expansion Tank is often mistakenly believed to be a coolant reservoir. This is a critical error. The expansion tank purpose is to be a sealed receptacle for receiving expanded coolant and returning it to the coolant system. It is the receptacle the pressure cap uses to store and return the expanded coolant. The need for an expansion tank is necessary in a closed & pressurized cooling system where there is a large fluctuation in coolant volume. If the expanded coolant were simply to be expelled externally from the cap and cooling system, owners would need to top off coolant on a frequent basis like older cars. The expansion tank captures the expelled coolant that has surpassed the Pressure Cap PSI rating (see below for Pressure Cap information). This fluctuation in coolant volume occurs as a result of heat transference. The liquid coolant volume expands as it passes through the engine (heat transferred to coolant), or when the liquid coolant volume decreases as it passes through the radiator (heat transfers to radiator from coolant). The Expansion Tank is also typically located at the highest point within the cooling system. This permits any accumulation of air or gases to collect in the expansion tank where it will be expelled to atmosphere if the closed cooling system is opened via the Pressure Cap at the top of the expansion tank. Purpose of the Pressure Cap The Pressure Cap has a dual purpose. For this purpose it is equipped with two spring valves. The first valve allows pressurized coolant access into the expansion tank; the second valve permits the return of the expanded coolant to the coolant system to maintain the volume of coolant in the cooling components (hoses, rad, heat exchanger etc.) and engine. When the pressure inside the cooling system increases above its psi rating, the pressure cap releases some of the coolant into the expansion tank (not out of the cap onto the ground). It does this by use of a spring loaded valve. Once enough coolant has exited the system to reduce the pressure, the valve closes again.