Cooling systems have not changed much since the earliest automobiles were created but they are absolutely critical to the longevity and reliability of an engine. This article will go over the basics of automotive cooling systems and talk about some specific BMW cooling systems including E36, Z3, E39 and E53 a little later on, as these models represent a good range of cooling system technology.
First though, I want to go over the idea of what "pressure" is and what "heat" is. Pressure is a simple mathematical formula, P = F/A (Pressure = Force/Area). Heat is a measurement of the kinetic movement of the molecules within an object. In simpler terms - heat means the molecules inside something are moving, the hotter something is, the more the molecules are moving. This is also the cause of thermal expansion - things that are hotter expand to a larger size because of the force of the molecules inside the material applying an "outwards" pressure. Heat can be transferred from one material to another and will do so until an equilibrium temperature is reached. Hot coffee, when left out, reaches an equilibrium temperature with the surrounding environment. The coffee decreases in temperature, which is to say the molecules inside the coffee slow down. In a similar way, hot engine blocks transfer heat into their cooling systems. Cooling systems, when operating correctly, are sealed systems. The volume of the cooling system is also static - which means the "Area" in the P=F/A formula is pre-defined.
The "Force" in that formula comes from the movement of the molecules in the coolant. This force increases as the coolant absorbs more heat from the engine, the source of this heat of course being the combustion of air/fuel in the combustion chamber. The force increases, the area stays the same therefore the pressure inside the cooling system will increase. This is why it is an exceptionally bad idea to open your expansion tank cap when the car is overheating. It's a bad idea when the car is at operating temperature, because you are exposing a system with a lot of heat and thus a lot of force, to ambient pressure. You rapidly change the "Area" in the formula from being a sealed, pre-defined value, to being for all intents and purposes, infinite - as you expose it to the open atmosphere. The system will rapidly and violently (especially if overheating) de-pressurize out of the opening.
This is also why it is critical to not overfill cooling systems. Cooling systems should only have their levels checked cold and should only ever be filled to the "kalt" (which is German for "cold") line, never above it. This fill line is never at the top of the tank. If the cooling system is overfilled, the pressure will increase inside the cooling system and you will quickly discover the weakest link in your cooling system via a leak or sometimes an explosion.
I will being explaining the basic operation of an automotive cooling system by starting with the most basic components: 1. Water Pump - The water pump has a very simple job, it pumps the coolant through the radiator and through the engine. Often belt-driven, sometimes electric on more modern engine
2. Radiator - The radiator also has a pretty simple job. It radiates heat, aka it transfers excess heat from the cooling system into the air passing through it. There are thin channels in the middle of the radiator which the coolant passes through, with thin metal fins attached to them. The fins serve to provide more surface area for the air passing through the radiator which helps with cooling ability
3. Expansion tank - the expansion tank is there to allow a certain amount of air in the cooling system so that as the coolant gets hot and expands, it has something to expand into. Air is compressible, fluids are not
4. Thermostat - the thermostat serves to regulate the flow of hot coolant out of an engine. It does this in order to keep the combustion chamber at the ideal temperature for combustion efficiency, improving performance and economy
That's basically it. The rest of the cooling system components are sort of added on to the basic system above, in order to allow for proper operation over a wider range of situations or for ease of maintenance. For instance, fans are fitted in order to provide airflow through the radiator in order to cool the coolant when the car is stationary or moving at a speed not fast enough to provide adequate airflow. Some cars have auxiliary water pumps, due to the enormous volume of coolant and coolant lines in their cooling systems, in order to allow the coolant to reach all the places it needs to go and maintain a proper flow rate while doing so. There are coolant temperature sensors integrated into the systems at various places in order to provide relays, the instrument cluster or the DME with data. There are bleed screws, often integrated into the expansion tank, which let you evacuate excess air out of the system. Heater core valves regulate the flow rate into the heater core - which are just really another radiator, but that evacuates it's heat output into the cabin
So now lets talk about detailed operational principles of specific BMW models. I will start with the E36, and the reason for starting here is simple - this was basically the last of this "traditional" style cooling system. The operation of this system extends really back to the '70s and earlier and is identical in operation to a ton of other car models
On the E36 there is a belt-driven water pump and a traditional thermostat. A temperature sensor mounted on the upper outlet side of the radiator feeds data to a set of relays which determine which of 2 speeds the auxiliary pusher fan, mounted in front of the radiator, should spin. A radiator-mounted expansion tank meant the system had to be manually bled.
This is all pretty typical and this system is shared with the M52 (non-TU) Z3 and all S52 powered Z3 M roadsters and M coupes as well as the E39 528i with the M52 non-TU engine
However the Z3 - which was an E36 chassis - was upgraded to M52TU, M54 and S54 engines and BMW changed part of the cooling system when they installed these engines. For the M52TU, M54 and S54 variants they made the system self-bleeding by moving the expansion tank to a location above the engine, allowing excess air in the system to naturally rise to the top during operation.
All M52TU and M54 engines also received electronically assisted thermostats, which allows the DME to close or open the thermostat earlier or later in order to adjust the temperature inside the engine. This allows it to increase the combustion chamber temperature to increase efficiency when cruising, and lower it to allow for more compression without pre-detonation when driving hard.
The E39 models with M52TU, M54 and M62TU (these include 525i, 528i, 530i an 540i) engines received the electronically assisted thermostats, but did not relocate the expansion tank for self-bleeding. However, the E39 models got something the Z3 did not - the PWM fan. All Z3 models received the older style E36 2-speed fan, but the E39 (and other models with these engines) received an infinitely variable PWM fan. PWM stands for pulse width modulation and uses short impulses of electricity to set the speed of the fan, determined based on several variables including but not limited to engine load, ambient temperature, A/C operation, throttl... Blah blah. Does anyone really care? I'm boring myself just writing this. Listen, cooling systems are simple. Just bleed your system properly, don't overfill it and that "unreliable plastic" stuff will last forever. If you want to upgrade your cooling system get a bigger radiator. If you want to delete your mechanical fan (which I do not personally recommend) then at least use a lower temp fan switch to kick the aux fan on earlier. Don't change your thermostat if your car is naturally aspirated. It only makes sense to change your thermostat if you added a turbocharger to a naturally aspirated engine. The reason for that is a turbocharged engine creates a lot more heat a lot quicker than a naturally aspirated engine, so you lower the temp at the head to keep temp at the combustion chamber the same as if it was naturally aspirated (at least as close as you can). To reiterate the goal is not to "lower" the temperature of the combustion chamber, rather the goal is to keep the temperature at the combustion chamber the same, but with an increased thermal load you run a lower thermostat to allow for increased cooling capacity to carry away some of that extra heat. Also, shrouding is important. Chasing your tail on a minor overheating issue? Check to make sure the plastics and ducting is in place. Without proper airflow you won't have proper cooling and without proper ducting you won't have proper airflow. Air is lazy, it will go around a radiator instead of through it if it can. So don't let it. I feel this article is incomplete, I will circle back and add some images in the future... but seriously, cooling systems are pretty simple. Don't overthink it. Buy a pressure tester at harbor freight, it will be your best friend.