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 and the environment around the coffee heats up a very very small amount. In a similar way, hot engine blocks transfer heat into their cooling systems and the radiator moves that heat out of the system.
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 be 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 coolant through the engine and through the radiator. Often belt-driven, sometimes electric on more modern engines
2. Radiator - The radiator also has a pretty simple job. It transfers 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 fuel 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 during the bleeding process. Heater core valves regulate the flow rate into the heater core - which is just really another radiator, but that evacuates it's heat output into the cabin instead of the ambient air
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 means 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 normal 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 and A/C operation Then later on they upgraded to electric water pumps and really at the end of the day, 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. Afraid your fan clutch will explode? Replace your fan clutch with new (blades usually don't need to be replaced) and replace your engine mounts.
Don't change your thermostat if your car is naturally aspirated. It only makes sense to change your thermostat if you added forced induction 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. It is critical to note though, a lower temp thermostat on it's own does not increase cooling capacity in any way. If you want more cooling capacity - there is ONE thing for that, a bigger radiator 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. Also - I may as well add in my bleeding technique, as this seems to be one of the most common questions I see asked. I've never had to mess with raising the front of the car or anything like that. I've always bled on flat level ground. I will describe two methods, depending on whether or not you have a pressure tester...
Starting with a COLD engine, open expansion tank cap and bleed screw and fill the system as full as you can
Tighten the expansion tank cap and bleed screw then squeeze the upper and lower radiator hoses. Crack the bleed screw and squeeze a few more times. If you do not feel/observe water moving in the hoses, top up with more coolant (Do you have a pressure tester? Look at my hint below... it may save you a lot of time later in the process!)
Set the heat to the hottest setting and the fan to the lowest setting. Start the engine
As it warms up, be mindful to check for leaks (especially if you don't have a pressure tester!) and crack the bleed screw periodically. Once every minute or so, I will just go and crack it. Close it once only coolant comes out. If you crack it and nothing comes out, just close it again and wait another minute or so. I also like to squeeze the hoses as it warms up
Once near operating temperature, I start to rev the engine a little. Just a short dab of the throttle to get the water pump moving and once it is back to idle, I crack the bleed screw (I don't open the bleed screw while engine is above idle, but I like to crack it right afterwards, to catch any bubbles that are being moved through the system)
As engine reaches operating temp, I continue to rev it and crack the bleeder. Eventually at this point you should have solid coolant coming out after a few consecutive cracks of the bleed screw.
The car should then be left long enough to let the coolant reach ambient temp. Then check the fill level - do not overfill it. Just above the "Cold/Kalt" line is fine, or on those with the red bobber dipsticks, just "off the bottom" is fine.
If the radiator was removed, remove the upper radiator hose from the thermostat housing and from this hose directly into radiator until coolant drips out of the thermostat housing Do you have a pressure tester? I recommend doing a "cold bleed" during step 2. Connect the pressure tester, close bleed screw and pump it up to normal operating pressure. Crack the bleeder screw once pressurized and squeeze the upper and lower radiator hoses alternatingly. If you do not feel/observe water moving in the hoses, top up with more coolant. Repeatedly re-pressurize/crack bleeder/squeeze hoses until you observe water in the system. Pressurizing it at this step helps force the coolant into all the cavities and move the air out which saves time later. Be aggressive when squeezing the hoses and squeeze them in different ways.
At the last step if you do not see coolant coming out of the bleed screw and the engine is at operating temperature, there may be too little coolant in the system. Often this happens if the bleeder screw is left cracked too long earlier in the process (It should only be cracked in short increments to let the air out, once it is solid coolant it should be tightened). In this case I would recommend leaving it until the next morning to allow the coolant to cool so you can check the fill level accurately.
At the last step if it seems like you have a geyser coming out, your system may be overfilled - I would recommend leaving until cool the next morning, checking the fill level and removing any excess. I do not recommend trying to let it "burp" the excess out - the system doesn't self-regulate the volume very well, it will quite readily dump out enough that it will be low on coolant.
Simplified bleeding instructions for the savvy home mechanic:
Starting with a COLD engine, fill the cooling system completely
Tighten the expansion tank cap and bleed screw then squeeze the upper and lower radiator hoses, top up with additional coolant if no water felt in system
Set the heat to the hottest setting and the fan to the lowest setting. Start the engine
Crack the bleed screw periodically. Close it once only coolant comes out. Rev engine occasionally as car starts to warm up and crack bleeder once back at idle
As engine reaches operating temp, you should have only solid coolant coming out after a few consecutive cracks of the bleed screw
The car should then be left long enough to let the coolant reach ambient temp. Then check the fill level - do not overfill it. Just above the "Cold/Kalt" line is fine, or on those with the red bobber dipsticks, just "off the bottom" is fine